Tag Archives | system

15+ examples for yum update command

Yum is a package manager used on Red Hat, CentOS, and other Linux distributions that use RPM Package Manager. Yum is used to install, update, delete, or otherwise manipulate the packages installed on these Linux systems. In this tutorial, we will cover the yum update command – what it is, how to use it, and all the different commands you may need to know when you wish to upgrade the installed packages on your system. Yum update is the command used to update applications installed on a system. If the command is run without any package names specified, it will update every currently installed package on the system. When running this command, yum will begin by checking its repositories for updated version of the software your system currently has installed. The screenshot below shows the type of output you’ll typically see when first issuing the yum update command.

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How does yum update work?

Yum update is the command used to update applications installed on a system. If the command is run without any package names specified, it will update every currently installed package on the system.

yum update

When running this command, yum will begin by checking its repositories for updated version of the software your system currently has installed. The screenshot below shows the type of output you’ll typically see when first issuing the yum update command.

yum update command

As you can see, the output from yum first lists the repositories it’s querying, which are the default ones for CentOS: AppStream, Base, and Extras. Below that, yum lists the various packages which it has found updates for.

At the tail end of this output, yum will display the “Transaction Summary,” which shows the total number of packages that are to be installed and upgraded.

yum update summary

In this example, 166 packages are being upgraded, and 6 new packages are being installed.

In case you’re wondering why new packages are being installed when we are only supposed to be upgrading applications, some new software packages may have become part of this Linux distribution, or some upgraded applications may rely on extra packages that are not yet installed.

Once you review the list of software that yum plans to upgrade, you can confirm these changes by typing “y” and hitting enter.

Yum will then perform the upgrades, which may take some time depending on the speed of your connection and the system itself.

Once it has finished, you’ll get a final summary which will list all the packages that were successfully upgraded, as well as any errors that may have been encountered.

yum update complete

Update without gpg checking

GPG keys are used to verify the authenticity of an RPM package. The –nogpgcheck option in yum will instruct it to skip checking GPG signatures on packages. This is useful in cases where you have an unsigned package or you just don’t have the GPG key.

yum update --nogpgcheck

This is a quick solution if you encounter an error like “Package NameOfPackage.rpm is not signed .. install failed!” when running the normal yum update command. The –nogpgcheck option will ignore this warning and proceed with upgrading the package anyway.

Update from a local repo

It’s possible to set up local repositories for yum to query when it does an update. This is often done if you want to use yum to update packages that aren’t included in the default repos, or if you need to upgrade an offline system.

First, place all your updated RPM files in a new folder. In this example, we’ll use /root/rpms.

Next, navigate to the following directory where you can see all the repo files for yum:

cd /etc/yum.repos.d

Local repo files

To set up a local repo, create a new file in this directory.

vi MyRepo.repo

Inside your repo file, configure it in this format, changing the lines as necessary:

[MyRepo]

name=My Local Repo

baseurl=file:///root/rpms

enabled=1

gpgcheck=0

The big difference between a local repo and a remote repo is in the “baseurl” line, where the file:// protocol is specifying a local file, as opposed to the remote protocols http:// or ftp://

Once the file has been saved, apply the correct permissions:

chmod 644 MyRepo.repo

The repository should now be ready to use. Be sure clear yum’s cache before attempting a yum update command:

yum clean all

Show patches

Yum can display available security patches, without installing them, with this command:

yum updateinfo list security

List specific patches

If no output is returned, like in the screenshot above, this means there are no security patches available for any installed software on your system.

Update a single package

If you need to update a certain package without running an update for every application installed, just specify the name of the package in your yum update command.

yum update name-of-package

Multiple packages can be specified, separated by a space. You need to have the name of the package typed perfectly in order for yum to find it in its repositories; if you’re not sure of a package name, first check what packages are currently eligible for updates:

yum check-update

Update all but one package

If you need to run the yum update command but you wish to exclude a package from being updated, you can specify the –exclude option.

A common situation where administrators may find this necessary is with kernel updates, since these are major updates that could cause unpredictable errors on a production server. However, they may still want to run the command to update less sensitive applications.

To exclude a package (in this example, those related to the kernel):

yum update --exclude=kernel*

The asterisk acts as a wildcard, in case there are multiple related packages or you don’t know the full name of the package.

Alternatively:

yum update -x 'kernel*'

Exclude multiple packages

You can exclude multiple packages with more –exclude flags.

yum update --exclude=kernel* --exclude=httpd

Use this flag as in the example above, or the -x flag, as many times as needed.

Check when last yum update ran

To see a list of yum transactions, with the date and time they were ran, use the yum history command.

yum history

Check yum update history

In the screenshot above, you can see that the last time yum updated software was on January 4th.

Rollback (revert) update

A great feature of yum is that it allows you to undo a recent update, thus restoring the upgraded packages to their previous versions.

Each yum action (install, update, erase, etc) is assigned a transaction ID, and this ID must be specified when undoing a yum update. To see a list of transaction IDs for recent yum operations, use this command:

yum history

List yum history

In the screenshot above, you can see the last operation run with yum was to install the httpd package. Undoing an installation or an update works the same way, so in this example, we will undo this recent installation of httpd. As shown in the screenshot, this transaction has an ID of 7.

To undo this change and roll back the program to its previous version, issue this command:

yum history undo 7

Undo yum update

As usual, yum will summarize the changes to be made and ask if you’d like to proceed with a Y/N prompt. Enter Y and the specified transaction will be undone.

yum undo report

Clean up a failed yum update (Troubleshooting)

If one or more packages fail to upgrade successfully when you run the yum update command, the system can end up with duplicate packages installed (2 versions of the same program).

Sometimes, following the rollback instructions in the section above can fix the problem. If that doesn’t work, you can remove duplicate packages on your system with this command:

package-cleanup --dupes

Yum stores a cache of information for packages, metadata, and headers. If you encounter an error, clearing yum’s cache is a good first step in troubleshooting. Use the following command to do that:

yum clean all

yum clean command

Skip errors

When updating or installing a package, that package may require additional software in order to run correctly. Yum is aware of these dependencies and will try to resolve them during updates by installing or upgrading the extra packages that are needed.

If yum has trouble installing the necessary dependencies, it produces an error and doesn’t proceed further. This is a problem if you have other packages that need to be updated.

To instruct yum to proceed with updating other packages and skipping the ones with broken dependencies, you can specify the –skip-broken command in your yum update command.

yum update --skip-broken

Get a list of packages that need an update

Running the yum update command as normal, with no additional options, will output a list of available updates.

yum update

If you’d like to see some additional information about the package updates available, type this command:

yum updateinfo

To see information about security updates that are available for the system, type this command:

yum updateinfo security

Difference between yum check updates and list update

Although the two commands sound similar, so there is a difference between checking for updates and listing updates in yum.

yum list updates

The command to list updates, shown above, will list all the packages in the repositories that have an update available. Keep in mind that some of the packages in the repositories may not even be installed on your system.

yum check-update

The command to check for updates, seen above, is a way to check for updates without prompting for interaction from the user. This is the command you would opt for if you were coding a script to check for updates, for example.

The check-update command will return an exit value of 100 if there are packages that have updates available, and it will return an exit value of 0 if there are no available updates.

A value of 1 is returned if an error is encountered. Use these exit codes to code your script accordingly.

Notify when updates are available

There are a few packages that can help manage yum updates on your system. Some can even notify administrators when yum has updates that are available to be installed. One such service is called yum-cron.

Install yum-cron using yum:

yum install yum-cron

Set the yum-cron service to start at boot:

systemctl enable yum-cron.service

systemctl start yum-cron.service

Configure the settings for yum-cron inside the configuration file using vi or your preferred text editor:

vi /etc/yum/yum-cron.conf

In this file, you can specify if the updates should be automatically applied or not. If you’d only like to receive notifications, fill out the email information inside the configuration file. Yum-cron will then send you an email anytime there are updates available for your system.

apply_updates = no #don’t apply updates automatically

email_from = root@localhost

email_to = admin@example.com

email_host = localhost

What port does yum update use

Yum uses port 80 when checking for updates. If you look inside the repository files on your system, you’ll see that all of the links inside begin with http.

If you need to create a rule in your firewall to allow yum to function, you need to allow port 80.

Yum update vs upgrade

So far, we have only talked about the yum update command in this tutorial, but there’s another very similar command: yum upgrade.

yum upgrade

There is a small difference between these two commands. Yum update will update the packages on your system, but skip removing obsolete packages.

Yum upgrade will also update all the packages on your system, but it will also remove the obsolete packages.

This inherently makes yum update the safer option, since you don’t have to worry about accidentally removing a necessary package when updating your software.

Use some discretion when issuing the yum upgrade command, since it may not preserve some packages that you are still using.

At last, I hope you find the tutorial useful.

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Linux find command tutorial (with examples)

When it comes to locating files or directories on your system, the find command on Linux is unparalleled. It’s simple to use, yet has a lot of different options that allow you to fine-tune your search for files. Read on to see examples of how you can wield this command to find anything on your system. Every file is only a few keystrokes away once you know how to use the find command in Linux. You can tell the find command to look specifically for directories with the -type d option. This will make find command only search for matching directory names and not file names. Since hidden files and directories in Linux begin with a period, we can specify this search pattern in our search string in order to recursively list hidden files and directories.

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Find a directory

You can tell the find command to look specifically for directories with the -type d option. This will make find command only search for matching directory names and not file names.

find /path/to/search -type d -name "name-of-dir"

Find directory

Find hidden files

Since hidden files and directories in Linux begin with a period, we can specify this search pattern in our search string in order to recursively list hidden files and directories.

find /path/to/search -name ".*"

Find files of a certain size or greater than X

The -size option on find allows us to search for files of a specific size. It can be used to find files of an exact size, files that are larger or smaller than a certain size, or files that fit into a specified size range. Here are some examples:

Search for files bigger than 10MB in size:

find /path/to/search -size +10M

Search for files smaller than 10MB in size:

find /path/to/search -size -10M

Search for files that are exactly 10MB in size:

find /path/to/search -size 10M

Search for files that are between 100MB and 1GB in size:

find /path/to/search -size +100M -size -1G

Find from a list of files

If you have a list of files (in a .txt file, for example) that you need to search for, you can search for your list of files with a combination of the find and grep commands. For this command to work, just make sure that each pattern you want to search for is separated by a new line.

find /path/to/search | grep -f filelist.txt

The -f option on grep means “file” and allows us to specify a file of strings to be matched with. This results in the find command returning any file or directory names that match those in the list.

Find not in a list

Using that same list of files we mentioned in the previous example, you can also use find to search for any files that do not fit the patterns inside the text file. Once again, we’ll use a combination of the find and grep command; we just need an additional option specified with grep:

find /path/to/search | grep -vf filelist.txt

The -v option on grep means “inverse match” and will return a list of files that don’t match any of the patterns specified in our list of files.

Set the maxdepth

The find command will search recursively by default. This means that it will search the specified directory for the pattern you specified, as well as any and all subdirectories within the directory you told it to search.

For example, if you tell find to search the root directory of Linux (/), it will search the entire hard drive, no matter how many subdirectories of subdirectories exist. You can circumvent this behavior with the -maxdepth option.

Specify a number after -maxdepth to instruct find on how many subdirectories it should recursively search.

Search for files only in the current directory and don’t search recursively:

find . -maxdepth 0 -name "myfile.txt"

Search for files only in the current directory and one subdirectory deeper:

find . -maxdepth 1 -name "myfile.txt"

Find empty files (zero-length)

To search for empty files with find, you can use the -empty flag. Search for all empty files:

find /path/to/search -type f -empty

Search for all empty directories:

find /path/to/search -type d -empty

It is also very handy to couple this command with the -delete option if you’d like to automatically delete the empty files or directories that are returned by find.

Delete all empty files in a directory (and subdirectories):

find /path/to/search -type f -empty -delete

Find largest directory or file

If you would like to quickly determine what files or directories on your system are taking up the most room, you can use find to search recursively and output a sorted list of files and/or directories by their size.

How to show the biggest file in a directory:

find /path/to/search -type f -printf "%s\t%p\n" | sort -n | tail -1

Notice that the find command was sorted to two other handy Linux utilities: sort and tail. Sort will put the list of files in order by their size, and tail will output only the last file in the list, which is also the largest.

You can adjust the tail command if you’d like to output, for example, the top 5 largest files:

find /path/to/search -type f -printf "%s\t%p\n" | sort -n | tail -5

Alternatively, you could use the head command to determine the smallest file(s):

find /path/to/search -type f -printf "%s\t%p\n" | sort -n | head -5

If you’d like to search for directories instead of files, just specify “d” in the type option. How to show the biggest directory:

find /path/to/search -type d -printf "%s\t%p\n" | sort -n | tail -1

Find setuid set files

Setuid is an abbreviation for “set user ID on execution” which is a file permission that allows a normal user to run a program with escalated privileges (such as root).

This can be a security concern for obvious reasons, but these files can be easy to isolate with the find command and a few options.

The find command has two options to help us search for files with certain permissions: -user and -perm. To find files that are able to be executed with root privileges by a normal user, you can use this command:

find /path/to/search -user root -perm /4000

Find suid files

In the screenshot above, we included the -exec option in order to show a little more output about the files that find returns with. The whole command looks like this:

find /path/to/search -user root -perm /4000 -exec ls -l {} \;

You could also substitute “root” in this command for any other user that you want to search for as the owner. Or, you could search for all files with SUID permissions and not specify a user at all:

find /path/to/search -perm /4000

Find sgid set files

Finding files with SGID set is almost the same as finding files with SUID, except the permissions for 4000 need to be changed to 2000:

find /path/to/search -perm /2000

You can also search for files that have both SUID and SGID set by specifying 6000 in the perms option:

find /path/to/search -perm /6000

List files without permission denied

When searching for files with the find command, you must have read permissions on the directories and subdirectories that you’re searching through. If you don’t, find will output an error message but continue to look throughout the directories that you do have permission on.

Permission denied

Although this could happen in a lot of different directories, it will definitely happen when searching your root directory.

That means that when you’re trying to search your whole hard drive for a file, the find command is going to produce a ton of error messages.

To avoid seeing these errors, you can redirect the stderr output of find to stdout, and pipe that to grep.

find / -name "myfile.txt" 2>%1 | grep -v "Permission denied"

This command uses the -v (inverse) option of grep to show all output except for the lines that say “Permission denied.”

Find modified files within the last X days

Use the -mtime option on the find command to search for files or directories that were modified within the last X days. It can also be used to search for files older than X days, or files that were modified exactly X days ago.

Here are some examples of how to use the -mtime option on the find command:

Search for all files that were modified within the last 30 days:

find /path/to/search -type f -mtime -30

Search for all files that were modified more than 30 days ago:

find /path/to/search -type f -mtime +30

Search for all files that were modified exactly 30 days ago:

find /path/to/search -type f -mtime 30

If you want the find command to output more information about the files it finds, such as the modified date, you can use the -exec option and include an ls command:

find /path/to/search -type f -mtime -30 -exec ls -l {} \;

Sort by time

To sort through the results of find by modified time of the files, you can use the -printf option to list the times in a sortable way, and pipe that output to the sort utility.

find /path/to/search -printf "%T+\t%p\n" | sort

This command will sort the files older to newer. If you’d like the newer files to appear first, just pass the -r (reverse) option to sort.

find /path/to/search -printf "%T+\t%p\n" | sort -r

Difference between locate and find

The locate command on Linux is another good way to search for files on your system. It’s not packed with a plethora of search options like the find command is, so it’s a bit less flexible, but it still comes in handy.

locate myfile.txt

The locate command works by searching a database that contains all the names of the files on the system. The database that it searches through is updated with the upatedb command.

Since the locate command doesn’t have to perform a live search of all the files on the system, it’s much more efficient than the find command. But in addition to the lack of options, there’s another drawback: the database of files only updates once per day.

You can update this database of files manually by running the updatedb command:

updatedb

The locate command is particularly useful when you need to search the entire hard drive for a file, since the find command will naturally take a lot longer, as it has to traverse every single directory in real-time.

If searching a specific directory, known to not contain a large number of subdirectories, it’s better to stick with the find command.

CPU load of find command

When searching through loads of directories, the find command can be resource-intensive. It should inherently allow more important system processes to have priority, but if you need to ensure that the find command takes up fewer resources on a production server, you can use the ionice or nice command.

Monitor CPU usage of the find command:

top

Reduce the Input/Output priority of find command:

ionice -c3 -n7 find /path/to/search -name "myfile.txt"

Reduce the CPU priority of find command:

nice -n 19 find /path/to/search -name "myfile.txt"

Or combine both utilities to really ensure low I/O and low CPU priority:

nice -n ionice -c2 -n7 find /path/to/search -name "myfile.txt"

I hope you find the tutorial useful. Keep coming back.

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15+ examples for listing users in Linux

In this post, you will learn about listing users in Linux. Besides this, you will know other tricks about Linux users’ characteristics. There are 2 types of users in Linux, system users who are created by default with the system. On the other hand, there are regular users who are created by system administrators and can log in to the system and use it. Before we start listing users, we need to know where are these users saved on Linux? The users are stored in a text file on the system called the passwd file. This file is located in the /etc directory. The file is located on the following path:

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/etc/passwd

In this file, you can find all the information about the users in the system.

List all users

Listing users is a the first step to manage them. This way we will know how many they are and who they are. In Linux, almost everything can be done in various ways and this is no exception.

To list all users, you can use the cat command:

cat /etc/passwd

list all users in Linux

As you can see in the image, there is all the information about the users.

1- In the first field, you will see the user name.

2- Then, a representation of the encrypted password (The x character). The encrypted password is stored in /etc/shadow file.

3- The UID or the user ID.

4- The next field refers to the primary group of the user.

5- Then, it shows user ID info such as the address, email, etc.

6- After this, you will see the home directory of the user.

7- The last field is the shell used by that user.

However, although the information is quite useful but if you only want to list users’ names in a basic way, you can use this command:

cut -d: -f1 /etc/passwd

Listing users in Linux

Now we have the names only by printing the first field of te file only.

List & sort users by name

The above command serves the purpose of listing users on Linux. But what about listing the users in alphabetical order?

To do this, we will use the previous command, but we will add the sort command.

So, the command will be like this:

cut -d: -f1 /etc/passwd | sort

Sort by name

As you can see in the image, the users are shown sorted.

Linux list users without password

It is important to know users who have no password and to take appropriate action. To list users who do not have a password, just use the following command:

sudo getent shadow | grep -Po '^[^:]*(?=:.?:)'

User with no password

The used regex will list all users with no password.

List users by disk usage

If you have a big directory and you want to know which user is flooding it, you can use the du command to get the disk usage.

With this, you can detect which of these users are misusing the disk space.

For it, it is enough to use the following command:

sudo du -smc /home/* | sort -n

List users by disk usage

In this way, you will have the users ordered by the disk usage for the /home directory.

We used the -n for the sort command to sort the output by numbers.

List the currently logged users

To list the currently logged in users, we have several ways to do it. The first method we can use the users command:

users

Users currently logged

It will list the users with open sessions in the system.

But this information is a little basic however, we have another command that gives more details. The command is simply w.

w

Using the w command to list users currently logged

With this command, we can have more information such as the exact time when the session was started and the terminal session he has available.

Finally, there is a command called who. It is available to the entire Unix family. So you can use it on other systems like FreeBSD.

who

The who command

With who command, we also have some information about currently logged in users. Of course, we can add the option -a and show all the details.

who -a

The who command with options

So, this way you know everything about the logged in users.

Linux list of users who recently logged into the system

We saw how to get the currently logged in users, what about listing the login history of users?

You can use the last command to get more info about the logins that took place:

last

The last command

Or the logins of a particular user

last [username]

For example:

last angelo

last command with specific user

These are the user login activity and when it was started and how long it took.

List users’ logins on a specific date or time

What about listing users’ logins on a specific date or time? To achieve this, we use the last command but with the -t parameter:

last -t YYMMDDHHMMSS
For example:
last -t 20190926110509

List users by a specific date

And now all you have to do is choose an exact date & time to list who logged on that time.

List all users in a group

There are 2 ways to list the members of a group in Linux, the easiest and most direct way is to get the users from the /etc/group file like this:

cat /etc/group | grep likegeeks

This command will list users in the likegeeks group.

The other way is by using commands like the members command in Debian based distros. However, it is not installed by default in Linux distributions.

To install it in Ubuntu / Linux Mint 19, just use APT:

sudo apt install members

Or in the case of CentOS:
sudo dnf install members

Once it’s installed, you can run the command then the name of the group you want to list the users to:

members [group_name]

For example:
members avahi

Using the members command

This way you can list users for a group in a Debian based distro. What about a RedHat based distro like CentOS?

You can use the following command:

getent group likegeeks

List users with UID

In Unix systems, each user has a user identifier or ID. It serves to manage and administer accounts internally in the operating system.

Generally, UIDs from 0 to 1000 are for system users. And thereafter for regular users. Always on Unix systems, UID zero belongs to the root users (You can have more than one user with UID of zero).

So now we will list the users with their respective UID using awk.

The command that performs the task is the following:

awk -F: '{printf "%s:%s\n",$1,$3}' /etc/passwd

List users with the UID

As you can see, each user with his UID.

List root users

In a Unix-like system like Linux, there is usually only one root user. If there are many, how to list them?

To do this, we can use this command:

grep 'x:0:' /etc/passwd

root users in the system

Here we are filtering the file to get users with UID of zero (root users).

Another way by checking the /etc/group file:

grep root /etc/group

The root users in Linux

Here we are getting users in the group root from the /etc/group file.

Also, you can check if any user can execute commands as root by checking the /etc/sudoers file:

cat /etc/sudoers

Get the total number of users

To get the total number of users in Linux, you can count lines in /etc/passwd file using the wc command like this:

cut -d: -f1 /etc/passwd | wc -l

List total number of users in Linux

Great! 43 users. But this includes system and regular users. What about getting the number of regular users only?

Easy! Since we know from above that regular users have UID of 1000 or greater, we can use awk to get them:

awk -F: '$3 >= 1000 {print $1}' /etc/passwd

List regular users

Cool!

List sudo users

Linux systems have a utility called sudo that allows you to execute commands as if you were another user who is usually the root user.

This should be handled with care in a professional environment.

Also, it is very important to know which users can run the sudo command. For this, it is enough to list the users that belong to the sudo group.

members sudo

sudo group users

Users in this group can execute commands as super users.

List users who have SSH access

SSH allows users to access remote computers over a network. It is quite secure and was born as a replacement for Telnet.

On Linux by default, all regular users can log in and use SSH. If you want to limit this, you can use the SSH configuration file (/etc/ssh/ssh_config) and add the following directive:

AllowUsers user1 user2 user3
Also, you can allow groups instead of allowing users only using the AllowGroups directive:
AllowGroups group1 group2 group3

These directives define who can access the service. Don’t forget to restart the SSH service.

List users who have permissions to a file or directory

We can give more than one user permission to access or modify files & directories in two ways.

The first method is by adding users to the group of the file or the directory.

This way, we can list the group members using the members utility as shown above.

Okay, but what if we just want this user to have access to this specific file only (Not all the group permissions)?

Here we can set the ACL for this file using setfacl command like this:

setfacl -m u:newuser:rwx myfile

Here we give the user called newser the permission for the file called myfile the permissions of read & write & execute.

Now the file can be accessed or modified by the owner and the user called newuser. So how to list them?

We can list them using the getfacl command like this:

getfacl myfile

This command will list all users who have permissions for the file with their corresponding permissions.

List locked (disabled) users

In Linux, as a security measure, we can lock users. This as a precaution if it is suspected that the user is doing things wrong and you don’t want to completely remove the user and just lock him for investigation.

To lock a user, you can use the following command:

usermod -L myuser

Now the user named myuser will no longer to able to login or use the system.

To list all locked users of the system, just use the following command:

cat /etc/passwd | cut -d : -f 1 | awk '{ system("passwd -S " $0) }' | grep locked

This will print all locked users including system users. What about listing regular users only?

As we saw above, using awk we can get locked regular users like this:

awk -F: '$3 >= 1000 {print $1}' /etc/passwd | cut -d : -f 1 | awk '{ system("passwd -S " $0) }' | grep locked

Very easy!

Listing remote users (LDAP)

Okay, now can list all system users (local users), but what about remote users or LDAP users? Well, we can use a tool like ldapsearch, but is there any other way?

Luckily yes! You can list local & remote users with one command called getent

getent passwd

This command lists both local system users and LDAP or NIS users or any other network users.

You can pipe the results of this command to any of the above-mentioned commands the same way.

Also, the getent command can list group accounts like this:

getent group

You can check the man page of the command to know the other databases the command can search in.

Conclusion

Listing users in the Linux system was fun! Besides this, we have learned some tips about users and how to manage them in different ways.

Finally, this knowledge will allow a better administration of the users of the system.

I hope you find the tutorial useful. keep coming back.

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How to write practical shell scripts

In the last post, we talked about regular expressions and we saw how to use them in sed and awk for text processing, and we discussed before Linux sed command and awk command. During the series, we wrote small shell scripts, but we didn’t mix things up, I think we should take a small step further and write a useful shell script. However, the scripts in this post will help you to empower your scriptwriting skills. You can send messages to someone by phone or email, but one method, not commonly used anymore, is sending a message directly to the user’s terminal. We are going to build a bash script that will send a message to a user who is logged into the Linux system. For this simple shell script, only a few functions are required. Most of the required commands are common and have been covered in our series of shell scripting; you can review the previous posts.

Continue Reading →

Sending Messages

First, we need to know who is logged in. This can be done using the who command which retrieves all logged in users.

who

shell scripts who command

To send a message you need the username and his current terminal.

You need to know if messages are allowed or not for that user using the mesg command.

mesg

mesg command

If the result shows “is y” that means messaging is permitted. If the result shows “is n”, that means messaging is not permitted.

To check any logged user message status, use the who command with -T option.

who -T

If you see a dash (-) that means messages are turned off and if you see plus sign (+) that means messages are enabled.

To allow messages, type mesg command with the “y” option like this

mesg y

allow messages

Sure enough, it shows “is y” which means messages are permitted for this user.

Of course, we need another user to be able to communicate with him so in my case I’m going to connect to my PC using SSH and I’m already logged in with my user, so we have two users logged onto the system.

Let’s see how to send a message.

Write Command

The write command is used to send messages between users using the username and current terminal.

For those users who logged into the graphical environment (KDE, Gnome, Cinnamon or any), they can’t receive messages. The user must be logged onto the terminal

We will send a message to testuser user from my user likegeeks like this:

write testuser pts/1

write command

Type the write command followed by the user and the terminal and hit Enter.

When you hit Enter, you can start typing your message. After finishing the message, you can send the message by pressing the Ctrl+D key combination which is the end of file signal. I recommend you to review the post about signals and jobs.

Receive message

The receiver can recognize which user on which terminal sends the message. EOF means that the message is finished.

I think now we have all the parts to build our shell script.

Creating The Send Script

Before we create our shell script, we need to determine whether the user we want to send a message to him is currently logged on the system, this can be done using the who command to determine that.

logged=$(who | awk -v IGNORECASE=1 -v usr=$1 '{ if ($1==usr) { print $1 }exit }')

We get the logged in users using the who command and pipe it to awk and check if it is matching the entered user.

The final output from the awk command is stored in the variable logged.

Then we need to check the variable if it contains something or not:

if [ -z $logged ]; then

echo "$1 is not logged on."

echo "Exit"

exit

fi

I recommend you to read the post about the if statement and how to use it Bash Script.

Check logged user

The logged variable is tested to check if it is a zero or not.

If it is zero, the script prints the message, and the script is terminated.

If the user is logged, the logged variable contains the username.

Checking If The User Accepts Messages

To check if messages are allowed or not, use the who command with -T option.

check=$(who -T | grep -i -m 1 $1 | awk '{print $2}')

if [ "$check" != "+" ]; then

echo "$1 disable messaging."

echo "Exit"

exit

fi

Check message allowed

Notice that we use the who command with -T. This shows a (+) beside the username if messaging is permitted. Otherwise, it shows a (-) beside the username, if messaging is not permitted.

Finally, we check for a messaging indicator if the indicator is not set to plus sign (+).

Checking If Message Was Included

You can check if the message was included or not like this:

if [ -z $2 ]; then

echo "Message not found"

echo "Exit"

exit

fi

Getting the Current Terminal

Before we send a message, we need to get the user current terminal and store it in a variable.

terminal=$(who | grep -i -m 1 $1 | awk '{print $2}')

Then we can send the message:

echo $2 | write $logged $terminal

Now we can test the whole shell script to see how it goes:

$ ./senderscript likegeeks welcome

Let’s see the other shell window:

Send message

Good!  You can now send simple one-word messages.

Sending a Long Message

If you try to send more than one word:

$ ./senderscript likegeeks welcome to shell scripting

One word message

It didn’t work. Only the first word of the message is sent.

To fix this problem, we will use the shift command with the while loop.

shift

while [ -n "$1" ]; do

message=$message' '$1

shift

done

And now one thing needs to be fixed, which is the message parameter.

echo $whole_message | write $logged $terminal

So now the whole script should be like this:

If you try now:

$ ./senderscript likegeeks welcome to shell scripting

Complete message

Awesome!! It worked. Again, I’m not here to make a script to send the message to the user, but the main goal is to review our shell scripting knowledge and use all the parts we’ve learned together and see how things work together.

Monitoring Disk Space

Let’s build a script that monitors the biggest top ten directories.

If you add -s option to the du command, it will show summarized totals.

$ du -s /var/log/

The -S option is used to show the subdirectories totals.

$ du -S /var/log/

du command

You should use the sort command to sort the results generated by the du command to get the largest directories like this:

$ du -S /var/log/ | sort -rn

sort command

The -n to sort numerically and the -r option to reverse the order so it shows the bigger first.

The N command is used to label each line with a number:

sed '{11,$D; =}' |

sed 'N; s/\n/ /' |

Then we can clean the output using the awk command:

awk '{printf $1 ":" "\t" $2 "\t" $3 "\n"}'

Then we add a colon and a tab so it appears much better.

$ du -S /var/log/ |

sort -rn |

sed '{11,$D; =}' |

# pipe the first result for another one to clean it

sed 'N; s/\n/ /' |

# formated printing using printf

awk '{printf $1 ":" "\t" $2 "\t" $3 "\n"}'

Format output with sed and awk

Suppose we have a variable called  MY_DIRECTORIES that holds 2 folders.

MY_DIRECTORIES=”/home /var/log”

We will iterate over each directory from MY_DIRECTORIES variable and get the disk usage using du command.

So the shell script will look like this:

Monitor disk usage

Good!! Both directories /home and /var/log are shown on the same report.

You can filter files, so instead of calculating the consumption of all files, you can calculate the consumption for a specific extension like *.log or whatever.

One thing I have to mention here, in production systems, you can’t rely on disk space report instead, you should use disk quotas.

Quota package is specialized for that, but here we are learning how bash scripts work.

Again the shell scripts we’ve introduced here is for showing you how shell scripting work, there are a ton of ways to implement any task in Linux.

My post is finished! I tried to reduce the post length and make everything as simple as possible, hope you like it.

Keep coming back. Thank you.

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Linux Bash Scripting Part5 – Signals and Jobs

In the previous post, we talked about input, output, and redirection in bash scripts. Today we will learn how to run and control them on a Linux system. Till now, we can run scripts only from the command line interface. This isn’t the only way to run Linux bash scripts. This post describes the different ways to control your Linux bash scripts. In shell scripts, we talked about important things called Input, Output and Redirection. Everything is a file in Linux and that includes input and output. So we need to understand each one in detail.

 

Continue Reading →

Your Linux bash scripts don’t control these signals, you can program your bash script to recognize signals and perform commands based on the signal that was sent.

Stop a Process

To stop a running process, you can press Ctrl+C which generates SIGINT signal to stop the current process running in the shell.

sleep 100

Ctrl+C

Linux bash scripting Signals and Jobs stop process

Pause a Process

The Ctrl+Z keys generate a SIGTSTP signal to stop any processes running in the shell, and that leaves the program in memory.

sleep 100

Ctrl+Z

pause process

The number between brackets which is (1) is the job number.

If try to exit the shell and you have a stopped job assigned to your shell, the bash warns you if you.

The ps command is used to view the stopped jobs.

ps –l

ps -l

In the S column (process state), it shows the traced (T) or stopped (S) states.

If you want to terminate a stopped job you can kill its process by using kill command.

kill processID

Trap Signals

To trap signals, you can use the trap command. If the script gets a signal defined by the trap command, it stops processing and instead the script handles the signal.

You can trap signals using the trap command like this:

#!/bin/bash

trap "echo 'Ctrl-C was trapped'" SIGINT

total=1

while [ $total -le 3 ]; do

echo "#$total"

sleep 2

total=$(($total + 1))

done

Every time you press Ctrl+C, the signal is trapped and the message is printed.

trap signal

If you press Ctrl+C, the echo statement specified in the trap command is printed instead of stopping the script. Cool, right?

Trapping The Script Exit

You can trap the shell script exit using the trap command like this:

#!/bin/bash

# Add the EXIT signal to trap it

trap "echo Goodbye..." EXIT

total=1

while [ $total -le 3 ]; do

echo "#$total"

sleep 2

total=$(($total + 1))

done

trap exit

When the bash script exits, the Goodbye message is printed as expected.

Also, if you exit the script before finishing its work, the EXIT trap will be fired.

Modifying Or Removing a Trap

You can reissue the trap command with new options like this:

#!/bin/bash

trap "echo 'Ctrl-C is trapped.'" SIGINT

total=1

while [ $total -le 3 ]; do

echo "Loop #$total"

sleep 2

total=$(($total + 1))

done

# Trap the SIGINT

trap "echo ' The trap changed'" SIGINT

total=1

while [ $total -le 3 ]; do

echo "Second Loop #$total"

sleep 1

total=$(($total + 1))

done

modify trap

Notice how the script manages the signal after changing the signal trap.

You can also remove a trap by using 2 dashes trap -- SIGNAL

#!/bin/bash

trap "echo 'Ctrl-C is trapped.'" SIGINT

total=1

while [ $total -le 3 ]; do

echo "#$total"

sleep 1

total=$(($total + 1))

done

trap -- SIGINT

echo "I just removed the trap"

total=1

while [ $total -le 3 ]; do

echo "Loop #2 #$total"

sleep 2

total=$(($total + 1))

done

Notice how the script processes the signal before removing the trap and after removing the trap.

./myscript

Crtl+C

remove trap

The first Ctrl+C was trapped and the script continues running while the second one exits the script because the trap was removed.

Running Linux Bash Scripts in Background Mode

If you see the output of the ps command, you will see all the running processes in the background and not tied to the terminal.

We can do the same, just place ampersand symbol (&) after the command.

#!/bin/bash

total=1

while [ $total -le 3 ]; do

sleep 2

total=$(($total + 1))

done

./myscipt &

run in background

Once you’ve done that, the script runs in a separate background process on the system and you can see the process id between the square brackets.

When the script dies,  you will see a message on the terminal.

Notice that while the background process is running, you can use your terminal monitor for STDOUT and STDERR messages so if an error occurs, you will see the error message and normal output.

run script in background

The background process will exit if you exit your terminal session.

So what if you want to continue running even if you close the terminal?

Running Scripts without a Hang-Up

You can run your Linux bash scripts in the background process even if you exit the terminal session using the nohup command.

The nohup command blocks any SIGHUP signals. This blocks the process from exiting when you exit your terminal.

nohup ./myscript &

linux bash nohup command

After running the nohup command, you can’t see any output or error from your script. The output and error messages are sent to a file called nohup.out.

Note: when running multiple commands from the same directory will override the nohup.out file content.

Viewing Jobs

To view the current jobs, you can use the jobs command.

#!/bin/bash

total=1

while [ $total -le 3 ]; do

echo "#$count"

sleep 5

total=$(($total + 1))

done

Then run it.

./myscript

Then press Ctrl+Z to stop the script.

linux bash view jobs

Run the same bash script but in the background using the ampersand symbol and redirect the output to a file just for clarification.

./myscript > outfile &

linux bash list jobs

The jobs command shows the stopped and the running jobs.

jobs –l

-l parameter to view the process ID

 Restarting Stopped Jobs

The bg command is used to restart a job in background mode.

./myscript

Then press Ctrl+Z

Now it is stopped.

bg

linux bash restart job

After using bg command, it is now running in background mode.

If you have multiple stopped jobs, you can do the same by specifying the job number to the bg command.

The fg command is used to restart a job in foreground mode.

fg 1

Scheduling a Job

The Linux system provides 2 ways to run a bash script at a predefined time:

  • at command.
  • cron table.

The at command

This is the format of the command

at [-f filename] time

The at command can accept different time formats:

  • Standard time format like 10:15.
  • An AM/PM indicator like 11:15PM.
  • A specifically named time like now, midnight.

You can include a specific date, using some different date formats:

  • A standard date format, such as MMDDYY or DD.MM.YY.
  • A text date, such as June 10 or Feb 12, with or without the year.
  • Now + 25 minutes.
  • 05:15AM tomorrow.
  • 11:15 + 7 days.

We don’t want to dig deep into the at command, but for now, just make it simple.

at -f ./myscript now

linux bash at command

The -M parameter is used to send the output to email if the system has email, and if not, this will suppress the output of the at command.

To list the pending jobs, use atq command:

linux bash at queue

Remove Pending Jobs

To remove a pending job, use the atrm command:

atrm 18

delete at queue

You must specify the job number to the atrm command.

Scheduling Scripts

What if you need to run a script at the same time every day or every month or so?

You can use the crontab command to schedule jobs.

To list the scheduled jobs, use the -l parameter:

crontab –l

The format for crontab is:

minute,Hour, dayofmonth, month, and dayofweek

So if you want to run a command daily at 10:30, type the following:

30 10 * * * command

The wildcard character (*) used to indicate that the cron will execute the command daily on every month at 10:30.

To run a command at 5:30 PM every Tuesday, you would use the following:

30 17 * * 2 command

The day of the week starts from 0 to 6 where Sunday=0 and Saturday=6.

To run a command at 10:00 on the beginning of every month:

00 10 1 * * command

The day of the month is from 1 to 31.

Let’s keep it simple for now and we will discuss the cron in great detail in future posts.

To edit the cron table, use the -e parameter like this:

crontab –e

Then type your command like the following:

30 10 * * * /home/likegeeks/Desktop/myscript

This will schedule our script to run at 10:30 every day.

Note: sometimes you see error says Resource temporarily unavailable.

All you have to do is this:

rm -f /var/run/crond.pid

You should be a root user to do this.

Just that simple!

You can use one of the pre-configured cron script directories like:

/etc/cron.hourly

/etc/cron.daily

/etc/cron.weekly

/etc/cron.monthly

Just put your bash script file on any of these directories and it will run periodically.

Starting Scripts at Login

In the previous posts, we’ve talked about startup files, I recommend you to review the previous.

$HOME/.bash_profile

$HOME/.bash_login

$HOME/.profile

To run your scripts at login, place your code in $HOME/.bash_profile.

Starting Scripts When Opening the Shell

OK, what about running our bash script when the shell opens? Easy.

Type your script on .bashrc file.

And now if you open the shell window, it will execute that command.

I hope you find the post useful. keep coming back.

Thank you.

0

Linux Bash Scripting Part5 – Signals and Jobs

In the previous post, we talked about input, output, and redirection in bash scripts. Today we will learn how to run and control them on Linux system. Till now, we can run scripts only from the command line interface. This isn’t the only way to run Linux bash scripts. This post describes the different ways to control your Linux bash scripts. These are the most common Linux system signals:

 

Continue Reading →

Linux Signals

These are the most common Linux system signals:

Num        Name                    Job

1              SIGHUP               Process hangs up.

2             SIGINT                 Process Interruption.

3             SIGQUIT              Proces quit or stop.

9             SIGKILL               Process termination.

15           SIGTERM             Process termination.

17           SIGSTOP              Process stopping without termination.

18           SIGTSTP              Process stopping or pausing without termination.

19           SIGCONT             Process continuation after stopping.

Your Linux bash scripts don’t control these signals, you can program your bash script to recognize signals and perform commands based on the signal that was sent.

Stop a Process

To stop a running process, you can press Ctrl+C which generates SIGINT signal to stop the current process running in the shell.

$ sleep 100

Ctrl+C

stop process

Pause a Process

The Ctrl+Z keys generate a SIGTSTP signal to stop any processes running in the shell, and that leaves the program in memory.

$ sleep 100

Ctrl+Z

pause process

The number between brackets which is (1) is the job number.

If try to exit the shell and you have a stopped job assigned to your shell, the bash warns you if you.

The ps command is used to view the stopped jobs.

ps –l

ps -l

In the S column (process state), it shows the traced (T) or stopped (S) states.

If you want to terminate a stopped job you can kill its process by using kill command.

kill processID

Trap Signals

To trap signals, you can use the trap command. If the script gets a signal defined by the trap command, it stops processing and instead the script handles the signal.

You can trap signals using the trap command like this:

#!/bin/bash

trap "echo 'Ctrl-C was trapped'" SIGINT

total=1

while [ $total -le 3 ]

do

echo "#$total"

sleep 2

total=$(( $total + 1 ))

done

Every time you press Ctrl+C, the signal is trapped and the message is printed.

trap signal

If you press Ctrl+C, the echo statement specified in the trap command is printed instead of stopping the script. Cool, right?

Trapping The Script Exit

You can trap the shell script exit using the trap command like this:

#!/bin/bash

# Add the EXIT signal to trap it

trap "echo Goodbye..." EXIT

total=1

while [ $total -le 3 ]

do

echo "#$total"

sleep 2

total=$(( $total + 1 ))

done

trap exit

When the bash script exits, the Goodbye message is printed as expected.

Also, if you exit the script before finishing its work, the EXIT trap will be fired.

Modifying Or Removing a Trap

You can reissue the trap command with new options like this:

#!/bin/bash

trap "echo 'Ctrl-C is trapped.'" SIGINT

total=1

while [ $total -le 3 ]

do

echo "Loop #$total"

sleep 2

total=$(( $total + 1 ))

done

# Trap the SIGINT

trap "echo ' The trap changed'" SIGINT

total=1

while [ $total -le 3 ]

do

echo "Second Loop #$total"

sleep 1

total=$(( $total + 1 ))

done

modify trap

Notice how the script manages the signal after changing the signal trap.

You can also remove a trap by using 2 dashes trap SIGNAL
#!/bin/bash

trap "echo 'Ctrl-C is trapped.'" SIGINT

total=1

while [ $total -le 3 ]

do

echo "#$total"

sleep 1

total=$(( $total + 1 ))

done

trap -- SIGINT

echo "I just removed the trap"

total=1

while [ $total -le 3 ]

do

echo "Loop #2 #$total"

sleep 2

total=$(( $total + 1 ))

done

Notice how the script processes the signal before removing the trap and after removing the trap.

$ ./myscript

Crtl+C

remove trap

The first Ctrl+C was trapped and the script continues running while the second one exits the script because the trap was removed.

Running Linux Bash Scripts in Background Mode

If you see the output of the ps command, you will see all the running processes in the background and not tied to the terminal.

We can do the same, just place ampersand symbol (&) after the command.

#!/bin/bash

total=1

while [ $total -le 3 ]

do

sleep 2

total=$(( $total + 1 ))

done

$ ./myscipt &

run in background

Once you’ve done that, the script runs in a separate background process on the system and you can see the process id between the square brackets.

When the script dies,  you will see a message on the terminal.

Notice that while the background process is running, you can use your terminal monitor for STDOUT and STDERR messages so if an error occurs, you will see the error message and normal output.

run script in background

The background process will exit if you exit your terminal session.

So what if you want to continue running even if you close the terminal?

Running Scripts without a Hang-Up

You can run your Linux bash scripts in the background process even if you exit the terminal session using the nohup command.

The nohup command blocks any SIGHUP signals. This blocks the process from exiting when you exit your terminal.

$ nohup ./myscript &

linux bash nohup command

After running the nohup command, you can’t see any output or error from your script. The output and error messages are sent to a file called nohup.out.

Note: when running multiple commands from the same directory will override the nohup.out file content.

Viewing Jobs

To view the current jobs, you can use the jobs command.

#!/bin/bash

total=1

while [ $total -le 3 ]

do

echo "#$count"

sleep 5

total=$(( $total + 1 ))

done

Then run it.

$ ./myscript

Then press Ctrl+Z to stop the script.

linux bash view jobs

Run the same bash script but in the background using the ampersand symbol and redirect the output to a file just for clarification.

v$ ./myscript > outfile &

linux bash list jobs

The jobs command shows the stopped and the running jobs.

jobs –l

-l parameter to view the process ID

Restarting Stopped Jobs

The bg command is used to restart a job in background mode.

$ ./myscript

Then press Ctrl+Z

Now it is stopped.

$ bg

linux bash restart job

After using bg command, it is now running in background mode.

If you have multiple stopped jobs, you can do the same by specifying the job number to the bg command.

The fg command is used to restart a job in foreground mode.

$ fg 1

Scheduling a Job

The Linux system provides 2 ways to run a bash script at a predefined time:

  • at command.
  • cron table.

The at command

This is the format of the command

at [-f filename] time

The at command can accept different time formats:

  • Standard time format like 10:15.
  • An AM/PM indicator like 11:15PM.
  • A specifically named time like now, midnight.

You can include a specific date, using some different date formats:

  • A standard date format, such as MMDDYY or DD.MM.YY.
  • A text date, such as June 10 or Feb 12, with or without the year.
  • Now + 25 minutes.
  • 05:15AM tomorrow.
  • 11:15 + 7 days.

We don’t want to dig deep into the at command, but for now, just make it simple.

$ at -f ./myscript now

linux bash at command

The -M parameter is used to send the output to email if the system has email, and if not, this will suppress the output of the at command.

To list the pending jobs, use atq command:

linux bash at queue

Remove Pending Jobs

To remove a pending job, use the atrm command:

$ atrm 18

delete at queue

You must specify the job number to the atrm command.

Scheduling Scripts

What if you need to run a script at the same time every day or every month or so?

You can use the crontab command to schedule jobs.

To list the scheduled jobs, use the -l parameter:

$ crontab –l

The format for crontab is:

minute,Hour, dayofmonth, month, and dayofweek

So if you want to run a command daily at 10:30, type the following:

30 10 * * * command

The wildcard character (*) used to indicate that the cron will execute the command daily on every month at 10:30.

To run a command at 5:30 PM every Tuesday, you would use the following:

30 17 * * 2 command

The day of the week starts from 0 to 6 where Sunday=0 and Saturday=6.

To run a command at 10:00 on the beginning of every month:

00 10 1 * * command

The day of the month is from 1 to 31.

Let’s keep it simple for now and we will discuss the cron in great detail in future posts.

To edit the cron table, use the -e parameter like this:

crontab –e

Then type your command like the following:

30 10 * * * /home/likegeeks/Desktop/myscript

This will schedule our script to run at 10:30 every day.

Note: sometimes you see error says Resource temporarily unavailable.

All you have to do is this:

$ rm -f /var/run/crond.pid

You should be a root user to do this.

Just that simple!

You can use one of the pre-configured cron script directories like:

/etc/cron.hourly

/etc/cron.daily

/etc/cron.weekly

/etc/cron.monthly

Just put your bash script file on any of these directories and it will run periodically.

Starting Scripts at Login

In the previous posts, we’ve talked about startup files, I recommend you to review the previous.

$HOME/.bash_profile

$HOME/.bash_login

$HOME/.profile

To run your scripts at login, place your code in  $HOME/.bash_profile.

Starting Scripts When Opening the Shell

OK, what about running our bash script when the shell opens? Easy.

Type your script on .bashrc file.

And now if you open the shell window, it will execute that command.

I hope you find the post useful. keep coming back.

Thank you.

likegeeks.com

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