Archive | GNU/Linux

mount | column –t

OK, in this example, we see the output is well formatted because the separator between them is spaces.

What if the separators are something else, like colons :

The /etc/passwd file is a good example.

Just specify the separator with -s parameter like this:

cat /etc/passwd | column -t -s :

 Run Until Success

If you search google about that trick, you will find a lot of questions about people asking how to repeat the command till it returns success and runs properly, like ping the server till it becomes alive or check if a file with a specific extension is uploaded at specific directory or maybe check if a specific URL becomes available or maybe any geeky thing, the list is very long.

You can use the while true loop to achieve that:

We use > /dev/null 2>&1  to redirect normal output and errors to /dev/null.

Actually, this is one of coolest Linux Command Line Tricks for me.

Sort Processes by (Memory – CPU) Usage

To sort by memory usage:

To sort by CPU usage:

Check Your Architecture

getconf  LONG_BIT

Monitor Multiple Log Files Concurrently

You can use the tail command to watch your logs and that’s fine, but sometimes you may need to monitor multiple log files simultaneously to take some actions.

Using multitail command which supports text highlighting, filtering, and many other features that you may need.

You can install it if it is not found on your system like this:

apt–get install multitail

Return to Your Previous Directory

It’s not a trick but some people forget it, others use it every minute.

Just type cd –  and you will return back to the previous directory.

Make Non-Interactive as Interactive Shell Session

To do this, put our settings in ~/.bashrc  from ~/.bash_profile.

Watch Command Output

By using watch command, you can watch any output of any command, for example, you can watch the free space and how it is growing:

watch df –h

You can imagine what you can do with any variant data that you can watch using watch command.

Run Your Program After Session Killing

When you run any program in the background and close your shell, definitely it will be killed, what about if it continues running after closing the shell.

This can be done using the nohup command which stands for no hang up.

nohup wget site.com/file.zip

This command is really one of the most useful Linux command line tricks for most webmasters.

A file will be generated in the same directory with the name nohup.out contains the output of the running program.

Cool command right?

Answer bot Using Yes & No Commands

It’s like an answer bot for those commands whose require the user to say yes.

That can be done using the yes command:

yes | apt–get update

Or maybe you want to automate saying no instead, this can be done using the following command:

yes no | command

Create a File With a Specific Size

Use the dd command to create a file with a specific size:

dd if=/dev/zero of=out.txt bs=1M count=10

This will create a file with 10-megabyte size filled with zeros.

Run Last Command as Root

Sometimes you forget to type sudo before your command that requires root privileges to run, you don’t have to rewrite it, just type:

sudo !!

Record your Command Line Session

If you want to record what you’ve typed in your shell screen, you can use the script command which will save all of your typings to a file named typescript.

script

Once you type exit, all of your commands will be written to that file so you can review them later.

Replacing Spaces with Tabs

You can replace any character with any other character using tr command which is very handy.

cat geeks.txt | tr ‘:[space]:’ ‘\t’ > out.txt

This command will replace the spaces with tabs.

Convert Character Case

cat my_file | tr a–z A–Z > output.txt

This command converts the content of the file to upper case using the tr command.

Powerful xargs Command

We can say that xargs command is one of the most important Linux command line tricks, you can use this command to pass outputs between commands as arguments, for example, you may search for png files and compress them or do anything with them.

find . –name “*.png” –type f –print | xargs tar –cvzf pics.tar.gz

Or maybe you have a list of URLs in a file and you want to download them or process them in a different way:

cat links.txt | xargs wget

The cat command result is passed to the end of xargs command.

What if your command needs the output in the middle?

Just use {} combined with –i parameter to replace the arguments in the place where the result should go like this:

ls /etc/*.conf | xargs –i cp {} /home/likegeeks/Desktop/out

This is only some of the Linux command line tricks, but there are some more geeky things that you can do using other commands like awk command and sed command.

If you know any geeky command I didn’t mention, you can type it in a comment and share it with others.

I’m going to make another post about those Linux command line tricks so we can remember all what we’ve forgotten.

Thank you.

likegeeks.com

Global variables

The Linux system sets some global environment variables when you log into your system and they are always CAPITAL LETTERS to differentiate them from user-defined environment variables.

To see these global variables, type printenv command:

As you can see, there are a lot of global environment variables, to print only one of them, type echo command followed by $VariableName. Ex: to print HOME variable type echo $HOME.

Local variables

Actually, the Linux system also defines some standard local environment variables for you by default.

To view the global and local variables for the shell you are running and available to that shell, type the set command.

Setting Local Environment Variables

To declare your own environment variables directly from the shell, type variableName you want, followed by an equal sign and the variable value WITHOUT any spaces:

mysite=likegeeks

And to print the variable value, use the echo command:

echo $mysite

Sure enough, it prints likegeeks.

OK, what if your variable is more than one word; may be a long string, you can put the string between single quotations:

mysite=’likegeeks is a website that offers tech tutorials for geeks’

And if we type echo $mysite

If you forget the single quotation, the shell will assume that the second word is another command and will show an error.

As you can see, I use lower case characters for my variable not upper case and this is recommended  NOT required, this helps you distinguishing your environment variables from the system environment variables.

Once you have set your local variable, it will be visible in the currently running shell scope and that means if you start another shell window the variable will not be available in that new window.

Declare Global Environment Variables

To declare a global environment variable, you have to declare a local environment variable then use the export command like this:

myvar=’I will do it likegeeks’

echo $myvar

export myvar

As you can see I don’t use dollar sign with the export command so make sure of that.

But there is something, when I close the shell and open it again the variable is gone, so how to make it persistent?

Persisting Environment Variables

Just edit $HOME/.bashrc  and type   export myvar=‘welcome to likegeeks’  and save the file:

Removing Environment Variables

This can be done by using the unset command:

Default Environment Variables

As we know, the system defines some variables for us, one of those variables is the PATH variable, this variable holds some paths.

These paths are the default paths that the shell uses to look for a command when you type it in the shell.

Modifying the PATH Environment Variable

If you add a folder path to the PATH variable, the shell will look in that folder for any executable to run when you type any command.

Just append the path variable followed by a colon and the new directory like this:

And if you want to persist the PATH variable, you have to edit the .bashrc file and add type it like this:

There is a useful trick but RISKY that some sysadmins do which is adding a period . to the path variable. By doing this, the shell will search for executables in the current directory you are in wherever you are.

This is relatively risky, you might give the attacker the opportunity to run a malicious script or malware in his current directory, so if you do this trick, you should know what you are doing.

System Environment Variables Paths

You can start a bash shell with one of the following ways:

• Login shell.
• The interactive shell.
• The non-interactive shell.

login shell

When you log onto the system, the search for those startup files to process the commands from them:

• /etc/profile
• $HOME/.bash_login
• $HOME/.bash_profile
• $HOME/.profile

/etc/profile runs on every startup with every user, the other 3 files run for every specific user, you can call them user specific environment variables.

Interactive shell

When you go to the rescue mode, this is the interactive shell.

If you start an Interactive shell, the system will not look for /etc/profile but instead will look for .bashrc in your HOME directory.

Non-interactive shell

This shell runs by the system itself to run a shell script.

Users can customize the above-mentioned files to include environment variables, just edit the desired file and type the variable you want and save it.

Variable Arrays

You can store many values as you want in one environment variable which is called array.

myvar=(first second third fourth)

Now if you check the value of that array using echo command, you will find it returns the first element only.

To get a specific element, just reference it by its position, and the positions start from zero so to get the third one, type it like this:

echo ${myvar[2]}

To display the entire array type asterisk instead of a number:

echo ${mytest[*]}

You can remove an element of the array using unset command:

unset mytest[2]

Or you can remove the whole array:

unset myvar

likegeeks.com