Tag Archives | Bash Scripting

31+ Examples for sed Linux Command in Text Manipulation

In the previous post, we talked about bash functions and how to use them from the command line directly and we saw some other cool stuff. Today we will talk about a very useful tool for string manipulation called sed or sed Linux command. Sed is used to work with text files like log files, configuration files, and other text files. In this post, we are going to focus on sed Linux command which is used for text manipulation, which is a very important step in our bash scripting journey. Linux system provides some tools for text processing, one of those tools is sed. We will discuss the 31+ examples with pictures to show the output of every example.

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Understand sed Linux Command

The sed command is a non-interactive text editor. Sed Linux command edits data based on the rules you provide, you can use it like this:

sed options file

You are not limited to use sed to manipulate files, you apply it to the STDIN directly like this:

echo "Welcome to LikeGeeks page" | sed 's/page/website/'

sed Linux command

The s command replaces the first text with the second text pattern. In this case, the string “website” was replaced with the word “page”, so the result will be as shown.

The above example was a very basic example to demonstrate the tool. We can use sed Linux command to manipulate files as well.

This is our file:

sed manipulate file

sed 's/test/another test/' ./myfile

The results are printed to the screen instantaneously, you don’t have to wait for processing the file to the end.

If your file is huge enough, you will see the result before the processing is finished.

Sed Linux command doesn’t update your data. It only sends the changed text to STDOUT. The file still untouched. If you need to overwrite the existing content, you can check our previous post which was talking about redirections.

Using Multiple sed Linux Commands in The Command Line

To run multiple sed commands, you can use the -e option like this:

sed -e 's/This/That/; s/test/another test/' ./myfile

sed multiple commands

Sed command must be separated by a semicolon without any spaces.

Also, you can use a single quotation to separate commands like this:

sed -e '

> s/This/That/

> s/test/another test/' myfile

sed separate commands

The same result, no big deal. 

Reading Commands From a File

You can save your sed commands in a file and use them by specifying the file using -f option.

cat mycommands


s/test/another test/

sed -f mycommands myfile

read commands from file

Substituting Flags

Look at the following example carefully:

cat myfile

sed 's/test/another test/' myfile

sed substitute flag

The above result shows the first occurrence in each line is only replaced. To substitute all occurrences of a pattern, use one of the following substitution flags.

The flags are written like this:


There are four types of substitutions:

  • g, replace all occurrences.
  • A number, the occurrence number for the new text that you want to substitute.
  • p, print the original content.
  • w file: means write the results to a file.

You can limit your replacement by specifying the occurrence number that should be replaced like this:

sed 's/test/another test/2' myfile

sed number flag

As you can see, only the second occurrence on each line was replaced.

The g flag means global, which means a global replacement for all occurrences:

sed 's/test/another test/g' myfile

sed global flag

The p flag prints each line contains a pattern match, you can use the -n option to print the modified lines only.

cat myfile

sed -n 's/test/another test/p' myfile

sed supress lines

The w flag saves the output to a specified file:

sed 's/test/another test/w output' myfile

send output to file

The output is printed on the screen, but the matching lines are saved to the output file.

Replace Characters

Suppose that you want to search for bash shell and replace it with csh shell in the /etc/passwd file using sed, well, you can do it easily:

sed 's/\/bin\/bash/\/bin\/csh/' /etc/passwd

Oh!! that looks terrible.

Luckily, there is another way to achieve that. You can use the exclamation mark (!) as string delimiter like this:

sed 's!/bin/bash!/bin/csh!' /etc/passwd

Now it’s easier to read.

Limiting sed

Sed command processes your entire file. However, you can limit the sed command to process specific lines, there are two ways:

  • A range of lines.
  • A pattern that matches a specific line.

You can type one number to limit it to a specific line:

sed '2s/test/another test/' myfile

sed restricted

Only line two is modified.

What about using a range of lines:

sed '2,3s/test/another test/' myfile

replace range of lines

Also, we can start from a line to the end of the file:

sed '2,$s/test/another test/' myfile

sed replace to the end

Or you can use a pattern like this:

sed '/likegeeks/s/bash/csh/' /etc/passwd

sed pattern match


You can use regular expressions to write this pattern to be more generic and useful.

Delete Lines

To delete lines, the delete (d) flag is your friend.

The delete flag deletes the text from the stream, not the original file.

sed '2d' myfile

sed delete line

Here we delete the second line only from myfile.

What about deleting a range of lines?

sed '2,3d' myfile

delete multiple line

Here we delete a range of lines, the second and the third.

Another type of ranges:

sed '3,$d' myfile

delete to the end

Here we delete from the third line to the end of the file.

All these examples never modify your original file.

sed '/test 1/d' myfile

delete pattern match

Here we use a pattern to delete the line if matched on the first line.

If you need to delete a range of lines, you can use two text patterns like this:

sed '/second/,/fourth/d' myfile

delete range of lines

From the second to the fourth line are deleted.

Insert and Append Text

You can insert or append text lines using the following flags:

  • The (i) flag.
  • The  (a) flag.

echo "Another test" | sed 'i\First test '

sed insert text

Here the text is added before the specified line.

echo "Another test" | sed 'a\First test '

sed append

Here the text is added after the specified line.

Well, what about adding text in the middle?

Easy, look at the following example:

sed '2i\This is the inserted line.' myfile

sed insert line

And the appending works the same way, but look at the position of the appended text:

sed '2a\This is the appended line.' myfile

sed append line

The same flags are used but with a location of insertion or appending.

Modifying Lines

To modify a specific line, you can use the (c) flag like this:

sed '3c\This is a modified line.' myfile

sed modify line

You can use a regular expression pattern and all lines match that pattern will be modified.

sed '/This is/c Line updated.' myfile

sed pattern match

Transform Characters

The transform flag (y) works on characters like this:

sed 'y/123/567/' myfile

sed transform character

The transformation is applied to all data and cannot be limited to a specific occurrence.

Print Line Numbers

You can print line number using the (=) sign like this:

sed '=' myfile

sed line numbers

However, by using -n combined with the equal sign, the sed command displays the line number that contains matching.

sed -n '/test/=' myfile

hide lines

Read Data From a File

You can use the (r) flag to read data from a file.

You can define a line number or a text pattern for the text that you want to read.

cat newfile

sed '3r newfile' myfile

read data from file

The content is just inserted after the third line as expected.

And this is using a text pattern:

sed '/test/r newfile' myfile

read match pattern

Cool right?

Useful Examples

We have a file that contains text with a placeholder and we have another file that contains the data that will be filled in that placeholder.

We will use the (r) and (d) flags to do the job.

The word DATA in that file is a placeholder for a real content which is stored in another file called data.

We will replace it with the actual content:

Sed '/DATA>/ {

r newfile

d}' myfile

repalce placeholder

Awesome!! as you can see, the placeholder location is filled with the data from the other file.

This is just a very small intro about sed command. Actually, sed Linux command is another world by itself.

The only limitation is your imagination.

I hope you enjoy what’ve introduced today about the string manipulation using sed Linux command.

Thank you.


Bash Scripting Part6 – Create and Use Bash Functions

Before we talk about bash functions, let’s discuss this situation. When writing bash scripts, you’ll find yourself that you are using the same code in multiple places. If you get tired of writing the same lines of code again and again in your bash script, it would be nice to write the block of code once and call it anywhere in your bash script. The bash shell allows you to do just that with Functions. Bash functions are blocks of code that you can reuse them anywhere in your code. Anytime you want to use this block of code in your script, you simply type the function name given to it. We are going to talk about how to create your own bash functions and how to use them in shell scripts.

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Creating a function

You can define a function like this:

functionName() {


The brackets () is required to define the function.

Also, you can define the function using the function keyword, but this keyword is deprecated for POSIX portability.

function functionName() {

# Deprecated definition but still used and working


In the second definition, the brackets are not required.

Using Functions


myfunc() {

echo "Using functions"



while [ $total -le 3 ]; do


total=$(($total + 1))


echo "Loop finished"


echo "End of the script"

Here we’ve created a function called myfunc and in order to call it, we just typed its name.

bash functions

The function can be called many times as you want.

Notice: If you try to use a function which is not defined, what will happen?



while [ $total -le 3 ]; do


total=$(($total + 1))


echo "Loop End"

myfunc() {

echo "Using function ..."


echo "End of the script"

call before declare

Oh, it’s an error because there no such function.

Another notice: bash function name must be unique. Otherwise, the new function will cancel the old function without any errors.


myfunc() {

echo "The first function definition"



function myfunc() {

echo "The second function definition"



echo "End of the script"

override definition

As you can see, the second function definition takes control from the first one without any error so take care when defining functions.

Using the return Command

The return command returns an integer from the function.

There are two ways of using the return command; the first way is like this:


myfunc() {

read -p "Enter a value: " value

echo "adding value"

return $(($value + 10))



echo "The new value is $?"

return command

The myfunc function adds 10 to the  $value variable then show the sum using the $? Variable.

Don’t execute any commands before getting the value of the function, because the variable $? returns the status of the last line.

This return method returns integers. what about returning strings?

Using Function Output

The second way of returning a value from a bash function is command substitution. This way, you can return anything from the function.


myfunc() {

read -p "Enter a value: " value

echo $(($value + 10))



echo "The value is $result"

bash functions output

Passing Parameters

We can deal with bash functions like small snippets that can be reused and that’s OK, but we need to make the function like an engine, we give it something and it returns a result based on what we provide.

You can use the environment variables to process the passed parameters to the function. The function name is declared as $0 variable, and the passed parameters are $1, $2, $3, etc.

You can get the number of passed parameters to the function using the ($#) variable.

We pass parameters like this:

myfunc $val1 10 20

The following example shows how to use the ($#) variable:


addnum() {

if [ $# -gt 2 ]; then

echo "Incorrect parameters passed" # If parameters no equal 2


echo $(($1 + $2)) # Otherwise add them



echo -n "Adding 10 and 15: "

value=$(addnum 10 15)

echo $value

echo -n "Adding three numbers: "

value=$(addnum 10 15 20)

echo $value

pass parameters

The addnum function gets the passed parameters count. If greater than 2 passed, it returns -1.

If there’s one parameter, the addnum function adds this parameter twice. If 2 parameters passed, the addnum function adds them together, and if you try to add three parameters it will return -1.

If you try to use the passed parameters inside the function, it fails:


myfunc() {

echo $(($1 + $2 + $3 + $4))


if [ $# -eq 4 ]; then


echo "Total= $value"


echo "Passed parameters like this: myfunc a b c d"


unknown parameters

Instead, you have to send them to the function like this:


myfunc() {

echo $(($1 + $2 + $3 + $4))


if [ $# -eq 4 ]; then

value=$(myfunc $1 $2 $3 $4)

echo "Total= $value"


echo "Passed parameters like this: myfunc a b c d"


bash functions parameters

Now it works!!

Processing Variables in Bash Functions

Every variable we use has a scope, the scope is variable visibility to your script.

You can define two types of variables:

  • Global
  • Local

Global Variables

They are visible and valid anywhere in the bash script. You can even get its value from inside the function.

If you declare a global variable within a function, you can get its value from outside the function.

Any variable you declare is a global variable by default. If you define a variable outside the function, you call it inside the function without problems:


myfunc() {

input=$(($input + 10))


read -p "Enter a number: " input


echo "The new value is: $input"

global variables

If you change the variable value inside the function, the value will be changed outside of the function.

So how to overcome something like this? Use local variables.

Local Variables

If you will use the variable inside the function only, you can declare it as a local variable using the local keyword  like this:

local tmp=$(( $val + 10 ))

So if you have two variables, one inside the function and the other is outside the function and they have the identical name, they won’t affect each other.


myfunc() {

local tmp=$(($val + 10))

echo "The Temp from inside function is $tmp"




echo "The temp from outside is $tmp"

local variables

When you use the $tmp variable inside the myfunc function, it doesn’t change the value of the $tmp which is outside the function.

Passing Arrays As Parameters

What will happen if you pass an array as a parameter to a function:


myfunc() {

echo "The parameters are: $@"


echo "The received array is ${arr[*]}"


my_arr=(5 10 15)

echo "The old array is: ${my_arr[*]}"

myfunc ${my_arr[*]}

pass arrays

The function only takes the first value of the array variable.

You should disassemble the array into its single values, then use these values as function parameters. Finally, pack them into an array in the function like this:


myfunc() {

local new_arr


echo "Updated value is: ${new_arr[*]}"


my_arr=(4 5 6)

echo "Old array is ${my_arr[*]}"

myfunc ${my_arr[*]}

pass arrays solution

The array variable was rebuilt thanks to the function.

Recursive Function

This feature enables the function to call itself from within the function itself.

The classic example of a recursive function is calculating factorials. To calculate the factorial of 3, use the following equation:

3! = 1 * 2 * 3

Instead, we can use the recursive function like this:

x! = x * (x-1)!

So to write the factorial function using bash scripting, it will be like this:


fac_func() {

if [ $1 -eq 1 ]; then

echo 1


local tmp=$(($1 - 1))

local res=$(fac_func $tmp)

echo $(($res * $1))



read -p "Enter value: " val

res=$(fac_func $val)

echo "The factorial of $val is: $res"

bash recursive function

Using recursive bash functions is so easy!

Creating Libraries

Now we know how to write functions and how to call them, but what if you want to use these bash functions or blocks of code on different bash script files without copying and pasting it on your files.

You can create a library for your functions and point to that library from any file as you need.

By using the source command, you can embed the library file script inside your shell script.

The source command has an alias which is the dot. To source a file in a shell script, write the following line:

. ./myscript

Let’s assume that we have a file called myfuncs that contains the following:

addnum() {

echo $(($1 + $2 + $3 + $4))


Now, we will use it in another bash script file like this:


. ./myfuncs

result=$(addnum 10 10 5 5)

echo "Total = $result"

source command

Awesome!! We’ve used the bash functions inside our bash script file, we can also use them in our shell directly.

Use Bash Functions From Command Line

Well, that is easy, if you read the previous post which was about the signals and jobs you will have an idea about how to source our functions file in the bashrc file and hence we can use the functions directly from the bash shell. Cool

Edit the bashrc file at /home/username and add this line:

. /home/likegeeks/Desktop/myfuncs

Make sure you type the correct path.

Now the function is available for us to use in the command line directly:

addnum 10 20

use from shell

Note: you may need to log out and log in to use the bash functions from the shell.

Another note: if you make your function name like any of the built-in commands you will overwrite the default command so you should take care of that.

I hope you like the post. Keep coming back.

Thank you.


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:


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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


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


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:


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


while [ $total -le 3 ]


echo "#$total"

sleep 2

total=$(( $total + 1 ))


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:


# Add the EXIT signal to trap it

trap "echo Goodbye..." EXIT


while [ $total -le 3 ]


echo "#$total"

sleep 2

total=$(( $total + 1 ))


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:


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


while [ $total -le 3 ]


echo "Loop #$total"

sleep 2

total=$(( $total + 1 ))


# Trap the SIGINT

trap "echo ' The trap changed'" SIGINT


while [ $total -le 3 ]


echo "Second Loop #$total"

sleep 1

total=$(( $total + 1 ))


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

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


while [ $total -le 3 ]


echo "#$total"

sleep 1

total=$(( $total + 1 ))


trap -- SIGINT

echo "I just removed the trap"


while [ $total -le 3 ]


echo "Loop #2 #$total"

sleep 2

total=$(( $total + 1 ))


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

$ ./myscript


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.



while [ $total -le 3 ]


sleep 2

total=$(( $total + 1 ))


$ ./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.



while [ $total -le 3 ]


echo "#$count"

sleep 5

total=$(( $total + 1 ))


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:





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.




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.