Bash Shell: Remove (Trim) White Spaces From String / Variable

Posted on in Categories , , last updated January 18, 2013

I have a bash variable as follows:

output="$(awk -F',' '/Name/ {print $9}' input.file)"

How do trim leading and trailing whitespace from bash variable called $output? How do I trim trailing whitespace from $output?

You can use sed, awk, cut, tr, and other utilities to remove whitespace from $output.

Sample data


Define a variable called ouput:

output=" This is a test"

Use echo statement to display $output:

echo "=${output}="
Sample outputs:

=    This is a test=

sed Example

The syntax is:

echo "${output}" | sed -e 's/^[ \t]*//'

Sample outputs:

This is a test

Bash Example

The syntax is to remove leading whitespaces:

${var##*( )}

For example:

# Just remove leading whiltespace
#turn it on
shopt -s extglob
 
output="    This is a test"
output="${output##*( )}"
echo "=${output}="
 
# turn it off
shopt -u extglob

Sample outputs:

=This is a test=

To trim leading and trailing whitespace using bash, try:

#turn it on
shopt -s extglob
output="    This is a test    "
 
### Trim leading whitespaces ###
output="${output##*( )}"
 
### trim trailing whitespaces  ##
output="${output%%*( )}
echo "=${output}="
 
# turn it off
shopt -u extglob

Sample outputs:

=This is a test=

awk Example

The syntax is:

output="    This is a test    "
echo "=${output}="
 
## Use awk to trim leading and trailing whitespace
echo "${output}" | awk '{gsub(/^ +| +$/,"")} {print "=" $0 "="}'

Sample outputs:

=This is a test=

16 comment

  1. I tend to just stick to sed for this typeof thing:

    [email protected]:~$ output=" This is a test  "
    [email protected]:~$ echo "=${output}="
    = This is a test  =
    [email protected]:~$ output=$( echo "${output}"| sed -e 's/^ *//' -e 's/ *$//' );
    [email protected]:~$ echo "=${output}="
    =This is a test=
    [email protected]:~$ 
    

    Since a lot of my work needs to live on BSD, AIX and Solaris, as well as on Linux, I tend to script very defensively and portably. It has saved my butt more times than bears mentioning…

  2. If the content is a single line of text, why not just use echo ?

    $VAR=$(echo $VAR)
    

    notice there are no double-quotes for the inner $VAR

  3. This is the BEST site in the world. I can’t tell you how many times I have wound up here to find the answer to my question or problem. Great resource, thanks!!!

  4. Hello..
    Could you please help me out….
    Here is my scenario:
    1. i am listing some of the processes which are active and sending the output to temp.txt.
    2. i got many processes, if i want to kill/doing some thing with any inactive process it should throw an error message.
    3. how do i work with pattern matching commands in temp.txt?
    Please suggest me the better answer..

  5. I’m not sure why the examples use extglob in bash. It’s completely unneeded when using the ## or %% operators. ${VAR%% } will remove trailing whitespace, as long as it’s not mixed tabs and spaces.

  6. You can also pipe to tr this:

    [email protected]:~$ echo "=    This is a test="|tr -s " "
    = This is a test=
  7. "${output##*( )}" output="${output%%*( )} 

    Is this bash version specific ? Any reference for the same ?

    Thanks

  8. I extend the ‘Bash Example’ as follows:
    useful for whitespaces the ${var##*(tab|space)} form, where the tab is itself, and the space is also itself, it look like ${var##*( | )} or ${var%%( | )} typed in editor.

    Thanks, and best regards, Miki

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