My bash shell script read a config file line-by-line using a bash while loop. I need to check if a string stored in $var starts with some value such as # character. If so I need to ignore that value and read the next line from my config file. How do I check if variable begins with # in bash shell scripting running on a Linux or Unix-like operating systems?
Introduction – In bash, we can check if a string begins with some value using regex comparison operator =~. One can test that a bash variable starts with a string or character in bash efficiently using any one of the following methods.
How to check if a string begins with some value in bash
Let us define a shell variable called vech as follows:
vech="Bus"
To check if string “Bus” stored in $vech starts with “B”, run:
[[ $vech = B* ]] && echo "Start with B"
The [[ used to execute the conditional command. It checks if $vech starts with “B” followed by any character. Set vech to something else and try it again:
vech="Car"
[[ $vech = B* ]] && echo "Start with B"
[[ $vech = B* ]] && echo "Start with B" || echo "Not matched"
Bash check if string starts with character using if statement
if..else..fi allows to make choice based on the success or failure of a command:
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<span style="color: #666666; font-style: italic;">#!/bin/bash</span> <span style="color: #007800;">input</span>=<span style="color: #ff0000;">"xBus"</span> <span style="color: #000000; font-weight: bold;">if</span> <span style="color: #7a0874; font-weight: bold;">[</span><span style="color: #7a0874; font-weight: bold;">[</span> <span style="color: #007800;">$input</span> = B<span style="color: #000000; font-weight: bold;">*</span> <span style="color: #7a0874; font-weight: bold;">]</span><span style="color: #7a0874; font-weight: bold;">]</span> <span style="color: #000000; font-weight: bold;">then</span> <span style="color: #7a0874; font-weight: bold;">echo</span> <span style="color: #ff0000;">"Start with B"</span> <span style="color: #000000; font-weight: bold;">else</span> <span style="color: #7a0874; font-weight: bold;">echo</span> <span style="color: #ff0000;">"No match"</span> <span style="color: #000000; font-weight: bold;">fi</span> |
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#!/bin/bash
input=”xBus” if [[ $input = B* ]]
then
echo “Start with B”
else
echo “No match”
fi
Bash check if a variable string begins with # value
The syntax is as follows to read a file line-by-line:
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<span style="color: #666666; font-style: italic;">#!/bin/bash</span> <span style="color: #007800;">input</span>=<span style="color: #ff0000;">"/path/to/txt/file"</span> <span style="color: #000000; font-weight: bold;">while</span> <span style="color: #007800;">IFS</span>= <span style="color: #c20cb9; font-weight: bold;">read</span> <span style="color: #660033;">-r</span> var <span style="color: #000000; font-weight: bold;">do</span> <span style="color: #7a0874; font-weight: bold;">echo</span> <span style="color: #ff0000;">"<span style="color: #007800;">$var</span>"</span> <span style="color: #000000; font-weight: bold;">done</span> <span style="color: #000000; font-weight: bold;"><</span> <span style="color: #ff0000;">"<span style="color: #007800;">$input</span>"</span> |
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#!/bin/bash
input=”/path/to/txt/file”
while IFS= read -r var
do
echo “$var”
done < “$input”
Let us add check to see if $var starts with “#” in bash:
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<span style="color: #666666; font-style: italic;">#!/bin/bash</span> <span style="color: #007800;">input</span>=<span style="color: #ff0000;">"/path/to/txt/file"</span> <span style="color: #000000; font-weight: bold;">while</span> <span style="color: #007800;">IFS</span>= <span style="color: #c20cb9; font-weight: bold;">read</span> <span style="color: #660033;">-r</span> var <span style="color: #000000; font-weight: bold;">do</span> <span style="color: #666666; font-style: italic;">#</span> <span style="color: #666666; font-style: italic;"># if value of $var starts with #, ignore it</span> <span style="color: #666666; font-style: italic;">#</span> <span style="color: #7a0874; font-weight: bold;">[</span><span style="color: #7a0874; font-weight: bold;">[</span> <span style="color: #007800;">$var</span> =~ ^<span style="color: #666666; font-style: italic;">#.* ]] && continue</span> <span style="color: #7a0874; font-weight: bold;">echo</span> <span style="color: #ff0000;">"<span style="color: #007800;">$var</span>"</span> <span style="color: #000000; font-weight: bold;">done</span> <span style="color: #000000; font-weight: bold;"><</span> <span style="color: #ff0000;">"<span style="color: #007800;">$input</span>"</span> |
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#!/bin/bash
input=”/path/to/txt/file”
while IFS= read -r var
do
#
# if value of $var starts with #, ignore it
#
[[ $var =~ ^#.* ]] && continue
echo “$var”
done < “$input”
The continue statement resumes the next iteration of the enclosing while loop, so it skips rest of the commands when commented line found out.
How to use regex comparison operator =~ if string starts with character
The syntax is as follows to see if $var starts with “#”:
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<span style="color: #000000; font-weight: bold;">if</span> <span style="color: #7a0874; font-weight: bold;">[</span><span style="color: #7a0874; font-weight: bold;">[</span> <span style="color: #ff0000;">"<span style="color: #007800;">$var</span>"</span> =~ ^<span style="color: #666666; font-style: italic;">#.* ]]; then</span> <span style="color: #7a0874; font-weight: bold;">echo</span> <span style="color: #ff0000;">"yes"</span> <span style="color: #000000; font-weight: bold;">fi</span> |
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if [[ “$var” =~ ^#.* ]]; then
echo “yes”
fi
Therefor the following is an updated version:
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<span style="color: #000000; font-weight: bold;">while</span> <span style="color: #007800;">IFS</span>=<span style="color: #ff0000;">'|'</span> <span style="color: #c20cb9; font-weight: bold;">read</span> <span style="color: #660033;">-r</span> t u <span style="color: #000000; font-weight: bold;">do</span> <span style="color: #666666; font-style: italic;"># ignore all config line starting with '#'</span> <span style="color: #7a0874; font-weight: bold;">[</span><span style="color: #7a0874; font-weight: bold;">[</span> <span style="color: #007800;">$t</span> =~ ^<span style="color: #666666; font-style: italic;">#.* ]] && continue</span> <span style="color: #7a0874; font-weight: bold;">echo</span> <span style="color: #ff0000;">"Working on <span style="color: #007800;">$t</span> and <span style="color: #007800;">$u</span>"</span> <span style="color: #000000; font-weight: bold;">done</span> <span style="color: #000000; font-weight: bold;"><</span> <span style="color: #ff0000;">"<span style="color: #007800;">$INPUT_FILE</span>"</span> |
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while IFS=’|’ read -r t u
do
# ignore all config line starting with ‘#’
[[ $t =~ ^#.* ]] && continue
echo “Working on $t and $u”
done < “$INPUT_FILE”
How to check if a string begins with some value in bash
The case statement is good alternative to multilevel if-then-else-fi statement. It enable you to match several values against one variable. It is easier to read and write:
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<span style="color: #666666; font-style: italic;">#!/bin/bash</span> <span style="color: #666666; font-style: italic;"># set default to 'Bus' but accept the CLI arg for testing</span> <span style="color: #007800;">input</span>=<span style="color: #ff0000;">"<span style="color: #007800;">${1:-Bus}</span>"</span> <span style="color: #7a0874; font-weight: bold;">echo</span> <span style="color: #660033;">-n</span> <span style="color: #ff0000;">"<span style="color: #000099; font-weight: bold;">$</span>input starts with 'B' : "</span> <span style="color: #000000; font-weight: bold;">case</span> <span style="color: #ff0000;">"<span style="color: #007800;">$input</span>"</span> <span style="color: #000000; font-weight: bold;">in</span> B<span style="color: #000000; font-weight: bold;">*</span><span style="color: #7a0874; font-weight: bold;">)</span> <span style="color: #7a0874; font-weight: bold;">echo</span> <span style="color: #ff0000;">"yes"</span><span style="color: #000000; font-weight: bold;">;;</span> <span style="color: #000000; font-weight: bold;">*</span><span style="color: #7a0874; font-weight: bold;">)</span> <span style="color: #7a0874; font-weight: bold;">echo</span> <span style="color: #ff0000;">"no"</span><span style="color: #000000; font-weight: bold;">;;</span> <span style="color: #000000; font-weight: bold;">esac</span> |
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#!/bin/bash
# set default to ‘Bus’ but accept the CLI arg for testing
input=”${1:-Bus}”
echo -n “$input starts with ‘B’ : ”
case “$input” in
B*)
echo “yes”;;
*)
echo “no”;;
esac
Conclusion
You learned how to check if a string begins with some value using various methods. For more info read the bash command man page here.
Posted by: Vivek Gite
The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.
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