[Shell Script](EN) Check command substitution's result exists or not in conditional statement

Check command substitution’s result exists or not in conditional statement

Environment and Prerequisite

• Linux
• Bash shell(/bin/bash)

Situatiaton and Method

Situatiaton

• Below is situatiaton that conditional statement is needed to check command substitution’s result exists or not
• command substitution is allowing the output of a command to replace the command itself by using subshell.

#!/bin/sh

result=$(ls -al | grep sample)
...

Method

• Use below conditional statements.
• Handle different way by using -z option.
• (Method 1) You can check it by only adding variable in conditional statement. If its length is 0 or it is not set, then it is considered as false.
• (Method 2) Use -z option. If variable next to -z is not set or length is 0, then it is considered as true.

#!/bin/sh

result=$(ls -al | grep sample)

# Return true if its length is not zero.
if [ "${result}" ]
then
    echo "result is not empty"
else
    echo "result is empty"
fi

# Checks if the given string operand size is zero; if it is zero length, then it returns true.
if [ -z "${result}" ]
then
    echo "result is empty"
else
    echo "result is not empty"
fi

Example

• Add specific string if it not exists in file
• Consider file exists

$ cat test.txt
String

$ vi test.sh

#!/bin/sh

set -e

set +e
result=$(cat test.txt | grep "test string")
set -e

if [ -z "${result}" ]
then
    echo "test string" >> test.txt
fi

$ chmod +x test.sh
$ ./test.sh

$ cat test.txt
String
test string

etc

• In grep command, normally the exit status is 0 if a line is selected, 1 if no lines were selected, and 2 if an error occurred. So if you use set -e option, then you need to unset that option before using it.
• Reference: https://www.gnu.org/software/grep/manual/grep.html#Exit-Status

...
set +e
result=$(ls -al | grep sample)
set -e
...

Reference

• https://www.gnu.org/software/bash/manual/html_node/Command-Substitution.html
• https://en.wikipedia.org/wiki/Command_substitution
• https://www.tutorialspoint.com/unix/unix-string-operators.htm
• https://stackoverflow.com/questions/12137431/test-if-a-command-outputs-an-empty-string
• https://www.cyberciti.biz/faq/unix-linux-bash-script-check-if-variable-is-empty/
• https://www.gnu.org/software/grep/manual/grep.html#Exit-Status