Set Bash variable to output of command
The Problem
How do I store the output of a command in a Bash variable?
The Solution
Bash provides two alternate syntaxes for command substitution: `` and $(). Therefore, the following two Bash scripts are equivalent:
#!/bin/bash
OUTPUT=`ls`
echo "$OUTPUT"
#!/bin/bash
OUTPUT=$(ls)
echo "$OUTPUT"
Note that omitting the double quotes from the argument to echo in the above scripts will remove line breaks from the output.
Both methods of command substitution allow for command nesting, but the `` method requires nested backticks to be escaped with \, whereas the $() method does not. Thus the second method may be considered easier to read, especially for substitutions with more than one layer of nesting. The following two Bash scripts with nested commands are equivalent:
#!/bin/bash
OUTPUT=`echo ls output: \`ls \`pwd\`\`` # two layers of nesting
echo $OUTPUT # no linebreaks in content, so no need for ""s
#!/bin/bash
OUTPUT=$(echo ls output: $(ls $(pwd))) # two layers of nesting
echo $OUTPUT # no linebreaks in content, so no need for ""s
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