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37 lines
1.3 KiB
Markdown
37 lines
1.3 KiB
Markdown
# Extract Value From Command Output With Sed
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As part of a shell script, you may need to extract a value from one command to
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be used as part of a subsequent command.
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For instance, [I recently wrote a
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script](https://gist.github.com/jbranchaud/3cda6be6e1dc69c6f55435a387018dac)
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that needed to determine the version of the currently running Postges server.
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The `postgres` command can tell me that.
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```bash
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$ postgres -V
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postgres (PostgreSQL) 12.3
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```
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However, the output includes extra fluff that I don't need, namely the leading
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`postgres (PostgreSQL) ` part.
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The output of `postgres` can be piped into a `sed` command that can extract
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just what I need.
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```bash
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$ postgres -V | sed -n 's/postgres (PostgreSQL) \(.*\)/\1/p'
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12.3
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```
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The `sed` command receives this single line of output and attempts a
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substituation. It matches on `postgres (PostgresSQL) ` followed by a capture
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group (`\(.*\)`) for the remaining characters. This capture group matches the
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version part of the output. `sed` replaces everything in the first part of the
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substitution with `\1`, which is `12.3`, and outputs that.
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The output of this could then be piped to another command or captured in a
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variable to be used in the remainder of a script.
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[source](https://stackoverflow.com/a/24572880/535590)
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