Extract Value From Command Output With Sed

As part of a shell script, you may need to extract a value from one command to be used as part of a subsequent command.

For instance, I recently wrote a script that needed to determine the version of the currently running Postges server. The postgres command can tell me that.

$ postgres -V
postgres (PostgreSQL) 12.3

However, the output includes extra fluff that I don't need, namely the leading postgres (PostgreSQL) part.

The output of postgres can be piped into a sed command that can extract just what I need.

$ postgres -V | sed -n 's/postgres (PostgreSQL) \(.*\)/\1/p'
12.3

The sed command receives this single line of output and attempts a substituation. It matches on postgres (PostgresSQL) followed by a capture group (\(.*\)) for the remaining characters. This capture group matches the version part of the output. sed replaces everything in the first part of the substitution with \1, which is 12.3, and outputs that.

The output of this could then be piped to another command or captured in a variable to be used in the remainder of a script.

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