Lessons · Terminal · $(...) (a command's output)

Using a command's output inside another

$(command) is replaced by what the command printed. $((2 + 3)) does whole-number maths.

Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.

What it is for

Today's date in a file name, the number of files in a message, the result of one tool as an argument to the next.

Run the inner command on its own first and look at its output. Then wrap it in $(...) where that text should appear.

echo "there are $(ls | wc -l) files here"

The inner command runs first: there are 0 files here.

count=$(printf 'a\nb\n' | wc -l)

Capture a pipeline's output in a variable.

echo $count

echo $((count + 3))

5: arithmetic on it.

Put today's date into the file name.

cp app.log app-(date +%F).log

Backticks do the same job and are still seen in old scripts, but they cannot nest and are easy to misread. Use $(...).

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