Bash Command Substitution
Internal
Overview
Command substitution is invoked with:
$(command)
An equivalent, older and obsolete style is:
`command`
Command substitution allows the stdout of a command or function to replace the command or function itself. The command is executed in a subshell environment, and the stdout of the command, which would normally be sent to stdout of the executing shell, is used to replace the command. The output can be saved into a variable as follows:
local content
content=$(ps -ef)
The stdout is processed as follows: Any trailing newlines are deleted. Embedded newlines are not deleted, but they may be removed during word splitting. If the substitution appears within double quotes, word splitting and filename expansion are not performed on the results.
If stdout content is an empty string, the behavior is noop.
If the command produces additional stderr content, or only stderr content, the stderr content is redirected to the stderr of the invoking shell and it is not involved in command substitution.
It is possible to coalesce stdout and stderr in a single command substitution as follows. In this case, the stderr content is concatenated to the stdout content and command substitution is applied to the resulted string.
$(command 2>&1)
Special Cases
Command substitution:
$(cat file)
can be replaced with the faster:
$(< file)