Command substitution

Command substitution expands to the output of a command. It has two forms: the preferred $(command) form and the deprecated backquote form `command`.

For example, this runs dirname -- "$0" and passes its output to cd:

$ cd -P -- "$(dirname -- "$0")"

This changes the working directory to the directory containing the script, regardless of the current directory.

Syntax

The $(…) form evaluates the command inside the parentheses. It supports nesting and is easier to read than backquotes:

$ echo $(echo $(echo hello))
hello
$ echo "$(echo "$(echo hello)")"
hello

In the backquote form, backslashes escape $, `, and \. If backquotes appear inside double quotes, backslashes also escape ". These escapes are processed before the command is run. A backquote-form equivalent to the previous example is:

$ echo `echo \`echo hello\``
hello
$ echo "`echo \"\`echo hello\`\"`"
hello

The $(…) form can be confused with arithmetic expansion. Command substitution is only recognized if the code is not a valid arithmetic expression. For example, $((echo + 1)) is arithmetic expansion, but $((echo + 1); (echo + 2)) is command substitution. To force command substitution starting with a subshell, insert a space: $( (echo + 1); (echo + 2)).

Semantics

The command runs in a subshell, and its standard output is captured. Standard error is not captured unless redirected. Trailing newlines are removed, and the result replaces the command substitution in the command line.

Currently, yash-rs parses the command when the substitution is executed, not when it is parsed. This may change in the future, affecting when syntax errors are detected and when aliases are substituted.