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.

Compatibility

When a command substitution only contains redirections, some shells treat it specially. For example, $(<file) is equivalent to $(cat file) in bash and zsh, but yash-rs does not support this yet.