command_substitution.md

Command Substitution

execute commands in a subshell, substitute output

Examples

echo "Current path is $(pwd)"   # Preferred syntax
echo "Current path is `pwd`"    # Backtick syntax, not recommended
foo=$(pwd)                      # assign to variable
foo="$(pwd)"                    # Ok, but you don't need quotes in variable assignments
foo=$( <somefile )              # assign contents of a file to a variable, without using "cat"
foo=$(cp file /some/path 2>&1)  # capture both stdout and stderr
foo=$(                          # Multi-line
    pwd                         # Don't need to escape newlines
    ls                          # can contain comments
)

Notes

• trailing newlines are removed from command substitutions.
• if not quoted, the results undergo word splitting and pathname expansion.
• Word splitting also removes embedded newlines and other IFS characters.

Backticks vs $()

The backtick syntax is discouraged because

• backticks look similar to single quotes
• it's clumsy to "nest" ("inner" backticks need to be escaped)
• backtick syntax is based on simple character substitution,
• whereas every $()-construct opens an own, subsequent parsing step
• Everything inside $() is interpreted as if written normal on a command line. No special escaping is needed.