What is $() in Linux Shell Commands?
For example:
chmod 777 $(pwd) 
- 1Note that there's no such thing as the line shell command. There are a number of shell applications available for Linux.Stéphane Chazelas– Stéphane Chazelas2014-07-30 12:44:19 +00:00Commented Jul 30, 2014 at 12:44
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3 Answers
It's very similar to the backticks ``.
It's called command substitution (posix specification) and it invokes a subshell. The command in the braces of $() or between the backticks (`…`) is executed in a subshell and the output is then placed in the original command.
Unlike backticks, the $(...) form can be nested. So you can use command substitution inside another substitution.
There are also differences in escaping characters within the substitution. I prefer the $(...) form.
- 6backticks can be nested as well. Note that trailing newline characters are removed from the output of the command.Stéphane Chazelas– Stéphane Chazelas2014-07-30 12:50:48 +00:00Commented Jul 30, 2014 at 12:50
- 2@StéphaneChazelas how can backticks be nested? You mean by ecxessive escaping?chaos– chaos2014-07-30 12:53:29 +00:00Commented Jul 30, 2014 at 12:53
- 3Yes: like with
echo `echo \`echo foo\`` barStéphane Chazelas– Stéphane Chazelas2014-07-30 12:59:44 +00:00Commented Jul 30, 2014 at 12:59 - 2Any official reference saying that backticks are deprecated?vinc17– vinc172014-07-30 13:08:42 +00:00Commented Jul 30, 2014 at 13:08
- 1Found this answer: unix.stackexchange.com/questions/126927/… seems they are not deprecated. Changed my answer, thankschaos– chaos2014-07-30 13:17:02 +00:00Commented Jul 30, 2014 at 13:17
In POSIX or POSIX-like shells (ksh, bash, ash, zsh, yash...), it is like ``: the command inside $() is executed and replaced by its standard output. Word-splitting and filename generation are done unless $() is inside double-quotes. Thus
chmod 777 $(pwd) should be replaced with:
chmod 777 "$(pwd)" to avoid word-splitting and filename generation on the current working directory path.
Or even better (except under some shells, like zsh, in case the directory has been renamed):
chmod 777 "$PWD" Since $PWD is a special variable that holds the path to the current working directory in POSIX shells.
Or even better:
chmod 777 . Since the . entry in the current directory is a hard link to that directory itself.
This $() is used for executing a command mostly inside some other command.
chmod 777 $(pwd) pwd command gives the current working directory. So, when the whole thing is executed output of pwd will replace its position and serve as the argument to chmod , and the result is that all your present working directory get the permission 777 which I guess should never be used in production environment ;) .
