What does $() do?

Question

Trying to troubleshoot this issue:

https://unix.stackexchange.com/questions/128894/ssh-exchange-identification-connection-closed-by-remote-host-not-using-hosts-d

and part of one solution is to use:

$(which sshd)

which in my case outputs:

Could not load host key: /etc/ssh_host_rsa_key
Could not load host key: /etc/ssh_host_dsa_key

I keep a cheat sheet for all my bash commands and wanted to add in:

$()

It appears to be doing something to the sshd executable.

That's a bad suggestion on the linked page. There's almost no reason at all for that over just `sshd` there. Also using `which` instead of `type` is poor (though common). — Etan Reisner, Oct 22 '15 at 18:46
It is a shell built-in that is guaranteed to exist and has POSIX-specified behavior. `which` is none of those things. — Etan Reisner, Oct 22 '15 at 18:49
@EtanReisner The linked answer explains why it recommended this and why simply using `sshd` wouldn't work. — Random832, Oct 22 '15 at 19:08
@Random832 That reason doesn't cover not using `type` though it does cover not just using `sshd`. — Etan Reisner, Oct 22 '15 at 19:09
@EtanReisner: You don't want to use `type` in scripting, it's output aim for interactive use instead of scripting. POSIXly, you need `command -v sshd`. — cuonglm, Oct 23 '15 at 03:24
@cuonglm Yeah, `command -v` also works here and is better (I'm still converting my muscle memory from `type -p` to `command -v`). — Etan Reisner, Oct 23 '15 at 03:34

Oliver Gondža · Answer 1 · 2015-10-23T16:46:26.813

3

$() is a way to execute another process and collect its output. See http://wiki.bash-hackers.org/syntax/expansion/cmdsubst for more details.

When such expression is passed to bash, its output gets executed. It effectively call the command using its full path, as that is what which returns. The messages printed are from sshd process started by that expression.

Note that which locates executable scanning $PATH, same as when you execute the command. In other words, executing which output it is not going to affect which executable is run, only the full path to executable tracked by operating system.

edited Oct 23 '15 at 16:46

answered Oct 22 '15 at 18:44

Oliver Gondža

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It is worth noting that the command should execute regardless since `which` only knows to look in the same `PATH` that `bash` would use. You get the same set of commands to run either way. So the only effect of the `$(which foo)` should be that you can see the whole path to in `ps` or `top`. – chicks Oct 22 '15 at 20:03

score 0 · Answer 2 · answered Oct 22 '15 at 18:46

$(which sshd) will be replaced by the stdout resulting from running which sshd. which sshd will return the fully-qualified path of the executable invoked when invoking sshd:

which returns the pathnames of the files (or links) which would be executed in the current environment, had its arguments been given as commands in a strictly POSIX-conformant shell. It does this by searching the PATH for executable files matching the names of the arguments. It does not follow symbolic links.

Examples, as run on a command line, where > represents the input prompt:

COMMAND: which sshd
 OUTPUT: /usr/sbin/sshd

COMMAND: echo "The full path of sshd is $(which sshd)"
 OUTPUT: The full path of sshd is /usr/sbin/sshd

COMMAND: $(which sshd)
 OUTPUT: [[whatever output you get from running /usr/sbin/sshd]]

What does $() do?

2 Answers2