Bash: ( commands ) | command

Question

I stumbled upon Zenity, a command-line based GUI today. I noticed the there was some syntax of the form ( commands ) | command. Could anyone shed some light on what this is and where I can read more about it?

I found the below script within the docs

(
  echo "10" ; sleep 1
  echo "# Updating mail logs" ; sleep 1
  echo "50" ; sleep 1
  echo "This line will just be ignored" ; sleep 1
  echo "100" ; sleep 1
) |
 zenity --progress \
   --title="Update System Logs" \
   --text="Scanning mail logs..." \
   --percentage=0

A command-line graphical interface. Interesting concept. — CompuChip
– CompuChip, Commented Mar 7, 2014 at 8:31
@CompuChip: Well, it displays dialog boxes, not all that weird? — tripleee
– tripleee, Commented Mar 7, 2014 at 8:41
Terminal-based graphical interface would be more accurate, since it isn't limited to a single command line :) — chepner
– chepner, Commented Mar 7, 2014 at 12:33

Mike Holt · Accepted Answer · 2014-03-07 08:45:27Z

1

The parentheses are delimiting a subshell, which means the commands inside the parens are run in a separate process, and interpreted by a separate instance of the bash interpreter. In this case, it appears they are using a subshell just to group together all the echo and sleep commands so that they can then pipe the combined output of the entire group of commands through zenity. Which makes sense given that the goal in this example is to simulate a progress bar.

You can read more about subshells here: http://tldp.org/LDP/abs/html/subshells.html

answered Mar 7, 2014 at 8:45

Mike Holt

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tripleee · Accepted Answer · 2014-03-07 08:50:42Z

1

The parentheses create a subshell, with all the implications that it has for the current shell.

The subshell cannot change the environment of the parent shell; sometimes, you want a subshell so that you can, say, quickly cd to a different directory without affecting the working directory for the rest of the script
The subshell has a single standard input and a single standard output stream; this is frequently a reason to start a subshell
The parent shell waits while the subshell completes the commands (unless you run it in the background)

If it helps,, think of ( foo; bar ) as a quick way to say sh -c 'foo; bar'.

A related piece of syntax is the brace, which runs a compound command in the current shell, not a subshell.

test -f file.rc || { echo "$0: file.rc not found -- aborting" >&2; exit 127; }

The exit in particular causes the current shell to exit with a failure exit code, while a subshell which exits does not directly affect the rest of the parent shell script.

(Weirdly, POSIX requires a statement terminator before the closing brace, but not before the closing parenthesis.)

edited Mar 7, 2014 at 8:50

answered Mar 7, 2014 at 8:44

tripleee

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1 Comment

chepner Over a year ago

Braces are reserved words--not operator tokens like ;, &, and <newline>--in POSIX. I suspect this is to allow the parameter expansion syntax ${foo}, although that doesn't explain why $(...) can be parsed. As a result, an actual control operator is required to terminate the final command of a command group.

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