CODEWITHSUNDEEP
bashshellpipeexit
Qiu Yangfan
Qiu Yangfan

Reputation: 891

Quit from pipe in bash

For following bash statement:

tail -Fn0 /tmp/report | while true; do echo "pre"; exit; echo "past"; done

I got "pre", but didn't quit to the bash prompt, then if I input something into /tmp/report, I could quit from this script and get into bash prompt.

I think that's reasonable. the 'exit' make the 'while' statement quit, but the 'tail' still alive. If something input into /tmp/report, the 'tail' will output to pipe, then 'tail' will detect the pipe is close, then 'tail' quits.

  1. Am I right? If not, would anyone provide a correct interpretation?

  2. Is it possible to add anything into 'while' statement to quit from the whole pipe statement immediately? I know I could save the pid of tail into a temporary file, then read this file in the 'while', then kill the tail. Is there a simpler way?

  3. Let me enlarge my question. If use this tail|while in a script file, is it possible to fulfill following items simultaneously?

    • If Ctrl-C is inputed or signal the main shell process, the main shell and various subshells and background processes spawned by the main shell will quit
    • I could quit from tail|while only at a trigger case, and preserve other subprocesses keep running
    • It's better not use temporary file or pipe file.

Upvotes: 9

Views: 8599

Answers (2)

William Pursell
William Pursell

Reputation: 212248

You can (unreliably) get away with killing the process group:

tail -Fn0 /tmp/report | while :
do 
  echo "pre"
  sh -c 'PGID=$( ps -o pgid= $$ | tr -d \  ); kill -TERM -$PGID'
  echo "past"
done

This may send the signal to more processes than you want. If you run the above command in an interactive terminal you should be okay, but in a script it is entirely possible (indeed likely) the the process group will include the script running the command. To avoid sending the signal, it would be wise to enable monitoring and run the pipeline in the background to ensure that a new process group is formed for the pipeline:

#!/bin/sh

# In Posix shells that support the User Portability Utilities option
# this includes bash & ksh), executing "set -m" turns on job control. 
# Background processes run in a separate process group.  If the shell
# is interactive, a line containing their exit status is printed to
# stderr upon their completion.
set -m
tail -Fn0 /tmp/report | while :
do 
  echo "pre"
  sh -c 'PGID=$( ps -o pgid= $$ | tr -d \  ); kill -TERM -$PGID'
  echo "past"
done &
wait

Note that I've replaced the while [ 1 ] with while : because while [ 1 ] is poor style. (It behaves exactly the same as while [ 0 ]).

Upvotes: 2

Barmar
Barmar

Reputation: 780974

You're correct. The while loop is executing in a subshell because its input is redirected, and exit just exits from that subshell.

If you're running bash 4.x, you may be able to achieve what you want with a coprocess.

coproc TAIL { tail -Fn0 /tmp/report.txt ;}
while [ 1 ]
do
    echo "pre"
    break
    echo "past"
done <&${TAIL[0]}
kill $TAIL_PID

http://www.gnu.org/software/bash/manual/html_node/Coprocesses.html

With older versions, you can use a background process writing to a named pipe:

pipe=/tmp/tail.$$
mkfifo $pipe
tail -Fn0 /tmp/report.txt >$pipe &
TAIL_PID=$!
while [ 1 ]
do
    echo "pre"
    break
    echo "past"
done <$pipe
kill $TAIL_PID
rm $pipe

Upvotes: 8

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