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I have a command that I want to have run again automatically each time it terminates, so I ran something like this:

while [ 1 ]; do COMMAND; done;

but if I can't stop the loop with Ctrl-c as that just kills COMMAND and not the entire loop.

How would I achieve something similar but which I can stop without having to close the terminal?

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4  
If I'm in bash I just use Ctrl-Z to stop the job and then "kill %1" to kill it. –  Paul Cager Jul 4 '12 at 13:20
1  
Just wait... Linus was quoted as saying: “We all know Linux is great... it does infinite loops in 5 seconds.” -- so really... just wait a few more seconds, it should complete. –  lornix Jul 5 '12 at 8:40
    
@PaulCager worked for me too! Why does it work where Ctrl-C does not? –  Ciro Santilli Mar 15 at 19:19
    
@cirosantilli it kills the outer job (the bash "wrapper"). In some situations, it won't immediately kill the "COMMAND", for instance, if you background it, it may sneak past alive even if it's parent is dead. But the loop is dead, and that's the important part. –  orion Mar 15 at 23:01
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6 Answers 6

up vote 11 down vote accepted

Check the exit status of the command. If the command was terminated by a signal the exit code will be 128 + the signal number. From the GNU online documentation for bash:

For the shell’s purposes, a command which exits with a zero exit status has succeeded. A non-zero exit status indicates failure. This seemingly counter-intuitive scheme is used so there is one well-defined way to indicate success and a variety of ways to indicate various failure modes. When a command terminates on a fatal signal whose number is N, Bash uses the value 128+N as the exit status.

POSIX also specifies that the value of a command that terminated by a signal is greater than 128, but does not seem to specify its exact value like GNU does:

The exit status of a command that terminated because it received a signal shall be reported as greater than 128.

For example if you interrupt a command with control-C the exit code will be 130, because SIGINT is signal 2 on Unix systems. So:

while [ 1 ]; do COMMAND; test $? -gt 128 && break; done
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5  
It should be mentioned that this is not guaranteed, in fact, many applications will not do this. –  Chris Down Jul 4 '12 at 1:18
1  
@Kyle Jones: can you link to the POSIX / GNU docs that mention that? –  Ciro Santilli Mar 15 at 20:03
    
@cirosantilli Done. –  Kyle Jones Mar 15 at 22:44
    
@KyleJones thanks! Still in practice not working for COMMAND = paplay alert.ogg, perhaps because paplay handles the signal? –  Ciro Santilli Mar 16 at 6:49
    
@cirosantilli Yes, that's the reason. If a process handles the signal and quits, that's different than the process being terminated by an unhandled signal. –  Kyle Jones Mar 16 at 7:21
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You can stop and put your job in background while it's running using ctrl+z. Then you can kill your job with:

$ kill %1

Where [1] is your job number.

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  1. You can always kill a process using its PID, there's no need to close your terminal
  2. If you want to run something in an infinite loop like a daemon then you'd best put it in the background
  3. while : will create an infinite loop and saves you writing the [ 1 ]

    while :; do COMMAND; done &
    

This will print the PID. If you exit your prompt using ctrl+d then the background job won't quit, and you can later kill the job from anywhere using kill PID

If you lose track of your PID, you can use pstree -pa $USER or pgrep -fl '.*PROCESS.*' to help you find it

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If you run bash with -e it will exit on any error conditions:

#!/bin/bash -e
false # returns 1
echo This won't be printed
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This will definitely come in handy. –  Dean Jul 4 '12 at 7:21
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I would say it might be best to put your infinite loop in a script and handle signals there. Here's a basic starting point. I'm sure you'll want to modify it to suit. The script uses trap to catch ctrl-c (or SIGTERM), kills off the command (I've used sleep here as a test) and exits.

cleanup ()
{
kill -s SIGTERM $!
exit 0
}

trap cleanup SIGINT SIGTERM

while [ 1 ]
do
    sleep 60 &
    wait $!
done
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1  
Nice. Here's how I used this tip to make an autorestarting netcat wrapper: trap "exit 0" SIGINT SIGTERM; while true; do netcat -l -p 3000; done –  Douglas Jan 13 '13 at 13:00
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I generally just hold down Ctrl-C. Sooner or later it'll register between COMMAND's and thus terminate the while loop. Maybe there is a better way.

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Not sure why, but it fails for certain COMMANDs like paplay on a 1s file. –  Ciro Santilli Mar 15 at 11:07
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