how do I watch for a process to have died in shell script?

Question

I'm running a shell test program that I can view a progress bar but when I run it I keep getting a unary error . Is kill -0 a way to kill a subprocess in shell ? Or is there another method to test if my process has died? heres my code to run a progress bar until my command ends:

#!/bin/sh
# test my progress bar

spin[0]="-"
spin[1]="\\"
spin[2]="|"
spin[3]="/"

sleep  10 2>/dev/null &    # run as background process
pid=$!                     # grab process id

echo -n "[sleeping] ${spin[0]}"

while [ kill -0 $pid ]     # wait for process to end
do
  for i in "${spin[@]}"
  do
    echo -ne "\b$i"
    sleep 0.1
  done
done

enter code here

Answer 1

1. Is kill -0 a way to kill a subprocess in shell ?

On Linux OS, kill -0 is just a way to try to kill a process and see what happens, '0' is not a POSIX signal, it does nothing at all. If the process is running, kill will return 0, if not, it will return 1. ps $pid >/dev/null 2>&1 could do the same job. To kill a process, one generally use the SIGQUIT/3 (quit program) or SIGKILL/9 (terminate program) ; the process could trap the signal and make a clean exit, or it could ignore the signal so the OS has to terminate it 'quick and dirty'.

2. test and '['

• The square bracket '[' is an utility ( /bin/[ ), and expect something you didn't provide correctly.
• The syntax of while is while list; do list; done where list will return an exit code, so you don't have to use something else.

3. how do I watch for a process to have died in shell script?

Like you did, the code below will do the job:

#!/bin/bash

spin[0]="-"
spin[1]="\\"
spin[2]="|"
spin[3]="/"

sleep  10 2>/dev/null &    # run as background process
pid=$!                     # grab process id

echo -n "[sleeping] ${spin[0]}"

#while ps -p $pid >/dev/null 2>&1       # using ps
while kill -0 $pid >/dev/null 2>&1      # using kill
do
  for i in "${spin[@]}"
  do
    echo -ne "\b$i"
    sleep 0.5
  done
done

CAVEATS I use /bin/bash as interpreter, as some of the Bourne Shell (sh) could not support the use of an array (ie spin[n]).

Answer 2

It's probably cleaner to run the spinner in the background and kill it when the process (running in the foreground) terminates. Or, you could open another file descriptor and write something into it after the background process terminates, and have the main process block on a read. eg:

#!/bin/bash
# test my progress bar
spin[0]='-'
spin[1]='\'
spin[2]='|'
spin[3]='/'

{ { { sleep 10 2>/dev/null; echo >&5; } &    # run as background process
} 5>&1 1>&3 | { # wait for process to end
        while ! read -t 1; do
                printf "\r[sleeping] ${spin[ $(( i = ++i % 4 )) ]}"
        done
}
} 3>&1