while loop with timeout and enter-less input? SOLVED

[linx255]

I would like to be able to end the following while loop after 5 seconds:

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while true; do
  read -p "" -n 1 key
  case $key in
    [xX]) y="y"; break ;;
  esac
done

However, when I try changing the while statement to:

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t=$((SECONDS+5))
while [ $t -lt 5 ]; do

the read command then pauses each loop until ENTER is pressed, so this stalling renders the countdown useless.

Any ideas?
Thanks

[WharfRat]

I don't know I understand what you're looking for but paste this in its entirety in a terminal and press enter:

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SECOND=0
while [ $SECOND -lt 5 ]; do
  sleep 1s
  SECOND=$((SECOND+1))
done
echo "SECOND is now $SECOND"

[xenopeek]

Do away with the loop entirely. Use the timeout option for your read command:

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read -p "" -n 1 -t 5 key

From the bash manpage:

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-t timeout
       Cause read to time out and return failure if a complete line of input (or a specified number of char‐
       acters)  is  not read within timeout seconds.  timeout may be a decimal number with a fractional por‐
       tion following the decimal point.  This option is only effective if read is reading input from a ter‐
       minal,  pipe, or other special file; it has no effect when reading from regular files.  If read times
       out, read saves any partial input read into the specified variable  name.   If  timeout  is  0,  read
       returns  immediately, without trying to read any data.  The exit status is 0 if input is available on
       the specified file descriptor, non-zero otherwise.  The exit status is greater than 128 if the  time‐
       out is exceeded.

[lmuserx4849]

A couple of thoughts:

• Debug: At the top of your script add 'set -x' and run to see what is happening.
• Debug: Also add 'set -u' to catch any unset variables.
• Debug: Add some quick echos. For example, right after the read, what is the return code 'echo $?'
  The return codes are documented.
• Documentation: On the command line do a 'help read'. You can do that for all bash builtins. To see if it is a builtins do: 'type -a READ'. The same info, or more is in the bash man page. If you'd rather read the man page in html checkout 'man --html bash' (you may need to specify the brower, see 'man man').
• As mentioned, if you want a timeout on read use '-t'
• You can get fancier (do you need to??) and use a subprocess. See: http://mywiki.wooledge.org/ProcessManagement
  
  It would look something like:
  

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  #!/bin/bash
  #set -xu
  
  declare -- sleepPID=0
  declare -- subprocessPID=0
  
  function my_loop() {
    #lsof -p $$
    local -- key=''
    local -- running=true
  
    printf -- '%s\n' "${FUNCNAME}"
  
    # Handle ctrl-c. Not inherited
    trap 'running=false' INT
  
    while ${running}; do
      # FD are inherited
      read -u 3 -rp "==> " key
      case ${key} in
        [xX]) y="y"; break ;;
      esac
    done
  }
  
  # Handle ctrl-c
  function cleanup() {
    while kill -0 ${subprocessPID}; do
      kill ${subprocessPID}
      sleep 1
    done
  
    while kill -0 ${sleepPID}; do
      kill ${sleepPID}
      sleep 1
    done
  
    trap - INT
  }
  
  trap cleanup INT
  
  exec 3</dev/tty
  my_loop &
  subprocessPID=$!
  
  sleep 10s &
  sleepPID=$!
  
  printf -- 'Sleep pid is [%s]\n' ${sleepPID}
  printf -- 'Subprocess pid is [%s]\n' ${subprocessPID}
  
  wait ${sleepPID}
  kill ${subprocessPID}
  
  trap - INT
  

[linx255]

Thanks y'all !!! Y'all ROCK !!!

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read -p "" -n 1 -t 5 key

works perfectly!