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Note: script refers to the script command, which logs stdin and stdout to files.

I was trying to run a script with the script utility to save output for later inspection while preserving text colors on terminal.

  • When I try to suspend the current script with Ctrl+Z (SIGTSTP), the console prints ^Z and the script just won't stop.
  • I then tried Ctrl+S (SIGSTOP).  The script currently running does freeze (htop shows all process as Z mode), though it does not release the terminal.  Ctrl+Q does resume them.

I'm now confused why SIGSTOP Ctrl+S works but SIGTSTP (Ctrl+Z) doesn’t.

I know SIGTSTP can be trapped, but I don't see any reason to do that, and there's nothing about it in script’s manual page.  I tried to force the script interpreter to interactive mode, but the result makes me more confused.  Issuing Ctrl+Z (SIGTSTP) does suspend the running script, but script says scripts are finished and then exited, which kills all suspended child processes.

Is there a way to allow suspend normally in such situation?  Also, can someone explain exactly what happened?

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    Ctrl/S isn't SIGSTOP Commented Apr 15, 2024 at 13:39

1 Answer 1

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Just like any other pass-through tool (ssh, screen, tmux, etc.), script puts the calling terminal into raw mode so that characters such as Ctrl/Z no longer generate interrupts. It then passes these characters along and the inside terminal device processes them "as normal", generating the expected signal. Any process can choose to trap SIGTSTP but cannot catch SIGSTOP.

In these examples I've used $ to indicate the command line prompt:

$ script
Script started, output log file is 'typescript'.
$ sleep 5    # After starting this I hit Ctrl/Z

[1]+  Stopped                 sleep 5
$

If you send script itself the SIGTSTP signal, it appears to get confused (programming fault, maybe?), and SIGCONT will not resume it. However, sending SIGINT or some other terminating signal resumes script long enough to action the original SIGTSTP and suspend. Then attempting to resume the script command with fg terminates the session:

$ script
Script started, output log file is 'typescript'.
$ while date; do sleep 5; done
Mon, 15 Apr 2024 14:44:42
Mon, 15 Apr 2024 14:44:47

At this point script received SIGTSTP and ceased executing its internal process. Subsequently sending SIGCONT made no visible difference.

Sending SIGINT resulted in this:

Mon, 15 Apr 2
[1]+  Stopped                 script

And then resuming the command:

$ fg
script

Session terminated, killing shell... ...killed.
Script done.

It seems to me that you would be better sending SIGSTOP to the process inside script. Here's another example:

$ script
Script started, output log file is 'typescript'.
$ while date; do sleep 5; done
Mon, 15 Apr 2024 14:48:31
Mon, 15 Apr 2024 14:48:36
Mon, 15 Apr 2024 14:48:41
Mon, 15 Apr 2024 14:48:46    # Here I sent SIGSTOP to the shell running the "while" loop
Mon, 15 Apr 2024 14:49:07
Mon, 15 Apr 2024 14:49:12    # Here I hit Ctrl/C to break the loop

$ exit
Script done.
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  • My original intention is that ctrl-z could suspend both "script" and the running script so it returns console to me that I can inspect the log file. Commented Apr 15, 2024 at 16:53
  • That's not a correct description of what happens. script starts a new pty and puts the host tty in raw -echo mode. So it doesn't pass the SIGTSTP signal along, it just passes the ^Z character (which hasn't caused a SIGTSTP by the host tty because raw implies -isig) along to the guest pty, which does cause a SIGTSTP signal to be sent to the foreground process group of the guest pty. Commented Apr 15, 2024 at 17:02
  • @GoDaftWithEBK I don't think you can reliably do that Commented Apr 15, 2024 at 17:06
  • @Stéphane Chazelas So this is why running script does react to ^Z when in forced interactive mode(which enables monitor mode - job control). Though issuing the signal makes"script" somehow decide that the running script is finished so it exits, which then kills all the remaining process. Commented Apr 15, 2024 at 18:15

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