Unix Background process STOPPED abnormally

Question

I executed a perl script in background using the following command

nohup perl myPerlSCript.pl >debug_log &

After few minutes I got the status as

[1]+ Stopped

I wasn't expecting it to stop, nor do I know what stopped it. How can I debug this and find out why it stopped? I am actually interested in knowing the unix commands to debug.

Answer 1

There are several ways a process running in the background can be stopped. All of them involve one of these signals:

• SIGSTOP
• SIGTSTP
• SIGTTOU
• SIGTTIN

SIGSTOP is severe. It's unblockable, unignorable, unhandlable. It stops the process as surely as SIGKILL would kill it. The others can be handled by the background process to prevent stopping.

• A signal was sent by another process using kill(2), or by the process to itself using raise(3) or kill(2)
• The process attempted to write to the terminal, and the terminal option tostop is enabled (see output of stty -a). This generates SIGTTOU.
• The process attempted to change the terminal modes with tcsetattr(3) or an equivalent ioctl. (These are the same modes shown by stty.) This generates SIGTTOU regardless of the current state of the tostop flag.
• The process attempted to read from the terminal. This generates SIGTTIN.

This list is probably very incomplete.