What happens when you hit ctrl+z on a process?

Question

If I am running a long-running process, and when I stop it with Ctrl+Z, I get the following message in my terminal:

76381 suspended  git clone [email protected]:kevinburke/<large-repo>.git

What actually happens when the process is suspended? Is the state held in memory? Is this functionality implemented at the operating system level? How is the process able to resume execution right where it left off when I restart it with fg?

Answer 1

When you hit Ctrl+Z in a terminal, the line-discipline of the (pseudo-)terminal device driver (the kernel) sends a SIGTSTP signal to all the processes in the foreground process group of the terminal device.

That process group is an attribute of the terminal device. Typically, your shell is the process that defines which process group is the foreground process group of the terminal device.

In shell terminology, a process group is called a "job", and you can put a job in foreground and background with the fg and bg command and find out about the currently running jobs with the jobs command.

The SIGTSTP signal is like the SIGSTOP signal except that contrary to SIGSTOP, SIGTSTP can be handled by a process.

Upon reception of such a signal, the process is suspended. That is, it's paused and still there, only it won't be scheduled for running any more until it's killed or sent a SIGCONT signal to resume execution. The shell that started the job will be waiting for the leader of the process group in it. If it is suspended, the wait() will return indicating that the process was suspended. The shell can then update the state of the job and tell you it is suspended.

$ sleep 100 | sleep 200 & # start job in background: two sleep processes
[1] 18657 18658
$ ps -lj # note the PGID
F S   UID   PID  PPID  PGID   SID  C PRI  NI ADDR SZ WCHAN  TTY          TIME CMD
0 S 10031 18657 26500 18657 26500  0  85   5 -  2256 -      pts/2    00:00:00 sleep
0 S 10031 18658 26500 18657 26500  0  85   5 -  2256 -      pts/2    00:00:00 sleep
0 R 10031 18692 26500 18692 26500  0  80   0 -  2964 -      pts/2    00:00:00 ps
0 S 10031 26500 26498 26500 26500  0  80   0 - 10775 -      pts/2    00:00:01 zsh
$ jobs -p
[1]  + 18657 running    sleep 100 |
     running    sleep 200
$ fg
[1]  + running    sleep 100 | sleep 200
^Z
zsh: suspended  sleep 100 | sleep 200
$ jobs -p
[1]  + 18657 suspended  sleep 100 |
     suspended  sleep 200
$ ps -lj # note the "T" under the S column
F S   UID   PID  PPID  PGID   SID  C PRI  NI ADDR SZ WCHAN  TTY          TIME CMD
0 T 10031 18657 26500 18657 26500  0  85   5 -  2256 -      pts/2    00:00:00 sleep
0 T 10031 18658 26500 18657 26500  0  85   5 -  2256 -      pts/2    00:00:00 sleep
0 R 10031 18766 26500 18766 26500  0  80   0 -  2964 -      pts/2    00:00:00 ps
0 S 10031 26500 26498 26500 26500  0  80   0 - 10775 -      pts/2    00:00:01 zsh
$ bg %1
[1]  + continued  sleep 100 | sleep 200
$ ps -lj
F S   UID   PID  PPID  PGID   SID  C PRI  NI ADDR SZ WCHAN  TTY          TIME CMD
0 S 10031 18657 26500 18657 26500  0  85   5 -  2256 -      pts/2    00:00:00 sleep
0 S 10031 18658 26500 18657 26500  0  85   5 -  2256 -      pts/2    00:00:00 sleep
0 R 10031 18824 26500 18824 26500  0  80   0 -  2964 -      pts/2    00:00:00 ps
0 S 10031 26500 26498 26500 26500  0  80   0 - 10775 -      pts/2    00:00:01 zsh