Introduction to Unix

4.13. Job or Process Control

4.13.1. Background Processes

The default behavior of processes started from the command line is to run in the foreground, meaning that they take over the terminal and the shell command prompt is not seen again until the program ends. By putting the & at the end of command line entry places the new process in the background. Then the command is displayed so that additional commands may be entered in the same terminal.

See also

Examples in section 5.11 and 5.12 of the text book.

4.13.2. ps

ps

Report a snapshot of the current processes.

SYNOPSIS

ps [OPTIONS]

Note

The ps command is one that, unfortunately, has different options for the BSD and AT&T System V versions of Unix. The GNU version of ps, which is used on Linux system, supports both set of options. If the options begin with a dash (-), then it is assumed to be a System V option, without the dash, means that that BSD options are being used.

OPTIONS

-e

List all running processes

-u **userid**

List all processes owned by a specified user

-f

Display a full listing of the processes

-H

List processes as a tree showing parent – child process relationships

-a

List all processes associated with a terminal. This should be most, if not all, of the user level processes on the system.

4.13.3. kill

kill

Send a signal to a process

SYNOPSIS

kill [-s signal | -p] pid

kill -l [signal]

We usually think of using the kill command to terminate a process. Process termination occurs when the sent signal is not caught by the process. Processes may be programmed to catch and handle any signal except SIGKILL, 9. When a signal is caught, it interrupts what it is currently doing and executes a function known as the signal handler.

Signal	Value	Comment
SIGHUP	1	Hangup detected on controlling terminal
SIGINT	2	Interrupt from keyboard (Default)
SIGQUIT	3	Quit from keyboard
SIGILL	4	Illegal Instruction
SIGABRT	6	Abort signal from abort(3)
SIGFPE	8	Floating point exception
SIGKILL	9	Kill signal
SIGSEGV	11	Invalid memory reference
SIGPIPE	13	Broken pipe: write to pipe with no readers
SIGALRM	14	Timer signal from alarm(2)
SIGTERM	15	Termination signal
SIGUSR1	30,10,16	User-defined signal 1
SIGUSR2	31,12,17	User-defined signal 2
SIGCHLD	20,17,18	Child stopped or terminated
SIGCONT	19,18,25	Continue if stopped
SIGSTOP	17,19,23	Stop process
SIGTSTP	18,20,24	Stop typed at tty
SIGTTIN	21,21,26	tty input for background process

After starting a process, pressing <cntrl>-c on the keyboard sends SIGINT.

After starting a process, pressing <cntrl>-z on the keyboard sends SIGSTOP. Source code debugging programs also often use SIGSTOP.

Daemon processes, such as server programs, are often programmed to handle SIGHUP by re-reading their configuration files. Thus it is common for a system administrator to enact configuration changes by sending the SIGHUP signal rather than restarting the service.

4.13.4. jobs

jobs

List the active jobs from the current shell. Active jobs are current background and stopped processes started from current shell (not processes started from other running shells from the same user). The listed number for each job can be used with the fg and bg commands.

4.13.5. fg

fg

Move a background or stopped process to the foreground. Use the % key along with job number from the jobs command to identify the target job.

4.13.6. bg

bg

Restart a stopped process as background process. Use the % key along with job number from the jobs command to identify the target job.

In the following example, we use sleep as a convenient command to illustrate control of jobs. The first sleep process was started in the foreground, then stopped and restarted as a background process. The second sleep process was started in the background and then moved to the foreground.

1008 timber:~
$ sleep 120
^Z
[1]+  Stopped                 sleep 120
1009 timber:~
$ sleep 200 &
[2] 4110
1010 timber:~
$ jobs
[1]+  Stopped                 sleep 120
[2]-  Running                 sleep 200 &
1011 timber:~
$ bg %1
[1]+ sleep 120 &
1012 timber:~
$ jobs
[1]-  Running                 sleep 120 &
[2]+  Running                 sleep 200 &
1013 timber:~
$ fg %2
sleep 200

4.13.7. nohup

nohup

Prevent termination of the process when the user logs out.

SYNOPSIS

nohup command [ command options and arguments ] &

Normally, background processes are terminated when the user logs out. nohup prevents this from happening.