15.1. Job Control Commands

Certain of the following job control commands take a job identifier as an argument. See the table at end of the chapter.

jobs

Lists the jobs running in the background, giving the job number. Not as useful as ps.

   It is all too easy to confuse jobs and processes. Certain  builtins , such as kill, d isown, and wait accept either a job number or a process number as an argument. The  fg , bg and  jobs commands accept only a job number.

bash$ sleep 100 &
[1] 1384

bash $ jobs
[1]+  Running                 sleep 100 &

"1" is the job number (jobs are maintained by the current shell). "1384" is the PID or process ID number (processes are maintained by the system). To kill this job/process, either a  kill %1  or a kill 1384  works.

Thanks, S.C.

disown

Remove job(s) from the shell's table of active jobs.

fg, bg

The fg command switches a job running in the background into the foreground. The bg command restarts a suspended job, and runs it in the background. If no job number is specified, then the fg or bg command acts upon the currently running job.

wait

Suspend script execution until all jobs running in background have terminated, or until the job number or process ID specified as an option terminates. Returns the exit status of waited-for command.

You may use the wait command to prevent a script from exiting before a background job finishes executing (this would create a dreaded orphan process).

Example 15-26. Waiting for a process to finish before proceeding

#!/bin/bash

ROOT_UID=0   # Only users with $UID 0 have root privileges.
E_NOTROOT=65
E_NOPARAMS=66

if [ "$UID" -ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
# "Run along kid, it's past your bedtime."
exit $E_NOTROOT
fi  

if [ -z "$1" ]
then
echo "Usage: `basename $0` find-string"
exit $E_NOPARAMS
fi


echo "Updating 'locate' database..."
echo "This may take a while."
updatedb /usr &     # Must be run as root.

wait
# Don't run the rest of the script until 'updatedb' finished.
# You want the the database updated before looking up the file name.

locate $1

#  Without the 'wait' command, in the worse case scenario,
#+ the script would exit while 'updatedb' was still running,
#+ leaving it as an orphan process.

exit 0

Optionally, wait can take a job identifier as an argument, for example, wait%1 or wait $PPID. ¹ See the job id table.

    Within a script, running a command in the background with an ampersand (&) may cause the script to hang until ENTER is hit. This seems to occur with commands that write to stdout. It can be a major annoyance.

#!/bin/bash
# test.sh          

ls -l &
echo "Done."

bash$ ./test.sh
Done.
 [bozo@localhost test-scripts]$ total 1
 -rwxr-xr-x    1 bozo     bozo           34 Oct 11 15:09 test.sh
 _

 As Walter Brameld IV explains it:

    As far as I can tell, such scripts don't actually hang. It just
    seems that they do because the background command writes text to
    the console after the prompt. The user gets the impression that
    the prompt was never displayed. Here's the sequence of events:

    1. Script launches background command.
    2. Script exits.
    3. Shell displays the prompt.
    4. Background command continues running and writing text to the
       console.
    5. Background command finishes.
    6. User doesn't see a prompt at the bottom of the output, thinks script
       is hanging.

Placing a wait after the background command seems to remedy this.

#!/bin/bash
# test.sh          

ls -l &
echo "Done."
wait

bash$ ./test.sh
Done.
 [bozo@localhost test-scripts]$ total 1
 -rwxr-xr-x    1 bozo     bozo           34 Oct 11 15:09 test.sh 

Redirecting the output of the command to a file or even to /dev/null also takes care of this problem.           

suspend

This has a similar effect to Control-Z, but it suspends the shell (the shell's parent process should resume it at an appropriate time).

logout

Exit a login shell, optionally specifying an exit status.

times

Gives statistics on the system time elapsed when executing commands, in the following form:

0m0.020s 0m0.020s

This capability is of relatively limited value, since it is not common to profile and benchmark shell scripts.

kill

Forcibly terminate a process by sending it an appropriate terminate signal (see Example 17-6).

Example 15-27. A script that kills itself

#!/bin/bash
# self-destruct.sh

kill $$  # Script kills its own process here.
     # Recall that "$$" is the script's PID.

echo "This line will not echo."
# Instead, the shell sends a "Terminated" message to stdout.

exit 0   # Normal exit? No!

#  After this script terminates prematurely,
#+ what exit status does it return?
#
# sh self-destruct.sh
# echo $?
# 143
#
# 143 = 128 + 15
#             TERM signal

  kill -l lists all the signals (as does the file /usr/include/asm/signal.h). A kill -9 is a sure kill, which will usually terminate a process that stubbornly refuses to die with a plain kill. Sometimes, a kill -15 works. A zombie process, that is, a child process that has terminated, but that the parent process has not (yet) killed, cannot be killed by a logged-on user -- you can't kill something that is already dead -- but init will generally clean it up sooner or later.

killall

The killall command kills a running process by name, rather than by process ID. If there are multiple instances of a particular command running, then doing a killall on that command will terminate them all.

This refers to the killall command in /usr/bin, not the killall script in /etc/rc.d/init.d.

command

The command directive disables aliases and functions for the command immediately following it.

bash$ command ls

This is one of three shell directives that effect script command processing. The others are builtin   and enable. 

builtin

Invoking builtin BUILTIN_COMMAND runs the command BUILTIN_COMMAND as a shell builtin, temporarily disabling both functions and external system commands with the same name.

enable

This either enables or disables a shell builtin command. As an example, enable -n kill disables the shell builtin kill, so that when Bash subsequently encounters kill, it invokes the external command /bin/kill.

The -a option to enable lists all the shell builtins, indicating whether or not they are enabled. The -f filename option lets enable load a builtin as a shared library (DLL) module from a properly compiled object file. ².

autoload

This is a port to Bash of the ksh autoloader. With autoload in place, a function with an autoload declaration will load from an external file at its first invocation. ³ This saves system resources.

Note that autoload is not a part of the core Bash installation. It needs to be loaded in with enable -f (see above).

Table 15-1. Job identifiers

Notation	Meaning
%N	Job number [N]
%S	Invocation (command-line) of job begins with string S
%?S	Invocation (command-line) of job contains within it string S
%%	"current" job (last job stopped in foreground or started in background)
%+	"current" job (last job stopped in foreground or started in background)
%-	Last job
$!	Last background process

Notes

1. ^{^} This only applies to child processes, of course.
2. ^{^} The C source for a number of loadable builtins is typically found in the /usr/share/doc/bash-?.??/functions directory.
      Note that the -f option to enable is not portable to all systems.
3. ^ The same effect as autoload can be achieved with typeset -fu.