14.1. Job Control Commands

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

bash$ sleep 100 &
[1] 1384

bash $ jobs
[1]+  Running                 sleep 100 &

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

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.

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).

#!/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. See the job id table.

#!/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
 _
               

#!/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
               

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).

Exit a login shell, optionally specifying an exit status.

0m0.020s 0m0.020s

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

Forcibly terminate a process by sending it an appropriate terminate signal (see Example 16-6).

#!/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

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.

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

bash$ command ls
              

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.

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. [1].

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. [2] 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).