XXX 15.1. Job Control Commands¶

Exemple 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] <x9644.html#FTN.AEN9753>`__ See the job id table .

#!/bin/bash
# test.sh

ls -l &
echo "Done."

bash$ ./test.sh
Done.
 [bozo@localhost tes

t-scripts]$ total 1

-rwxr-xr-x

1 boz

o 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 tes

t-scripts]$ total 1

-rwxr-xr-x

1 boz

o 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.

#!/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

#!/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

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 ).

Exemple 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

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