2004.11_Command Line-Job Control in the Shell.pdf

LINUX USER

Command Line

Line”, we looked at screen , a tool that

gives you multiple virtual consoles in

a single terminal window. If you do not

have screen , and the number of consoles

you can launch is limited, it makes sense

to know how you can launch a program

as a background task, interrupt a

program, send a program into the back-

ground, or restore a program to the

foreground. Read on to discover how you

can keep control over your jobs.

Nice Job

The right commands make child’s play of job control in the shell, allowing you

to launch commands in the background, interrupt processes, monitor multiple

jobs, and restore specific jobs to the foreground. BY HEIKE JURZIK

Into the Background

Let’s imagine a situation where a com-

mand you have launched is taking a

while to complete. You sit there twid-

dling your thumbs while the terminal

waits for the program to complete. And

you cannot enter any other commands

while you are waiting.

It makes sense to move the process to

the background when you are launching

it. To do this, just add an ampersand (&)

to the command, for example:

prompt. To send the task into the back-

ground type bg (for “ b ack g round”):

[3]- Done

sleep 5

$ bg

[1]+ find . -name bla

> /tmp/list &

Shortcuts

Bash surprises even experienced users

with neat shortcuts and variables that

make the administrator’s life so much

easier.

For example, the exclamation mark

can be used as a variable to represent the

process ID of the last background

process to be launched.

The following commands first display

the process ID in addition to the job ID

( jobs -l ), and then run the echo command

to display the process ID for the last

background process to be launched ( $! ):

This output displays both the job ID and

the complete process name, followed by

the ampersand character. And it shows

you that the process is now running in

the background.

If you have more than one stopped job

running in the shell, the bg needs details

on the process you want to run in the

background. By default bg moves the

process with the highest job ID to the

background.

You can again use the jobs command

to check the job IDs. Then add a per-

cent sign and the job ID to the bg

command:

$ find . -name bla > /tmp/list &

[1] 664

This command displays a few messages

about the process you launched and then

immediately restores the prompt. The

message includes the job ID in angled

brackets, followed by the PID ( P rocess

ID entifier). The PID is unique through-

out the system, whereas the job ID is a

consecutive number assigned by the

shell.

The jobs command tells you which

jobs are running in the current shell:

$ sleep 100 &

$ jobs -l

[1]+ 1057 Stopped sleep 1000

[2] 1058 Running sleep 1000 &

[3]- 1066 Running sleep 100 &

$ echo $!

1066

$ jobs

[1]- Stopped find .

-name blub > /tmp/list

[2]+ Stopped find .

-name bla > /tmp/list

$ bg %2

[2]+ find . -name bla

>/tmp/list &

$ jobs

[1]+ Stopped find .

-name bla > /tmp/list

You can put this information to good use

to interrupt the last process. We do not

need the fg command (the command

would need the job ID) and [Ctrl-Z] this

time because we will be using the kill

command instead.

If you forget to add an ampersand when

launching a time-consuming command,

and you have the task running in the

foreground, you may become very impa-

tient and wish to work on something

else. In this situation, the commands to

send this task into the background are

quite simple in the Bash shell.

First, press [Ctrl-Z] to interrupt the

task. The output of the [Ctrl-Z] com-

mand tells you the job ID and the name

of the command you just interrupted

before the shell again displays the

Back to the Foreground

The fg (“ f ore g round”) command restores

a background job to the foreground.

Again, the command needs to know

which job you mean, for example:

Command Line

Although GUIs such as KDE or GNOME are

useful for various tasks, if you intend to get

the most out of your Linux machine, you will

need to revert to the good old command

line from time to time. Apart from that, you

will probably be confronted with various

scenarios where some working knowledge

will be extremely useful in finding your way

through the command line jungle.

fg %2

Restoring a job to the foreground means

tying up the shell while the foreground

process completes. A shell prompt tells

you when this is the case:

November 2004

www.linux-magazine.com

Job control in the shell

I n the previous issue of “Command

Command Line

LINUX USER

kill can do more than just

shoot processes down in

flames; in fact, it can send

them all kinds of signals. For

an overview, type kill -l (or

read the manpage man 7 sig-

nal ). In this case, we will be

using the -STOP signal:

The nice command assigns

processes a specific priority –

this is useful if you have a

program running in the back-

ground and do not want to

risk losing control over the

system load. Non-privileged

users are only permitted to

assign lower priorities to their

own tasks.

Processes are assigned a

nice value of 0 by default,

where -20 is the highest, and

19 the lowest priority. The

nice command is followed by

the priority and the com-

mand, as follows:

Sfmby!boe!fokpz!uif!Xfc!

$ kill -STOP $!

$ jobs -l

[1]- 1057 Stopped

sleep 1000

[2] 1058 Running

sleep 1000 &

[3]+ 1066 Stopped

(signal)

sleep 100 &

Shy Guy

The nohup command gives

processes the ability to con-

tinue running when you quit

the shell. To unhitch the

process from the shell, type

nohup , plus the command

name, and add an ampersand

to send the process to the

background. The following

output

nice -10 find . U

-name bla > /tmp/list

If you then call the ps com-

mand to view the process

status, you will notice that

the find call has been

“niced”:

$ ps auxwww

[...]

huhn 1200 0.2 0.6

1520 404 pts/7 RN+

23:02 0:00 find . -

name bla [...]

$ nohup sleep 1000 &

[1] 1116 nohup:

appending output to

`nohup.out'

The top command gives you

an even better way of discov-

ering a program’s nice level.

The top program lists

processes, and sorts them by

CPU usage. The fourth col-

umn, the one with the NI

heading, tells you the nice

level (Figure 1).

Pqfsb!cspxtfs!.!b!tbgfs!boe!gbtufs!

Joufsofu!fyqfsjfodf/

tells you that the process will

go on running even if you

type exit or press [Ctrl-D] to

quit the shell. Later on, you

can check the nohup.out file

to see what the program did

while you were away.

¦!Qpq.vq!ljmmfs

¦!Joufhsbufe!tfbsdi

■

¦!Ubccfe!cspxtjoh

¦!Tqbn.gjmufsfe!f.nbjm

¦!Bewbodfe!tfdvsjuz

¦!Dsptt.qmbugpsn

Hfu!Pqfsb!gps!Mjovy!bu;!

xxx/pqfsb/dpn0mjovynbh

Figure 1: The “top” command also tells you a program’s “nice” level.

www.linux-magazine.com November 2004

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