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This article grounds you in the basic Linux techniques for process management. Learn to:
- Manage foreground and background jobs
- Start processes that will run after you log out
- Monitor processes
- Select and sort processes for display
- Send signals to processes
This article helps you prepare for Objective 103.5 in Topic 103 of the Linux Professional Institute's Junior Level Administration (LPIC-1) exam 101. The objective has a weight of 4.
To get the most from the articles in this series, you should have a basic knowledge of Linux and a working Linux system on which you can practice the commands covered in this article. Sometimes different versions of a program will format output differently, so your results may not always look exactly like the listings and figures shown here. The results in the examples shown here were obtained on an Ubuntu 9.10 (Karmic Koala) distribution.
Manage foreground and background jobs
If you stop and reflect for a moment, it's pretty obvious that lots of things are running on your computer besides the terminal programs we've been discussing in earlier articles in this series. Indeed, if you are using a graphical desktop, you may have opened more than one terminal window at a time, or perhaps opened a file browser, Internet browser, game, spreadsheet, or other application. Previously our examples have shown commands entered at a terminal window. The command runs and you wait for it to complete before you do anything else. In this article, you will learn how to do more than one thing at a time using your terminal window.
When you run a command in your terminal window, you are running it in the foreground. Most such commands run quickly, but suppose you are running a graphical desktop and would like a digital clock displayed on the desktop. For now, let's ignore the fact that most graphical desktops already have one; we're just using this as an example.
If you have the X Window System installed, you probably also have some
utilities such as xclock or
xeyes. You'll probably find these in a package
named xorg-x11-apps or x11-apps if you don't have them installed already.
Either works for this exercise, but we'll use xclock. The man page
explains that you can launch a digital clock on your graphical desktop
using the command:
xclock -d -update 1
The -update 1 part requests updates
every second; otherwise, the clock updates only every minute. So let's run
this in a terminal window. You should see a clock like Figure 1, and your
terminal window should look like Listing 1. If you don't have xclock or
the X Window System, you'll see shortly how to create a poor man's
digital clock with your terminal, so you might want to follow along for
now and then retry these exercises with that clock.
Note: At the time of writing, there is a bug that affects xclock when desktop effects are enabled. The most noticeable effect is that the title bar does not change, even when given focus. If your xclock examples don't look like the ones in this article, you may want to try switching off desktop effects for a while.
Figure 1. A digital clock with xclock
Listing 1. Starting xclock
ian@attic4:~$ xclock -d -update 1
|
Unfortunately, your terminal window no longer has a prompt, so you really need to get control back. Fortunately, the Bash shell has a suspend key, Ctrl-z. Pressing this key combination gets you a terminal prompt again as shown in Listing 2.
Listing 2. Suspending xclock with Ctrl-z
ian@attic4:~$ xclock -d -update 1
^Z
[1]+ Stopped xclock -d -update 1
|
The clock is still on your desktop, but it has stopped running. Suspending
it did exactly that. In fact, if you drag another window over part of it,
that part of the clock won't even redraw. Notice the terminal output
message indicating "[1]+ Stopped". The 1 in
this message is a job number. You can restart the clock by typing
fg %1. You could also use the
command name or part of it by typing
fg %xclock or
fg %?clo. Finally, if you just type
fg with no parameters, you can restart the most
recently stopped job, job 1 in this case. Restarting it with
fg also brings the job right back to the
foreground, and you no longer have a shell prompt. What you need to do is
place the job in the background; a bg
command takes the same type of job specification as the
fg command and does exactly that.
Listing 3
shows how to bring the xclock job back to the foreground and suspend it
using two forms of the fg command.. You can
suspend it again and place it in the background; the clock continues to
run while you do other work at your terminal.
Listing 3. Placing xclock in the foreground or background
ian@attic4:~$ fg %1
xclock -d -update 1
^Z
[1]+ Stopped xclock -d -update 1
ian@attic4:~$ fg %?clo
xclock -d -update 1
^Z
[1]+ Stopped xclock -d -update 1
ian@attic4:~$ bg
[1]+ xclock -d -update 1 &
|
You may have noticed that when you placed the xclock job in the background, the message no longer said "Stopped" and that it was terminated with an ampersand (&). In fact, you don't need to suspend the process to place it in the background at all; just append an ampersand to the command and the shell will start the command (or command list) in the background. Let's start an analog clock with a wheat background using this method. You should see a clock like that in Figure 2 and terminal output like Listing 4.
Figure 2. An analog clock with xclock
Listing 4. Starting analog xclock in background with &
ian@attic4:~$ xclock -bg wheat -update 1&
[2] 4320
|
Notice that the message is slightly different this time. It represents a
job number and a process id (PID). We will cover PIDs and more about
status in a moment. For now, let's use the jobs
command to find out what jobs are running. Add the
-l option to list PIDs, and you see that job 2
indeed has PID 4320 as shown in Listing 5. Note also that job 2 has a plus
sign (+) beside the job number, indicating that it is the current
job. This job will come to the foreground if no job specification
is given with the fg command.
Listing 5. Displaying job and process information
ian@attic4:~$ jobs -l [1]- 3878 Running xclock -d -update 1 & [2]+ 4320 Running xclock -bg wheat -update 1 & |
Before we address some other issues related to background jobs, let's
create a poor man's digital clock. We use the
sleep command to cause a delay for two seconds,
and use the date command to print the
current date and time. We wrap these commands in a
while loop with a
do/done block to create an infinite loop.
Finally, we put the whole lot in parentheses to make a command list and put
the entire list in the background using an ampersand. You will learn more
about how to build more complex commands using loops and scripting in
later articles of this series. See our series roadmap for a
description of and link to each article in the series.
Listing 6. Poor man's digital clock
ian@attic4:~$ (while sleep 2; do date;done)&
[2] 4856
ian@attic4:~$ Tue Jan 19 09:23:30 EST 2010
Tue Jan 19 09:23:32 EST 2010
Tue Jan 19 09:23:34 EST 2010
fTue Jan 19 09:23:36 EST 2010
Tue Jan 19 09:23:38 EST 2010
gTue Jan 19 09:23:40 EST 2010
( while sleep 2; do
date;
done )
Tue Jan 19 09:23:42 EST 2010
Tue Jan 19 09:23:44 EST 2010
Tue Jan 19 09:23:46 EST 2010
^C
|
Our list is running as job 2 with PID 4856. Every two seconds, the date
command runs, and a date and time are printed on the terminal. The input
that you type is highlighted. A slow typist will have characters
interspersed with several lines of output before a full command can be
typed. In fact, notice how the 'f' 'g' that you type in to bring the
command list to foreground are a couple of lines apart. When you finally
get the fg command entered, bash displays the
command that is now running in your shell, namely, the command list, which
is still happily printing the time every two seconds.
Once you succeed in getting the job into the foreground, you can either terminate (or kill) it, or take some other action, In this case, let's use Ctrl-c to terminate our 'clock.'
You may wonder why this job is job 2. With the analog clock terminated, there was only one job running, which was job number 1. So the next available job number was assigned, and our poor man's clock became job 2.
Standard IO and background processes
The output from the date command in our previous
example is interspersed with echoed characters for the
fg command that we are trying to type. This
raises an interesting issue. What happens to a background process if it
needs input from stdin?
The terminal process under which we start a background application is called the controlling terminal. Unless redirected elsewhere, the stdout and stderr streams from the background process are directed to the controlling terminal. Similarly, the background task expects input from the controlling terminal, but the controlling terminal has no way of directing any characters you type to the stdin of a background process. In such a case, the Bash shell suspends the process, so that it is no longer executing. You may bring it to the foreground and supply the necessary input. Listing 7 illustrates a simple case where you can put a command list in the background. After a moment, press Enter and see the message that the process has stopped. Bring it to the foreground and provide a line of input followed by Ctrl-d to signal end of input file. The command list completes and you display the file we created.
Listing 7. Waiting for stdin
ian@attic4:~$ (date; cat - > bginput.txt;date)& [2] 5070 ian@attic4:~$ Tue Jan 19 10:33:13 EST 2010 [2]+ Stopped ( date; cat - > bginput.txt; date ) ian@attic4:~$ ian@attic4:~$ fg ( date; cat - > bginput.txt; date ) some text more text Tue Jan 19 10:33:31 EST 2010 ian@attic4:~$ cat bginput.txt some text more text |
In practice, you probably want to have standard IO streams for background processes redirected to or from a file. There is another related question: what happens to the process if the controlling terminal closes or the user logs off? The answer depends on the shell in use. If the shell sends a SIGHUP (or hangup) signal, then the application is likely to close. We cover signals shortly, but for now we'll consider another way around this problem.
The nohup command is used to start a command
that will ignore hangup signals and will append stdout and stderr to a
file. The default file is either nohup.out or $HOME/nohup.out. If the file
cannot be written, then the command will not run. If you want output to go
somewhere else, redirect stdout, or stderr as discussed in the article "Learn Linux 101: Streams, pipes
and redirects."
The nohup command will not execute a pipeline or
a command list. You can save a pipeline or list in a file and then run it
using the sh (default shell) or the
bash command. Another article in this series
will show you how to make the script file executable, but for now we'll
stick to running scripts by using the sh or the
bash command. Listing 8 shows how we might do
this for our poor man's digital clock. Needless to say, having the time
written to a file isn't particularly useful, and the file will keep
growing, so we'll set the clock to update every 30 seconds instead of
every second.
Listing 8. Using nohup with a command list in a script
ian@attic4:~$ echo "while sleep 30; do date;done">pmc.sh ian@attic4:~$ nohup sh pmc.sh& [2] 5485 ian@attic4:~$ nohup: ignoring input and appending output to `nohup.out' ian@attic4:~$ nohup bash pmc.sh& [3] 5487 ian@attic4:~$ nohup: ignoring input and appending output to `nohup.out' |
If we display the contents of nohup.out, we see lines, with each line approximately 30 seconds after the one that is two lines above it, as shown in Listing 9.
Listing 9. Output from nohup processes
ian@attic4:~$cat nohup.out Tue Jan 19 15:01:12 EST 2010 Tue Jan 19 15:01:26 EST 2010 Tue Jan 19 15:01:44 EST 2010 Tue Jan 19 15:01:58 EST 2010 Tue Jan 19 15:02:14 EST 2010 Tue Jan 19 15:02:28 EST 2010 Tue Jan 19 15:02:44 EST 2010 Tue Jan 19 15:02:58 EST 2010 |
Older versions of nohup did not write a status message to the controlling
terminal, so if you made a mistake, you might not immediately know. You
can see the old behavior if you redirect both stdout and stderr to a file
of your own choosing. Suppose you decided that it would be easier to
source the command using . rather than typing
sh or bash. Listing
10 shows what happens if you use nohup as we did before, but redirect both
stdout and stderr. After you enter the command, you see the message
indicating that job 4 has started with PID 5853. But press
Enter again, and you see another message saying that
the job has terminated with exit code 126.
Listing 10. Making mistakes with nohup
ian@attic4:~$ nohup . pmc.sh >mynohup.out 2>&1 & [4] 5853 ian@attic4:~$ [4]+ Exit 126 nohup . pmc.sh > mynohup.out 2>&1 |
Listing 11 shows the contents of mynohup.out. Not surprising, really. You
use nohup to run a command in the background, and you use
source (.) to run read commands from
a file and run them in the current shell. The important thing to remember
about this is that you may have to press Enter to allow
the shell to display the background job exit status, and you may have to
look at nohup's output file to see what really went
wrong.
Listing 11. Hidden message from
nohup
ian@attic4:~$ cat mynohup.out nohup: ignoring input nohup: cannot run command `.': Permission denied |
Now let's turn our attention to the status of our processes. If you are
following along and planning to take a break at this point, please stay
around as you now have two jobs that are creating ever larger files in
your file system. You can use the fg command to
bring each, in turn, to foreground, and then use Ctrl-c to terminate it,
but if you let them run for a little longer, you'll see other ways to
monitor and interact with them.
Earlier, we had a brief introduction to the jobs
command and saw how to use it to list the Process IDs (or PIDs) of our
jobs.
There is another command, the ps command, which
we use to display various pieces of process status information. Remember
"ps" as an acronym for "process status." The ps
command accepts zero or more PIDs as arguments and displays the associated
process status. If we use the jobs command with
the -p option, the output is simply the PID of
the process group leader for each job. We'll use this output as
arguments to the ps command as shown in Listing
12.
Listing 12. Status of background processes
ian@attic4:~$ jobs -p 3878 5485 5487 ian@attic4:~$ ps $(jobs -p) PID TTY STAT TIME COMMAND 3878 pts/1 S 0:06 xclock -d -update 1 5485 pts/1 S 0:00 sh pmc.sh 5487 pts/1 S 0:00 bash pmc.sh |
If you use ps with no options, you see a list of
processes that have your terminal as their controlling terminal as shown in
Listing 13. Notice that the pmc.sh commands do not show up in this list.
You'll see why in a moment.
Listing 13. Displaying status with ps
ian@attic4:~$ ps PID TTY TIME CMD 2643 pts/1 00:00:00 bash 3878 pts/1 00:00:06 xclock 5485 pts/1 00:00:00 sh 5487 pts/1 00:00:00 bash 6457 pts/1 00:00:00 sleep 6467 pts/1 00:00:00 sleep 6468 pts/1 00:00:00 ps |
Several options, including -f (full),
-j (jobs), and -l
(long) give control of how much information is displayed. If you do not
specify any PIDs, then another useful option is the
--forest option, which displays the commands in
a tree hierarchy, showing which process has which other process as a
parent. In particular, you see that the sleep
commands of the previous listing are children of the scripts you have
running in background. If you happened to run the command at a different
instant, you might see the date command listed
in the process status instead, but the odds are very small with this
script. We illustrate some of these options
in Listing 14.
Listing 14. More status information
ian@attic4:~$ ps -f UID PID PPID C STIME TTY TIME CMD ian 2643 2093 0 Jan18 pts/1 00:00:00 bash ian 3878 2643 0 09:17 pts/1 00:00:06 xclock -d -update 1 ian 5485 2643 0 15:00 pts/1 00:00:00 sh pmc.sh ian 5487 2643 0 15:01 pts/1 00:00:00 bash pmc.sh ian 6635 5485 0 15:41 pts/1 00:00:00 sleep 30 ian 6645 5487 0 15:42 pts/1 00:00:00 sleep 30 ian 6647 2643 0 15:42 pts/1 00:00:00 ps -f ian@attic4:~$ ps -j --forest PID PGID SID TTY TIME CMD 2643 2643 2643 pts/1 00:00:00 bash 3878 3878 2643 pts/1 00:00:06 \_ xclock 5485 5485 2643 pts/1 00:00:00 \_ sh 6657 5485 2643 pts/1 00:00:00 | \_ sleep 5487 5487 2643 pts/1 00:00:00 \_ bash 6651 5487 2643 pts/1 00:00:00 | \_ sleep 6658 6658 2643 pts/1 00:00:00 \_ ps |
Now that you have some basic tools for monitoring your processes using the
jobs and ps
commands, let's take a brief look at two other monitoring commands before
moving on to other ways to select and sort processes for display.
The free command displays the amount of free and
used memory in your system. By default the display is in kilobytes, but
you can override this using -b for bytes,
-k for kilobytes, -m
for megabytes, or -g for gigabytes. The
-t option displays a total line, and the
-s option along with a value refreshes the
info with the frequency specified. The number is in seconds but may be a
floating point value. Listing 15 shows two examples.
Listing 15. Using the free command
ian@attic4:~$ free
total used free shared buffers cached
Mem: 4057976 1543164 2514812 0 198592 613488
-/+ buffers/cache: 731084 3326892
Swap: 10241428 0 10241428
ian@attic4:~$ free -mt
total used free shared buffers cached
Mem: 3962 1506 2456 0 193 599
-/+ buffers/cache: 713 3249
Swap: 10001 0 10001
Total: 13964 1506 12457 |
The uptime command shows you a one-line display
that includes the current time, how long the system has been running, how
many users are currently logged on, and the system load averages for the
past 1, 5, and 15 minutes. Listing 16 shows an example.
Listing 16. Showing uptime information
ian@attic4:~$ uptime 17:41:17 up 20:03, 5 users, load average: 0.00, 0.00, 0.00 |
Select and sort processes for display
The ps commands discussed so far only list
processes that were started from your terminal session (note the SID, or
session id, column in the second example of Listing 14).
To see all the
processes with controlling terminals, use the -a
option. The -x option displays processes
without a controlling terminal, and the -e
option displays information for every process. Listing 17 shows
the full format for all the processes with a controlling terminal.
Listing 17. Displaying other processes
ian@attic4:~$ ps -af
UID PID PPID C STIME TTY TIME CMD
ian 3878 2643 0 09:17 pts/1 00:00:06 xclock -d -update 1
ian 5485 2643 0 15:00 pts/1 00:00:00 sh pmc.sh
ian 5487 2643 0 15:01 pts/1 00:00:00 bash pmc.sh
ian 7192 5485 0 16:00 pts/1 00:00:00 sleep 30
ian 7201 5487 0 16:00 pts/1 00:00:00 sleep 30
ian 7202 2095 0 16:00 pts/0 00:00:00 ps -af
|
Note the controlling terminal listed in the TTY column. For this listing, I
switched to the terminal window I opened originally (pts/0), so the
ps -af command is
running under pts/0, while the commands created for this
article are running under
pts/1.
There are many more options for ps, including a
number that provide significant control over what fields are displayed
and how they are displayed. Others provide control over the selection of
processes for display, for example, by selecting those processes for a
particular user (-u) or a particular command
(-C). In Listing 18, all processes
running the getty command are listed; we use the
-o option to specify the columns that will be
displayed. We've added the user option to the
normal list that you get with just plain ps, so
you can see which user runs
getty.
Listing 18. Who is running the getty
command?
ian@attic4:~$ ps -C getty -o user,pid,tty,time,comm USER PID TT TIME COMMAND root 1192 tty4 00:00:00 getty root 1196 tty5 00:00:00 getty root 1209 tty2 00:00:00 getty root 1219 tty3 00:00:00 getty root 1229 tty6 00:00:00 getty root 1731 tty1 00:00:00 getty |
Sometimes you will want to sort the output by particular fields, and you
can do that too using the --sort option to
specify the sort fields. The default is to sort in ascending order
(+), but you can also specify descending order
(-). Listing 19 shows the final
ps example where all processes are listed using
jobs format, and the output is sorted by session id and command name. For the
first, we use the default sort order, and for the second, we specify both
sorts orders explicitly.
Listing 19. Sorting the output from ps
ian@attic4:~$ ps -aj --sort -sid,+comm PID PGID SID TTY TIME CMD 5487 5487 2643 pts/1 00:00:00 bash 9434 9434 2643 pts/1 00:00:00 ps 5485 5485 2643 pts/1 00:00:00 sh 9430 5485 2643 pts/1 00:00:00 sleep 9433 5487 2643 pts/1 00:00:00 sleep 3878 3878 2643 pts/1 00:00:10 xclock 8019 8019 2095 pts/0 00:00:00 man 8033 8019 2095 pts/0 00:00:00 pager ian@attic4:~$ ps -aj --sort sid,comm PID PGID SID TTY TIME CMD 8019 8019 2095 pts/0 00:00:00 man 8033 8019 2095 pts/0 00:00:00 pager 5487 5487 2643 pts/1 00:00:00 bash 9435 9435 2643 pts/1 00:00:00 ps 5485 5485 2643 pts/1 00:00:00 sh 9430 5485 2643 pts/1 00:00:00 sleep 9433 5487 2643 pts/1 00:00:00 sleep 3878 3878 2643 pts/1 00:00:10 xclock |
As usual, see the man pages for ps for full
details on the many options and fields you may specify, or get a brief
summary by using ps --help.
If you run ps several times in a row to see
what is changing, you probably need the top
command instead. It displays a continuously updated process list, along
with useful summary information. Listing 20 shows the first few lines of a
top display. Use the q
subcommand to quit top.
Listing 20. Displaying processes using top
top - 16:07:22 up 18:29, 5 users, load average: 0.03, 0.02, 0.00
Tasks: 170 total, 1 running, 169 sleeping, 0 stopped, 0 zombie
Cpu(s): 2.1%us, 0.5%sy, 0.0%ni, 97.4%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 4057976k total, 1543616k used, 2514360k free, 194648k buffers
Swap: 10241428k total, 0k used, 10241428k free, 613000k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
6820 ian 20 0 506m 78m 26m S 1 2.0 0:23.97 firefox
1381 root 20 0 634m 40m 18m S 1 1.0 2:06.74 Xorg
2093 ian 20 0 212m 15m 10m S 1 0.4 0:13.53 gnome-terminal
6925 ian 20 0 1118m 298m 19m S 1 7.5 1:07.04 java
6855 ian 20 0 73416 11m 8808 S 1 0.3 0:05.01 npviewer.bin
7351 ian 20 0 19132 1364 980 R 0 0.0 0:00.07 top
1 root 20 0 19584 1888 1196 S 0 0.0 0:00.74 init
2 root 15 -5 0 0 0 S 0 0.0 0:00.01 kthreadd
|
The top command has a number of subcommands, of
which the most useful to start with are:
- h
- gets you help
- q
- quits the
topcommand
- f
- lets you add or remove fields from the display
- o
- orders the display order
- F
- selects fields to sort on
See the man pages for top for full details on
options, including how to sort by memory usage or other criteria. Listing
21 shows an example of the output sorted by virtual memory usage in
descending order.
Listing 21. Sorting the output of top
top - 16:21:48 up 18:43, 5 users, load average: 0.16, 0.06, 0.01 Tasks: 170 total, 3 running, 167 sleeping, 0 stopped, 0 zombie Cpu(s): 2.1%us, 0.8%sy, 0.0%ni, 96.6%id, 0.0%wa, 0.0%hi, 0.5%si, 0.0%st Mem: 4057976k total, 1588940k used, 2469036k free, 195412k buffers Swap: 10241428k total, 0k used, 10241428k free, 613056k cached PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 6925 ian 20 0 1171m 338m 21m S 0 8.5 1:44.10 java 1381 root 20 0 634m 40m 18m S 0 1.0 2:13.63 Xorg 6820 ian 20 0 506m 83m 26m S 3 2.1 0:51.28 firefox 2004 ian 20 0 436m 23m 15m S 0 0.6 0:01.55 nautilus 2031 ian 20 0 419m 13m 10m S 0 0.3 0:00.11 evolution-alarm 2118 ian 20 0 372m 10m 7856 S 0 0.3 0:00.06 evolution-data- 2122 ian 20 0 344m 13m 10m S 0 0.3 0:00.10 evolution-excha 2001 ian 20 0 331m 22m 14m S 0 0.6 0:13.61 gnome-panel 1971 ian 20 0 299m 9.9m 7244 S 0 0.3 0:05.00 gnome-settings- 1989 ian 20 0 288m 15m 11m S 0 0.4 0:11.95 metacity 1954 ian 20 0 265m 5460 3412 S 0 0.1 0:00.28 pulseaudio |
Let's now look at Linux signals, which are an asynchronous way to
communicate with processes. We have already mentioned the SIGHUP signal,
and we have used both Ctrl-c and Ctrl-z, which are other ways of sending
a signal to processes. The general way to send a signal is with the
kill command.
The kill command sends a signal to a specified
job or process. Listing 22 shows the use of the SIGTSTP and SIGCONT
signals to stop and resume a background job. Using the SIGTSTP signal is
equivalent to using the fg command to bring the
job to the foreground and then Ctrl-z to suspend it. Using SIGCONT is like
using the bg command.
Listing 22. Stopping and restarting background jobs
ian@attic4:~$ kill -s SIGTSTP %1
[1]+ Stopped xclock -d -update 1
ian@attic4:~$ jobs -l
[1]+ 3878 Stopped xclock -d -update 1
[2] 5485 Running nohup sh pmc.sh &
[3]- 5487 Running nohup bash pmc.sh &
ian@attic4:~$ kill -s SIGCONT 3878
ian@attic4:~$ jobs -l
[1] 3878 Running xclock -d -update 1 &
[2]- 5485 Running nohup sh pmc.sh &
[3]+ 5487 Running nohup bash pmc.sh &
|
We used the job specification (%1) to stop the xclock process in this
example, and then the process id (PID) to restart (continue) it. If you
stopped job %2 and then used tail with the
-f option to follow it, you would see that only
one process is updating the nohup.out file.
There are a number of other possible signals that you can display on your
system using kill -l. Some are
used to report errors such as illegal operation codes, floating point
exceptions, or attempts to access memory that a process does not have
access to. Notice that signals have both a number, such as 20, and a name,
such as SIGTSTP. You may use either the number prefixed by a - sign, or
the -s option and the signal name. On my system
I could have used kill -20 instead
of
kill -s SIGTSTP.
You
should always check the signal numbers on your system before assuming
which number belongs to which signal.
Signal handlers and process termination
You have seen that Ctrl-c terminates a process. In fact, it sends a SIGINT
(or interrupt) signal to the process. If you use kill
without any signal
name, it sends a SIGTERM signal. For most purposes, these two signals are
equivalent.
You have seen that the nohup command
makes a process
immune to the SIGHUP signal. In general, a process can implement a
signal handler to catch signals. So a process could
implement a signal handler to catch either SIGINT or SIGTERM. Since the
signal handler knows what signal was sent, it may choose to ignore SIGINT
and only terminate when it receives SIGTERM, for example. Listing 23 shows
how to send the SIGTERM signal to job %2. Notice that the process status
shows as "Terminated" right after we send the signal. This would show as
"Interrupt" if we used SIGINT instead. After a few moments, the process
cleanup has occurred and the job no longer shows in the job list.
Listing 23. Terminating a process with SIGTERM
ian@attic4:~$ kill -s SIGTERM %2 ian@attic4:~$ [2]- Terminated nohup sh pmc.sh ian@attic4:~$ jobs -l [1]- 3878 Running xclock -d -update 1 & [3]+ 5487 Running nohup bash pmc.sh & |
Signal handlers give a process great flexibility. A process can do its normal work and be interrupted by a signal for some special purpose. Besides allowing a process to catch termination requests and take possible action such as closing files or checkpointing transactions in progress, signals are often used to tell a daemon process to reread its configuration file and possibly restart operation. You might do this for the inetd process when you change network parameters, or the line printer daemon (lpd) when you add a new printer.
Terminating processes unconditionally
Some signals cannot be caught, such as some hardware exceptions. SIGKILL, the most likely one you will use, cannot be caught by a signal handler and unconditionally terminates a process. In general, you should need this only if all other means of terminating the process have failed.
Remember you saw that using nohup would
allow your processes to keep running after you log out. Well, let's do that
and then log back in again. After you log back in, check your
remaining poor man's clock process using jobs
and ps as we have done above. The output is
shown in Listing 24.
Listing 24. Logging back in
ian@attic4:~$ jobs -l ian@attic4:~$ ps -a PID TTY TIME CMD 10995 pts/0 00:00:00 ps |
We are running on pts/0 this time, but there is no sign of our
jobs, just the ps command. Not perhaps what we were
expecting. However, all is not lost. Suppose you can't remember
whether you terminated the nohup job that you started with bash or the one
you started with bash.
You saw above how to find the processes that were
running the getty command, so you can use the
same trick to display just the SID, PID, PPID, and command string. Then
you
can use the -js option to display all the
processes in the session. Listing 25 shows the result. Think about other
ways you might have found these processes, such as searching by username
and then filtering using grep.
Listing 25. Finding our lost commands
ian@attic4:~$ ps -C bash -C sh -o pid,sid,tname,cmd
PID SID TTY CMD
5487 2643 ? bash pmc.sh
7050 7050 pts/3 -bash
10851 10851 pts/0 bash
ian@attic4:~$ ps -js 2643
PID PGID SID TTY TIME CMD
5487 5487 2643 ? 00:00:00 bash
11197 5487 2643 ? 00:00:00 sleep
|
Note that the pmc.sh is still running but now it has a question mark (?) for the controlling TTY.
Given what you have now learned about killing processes, you should be able
to kill the remaining poor man's clock process using its PID and the
kill command.
Learn
- Use the developerWorks roadmap for
LPIC-1 to find the developerWorks articles to help you study for
LPIC-1 certification based on the April 2009 objectives.
- At the LPIC
Program site, find detailed objectives, task lists, and sample
questions for the three levels of the Linux Professional Institute's Linux
system administration certification. In particular, see their April 2009
objectives for LPI exam 101 and LPI exam 102. Always refer to the LPIC Program site for the
latest objectives.
- Review the entire LPI exam prep series on
developerWorks to learn Linux fundamentals and prepare for system
administrator certification based on earlier LPI exam objectives prior to
April 2009.
- In "Basic tasks for new Linux
developers" (developerWorks, March 2005), learn how to open a
terminal window or shell prompt and much more.
- The Linux
Documentation Project has a variety of useful documents,
especially its HOWTOs.
- Read more of Ian's articles on
developerWorks, and connect with him through his profile in My developerWorks.
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In the
developerWorks Linux zone,
find more resources for Linux developers, and scan our
most popular articles and
tutorials.
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and
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Connect with other developerWorks users while exploring the
developer-driven blogs, forums, groups, and wikis.
Ian Shields works on a multitude of Linux projects for the developerWorks Linux zone. He is a Senior Programmer at IBM at the Research Triangle Park, NC. He joined IBM in Canberra, Australia, as a Systems Engineer in 1973, and has since worked on communications systems and pervasive computing in Montreal, Canada, and RTP, NC. He has several patents and has published several papers. His undergraduate degree is in pure mathematics and philosophy from the Australian National University. He has an M.S. and Ph.D. in computer science from North Carolina State University. Learn more about Ian in in Ian's profile on My developerWorks.
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