background jobs
(Written by Paul Cobbaut, https://github.com/paulcobbaut/, with contributions by: Alex M. Schapelle, https://github.com/zero-pytagoras/)
background processes
jobs
Stuff that runs in background of your current shell can be displayed
with the jobs command. By default you will not have any
jobs running in background.
root@linux ~# jobs
root@linux ~#
This jobs command will be used several times in this section.
control-Z
Some processes can be suspended with the Ctrl-Z key
combination. This sends a SIGSTOP signal to the Linux kernel,
effectively freezing the operation of the process.
When doing this in vi(m), then vi(m) goes to the background. The
background vi(m) can be seen with the jobs command.
[student@linux ~]$ vi procdemo.txt
[5]+ Stopped vim procdemo.txt
[student@linux ~]$ jobs
[5]+ Stopped vim procdemo.txt
& ampersand
Processes that are started in background using the & character at the
end of the command line are also visible with the jobs command.
[student@linux ~]$ find / > allfiles.txt 2> /dev/null &
[6] 5230
[student@linux ~]$ jobs
[5]+ Stopped vim procdemo.txt
[6]- Running find / >allfiles.txt 2>/dev/null &
[student@linux ~]$
jobs -p
An interesting option is jobs -p to see the process id of background
processes.
[student@linux ~]$ sleep 500 &
[1] 4902
[student@linux ~]$ sleep 400 &
[2] 4903
[student@linux ~]$ jobs -p
4902
4903
[student@linux ~]$ ps `jobs -p`
PID TTY STAT TIME COMMAND
4902 pts/0 S 0:00 sleep 500
4903 pts/0 S 0:00 sleep 400
[student@linux ~]$
fg
Running the fg command will bring a background job to
the foreground. The number of the background job to bring forward is the
parameter of fg.
[student@linux ~]$ jobs
[1] Running sleep 1000 &
[2]- Running sleep 1000 &
[3]+ Running sleep 2000 &
[student@linux ~]$ fg 3
sleep 2000
bg
Jobs that are suspended in background can be started in background
with bg. The bg will send a SIGCONT signal.
Below an example of the sleep command (suspended with Ctrl-Z) being
reactivated in background with bg.
[student@linux ~]$ jobs
[student@linux ~]$ sleep 5000 &
[1] 6702
[student@linux ~]$ sleep 3000
[2]+ Stopped sleep 3000
[student@linux ~]$ jobs
[1]- Running sleep 5000 &
[2]+ Stopped sleep 3000
[student@linux ~]$ bg 2
[2]+ sleep 3000 &
[student@linux ~]$ jobs
[1]- Running sleep 5000 &
[2]+ Running sleep 3000 &
[student@linux ~]$
practice : background processes
1. Use the jobs command to verify whether you have any processes
running in background.
2. Use vi to create a little text file. Suspend vi in background.
3. Verify with jobs that vi is suspended in background.
4. Start find / > allfiles.txt 2>/dev/null in foreground. Suspend it
in background before it finishes.
5. Start two long sleep processes in background.
6. Display all jobs in background.
7. Use the kill command to suspend the last sleep process.
8. Continue the find process in background (make sure it runs again).
9. Put one of the sleep commands back in foreground.
10. (if time permits, a general review question...) Explain in detail where the numbers come from in the next screenshot. When are the variables replaced by their value ? By which shell ?
[student@linux ~]$ echo $$ $PPID
4224 4223
[student@linux ~]$ bash -c "echo $$ $PPID"
4224 4223
[student@linux ~]$ bash -c 'echo $$ $PPID'
5059 4224
[student@linux ~]$ bash -c `echo $$ $PPID`
4223: 4224: command not found
solution : background processes
1. Use the jobs command to verify whether you have any processes
running in background.
jobs (maybe the catfun is still running?)
2. Use vi to create a little text file. Suspend vi in background.
vi text.txt
(inside vi press ctrl-z)
3. Verify with jobs that vi is suspended in background.
[student@linux ~]$ jobs
[1]+ Stopped vim text.txt
4. Start find / > allfiles.txt 2>/dev/null in foreground. Suspend it
in background before it finishes.
[student@linux ~]$ find / > allfiles.txt 2>/dev/null
(press ctrl-z)
[2]+ Stopped find / > allfiles.txt 2> /dev/null
5. Start two long sleep processes in background.
sleep 4000 & ; sleep 5000 &
6. Display all jobs in background.
[student@linux ~]$ jobs
[1]- Stopped vim text.txt
[2]+ Stopped find / > allfiles.txt 2> /dev/null
[3] Running sleep 4000 &
[4] Running sleep 5000 &
7. Use the kill command to suspend the last sleep process.
[student@linux ~]$ kill -SIGSTOP 4519
[student@linux ~]$ jobs
[1] Stopped vim text.txt
[2]- Stopped find / > allfiles.txt 2> /dev/null
[3] Running sleep 4000 &
[4]+ Stopped sleep 5000
8. Continue the find process in background (make sure it runs again).
bg 2 (verify the job-id in your jobs list)
9. Put one of the sleep commands back in foreground.
fg 3 (again verify your job-id)
10. (if time permits, a general review question...) Explain in detail where the numbers come from in the next screenshot. When are the variables replaced by their value ? By which shell ?
[student@linux ~]$ echo $$ $PPID
4224 4223
[student@linux ~]$ bash -c "echo $$ $PPID"
4224 4223
[student@linux ~]$ bash -c 'echo $$ $PPID'
5059 4224
[student@linux ~]$ bash -c `echo $$ $PPID`
4223: 4224: command not found
The current bash shell will replace the $$ and $PPID while scanning the line, and before executing the echo command.
[student@linux ~]$ echo $$ $PPID
4224 4223
The variables are now double quoted, but the current bash shell will
replace $$ and $PPID while scanning the line, and before executing
the bash -c command.
[student@linux ~]$ bash -c "echo $$ $PPID"
4224 4223
The variables are now single quoted. The current bash shell will not
replace the $$ and the $PPID. The bash -c command will be executed
before the variables replaced with their value. This latter bash is the
one replacing the $$ and $PPID with their value.
[student@linux ~]$ bash -c 'echo $$ $PPID'
5059 4224
With backticks the shell will still replace both variable before the embedded echo is executed. The result of this echo is the two process id\'s. These are given as commands to bash -c. But two numbers are not commands!
[student@linux ~]$ bash -c `echo $$ $PPID`
4223: 4224: command not found