linux bash scripting
Linux

Linux bash scripting the awesome guide part5

In the previous post, we talked about input, output, and redirection in bash scripts. Today we will learn how to run and control them on Linux system. Till now, we can run scripts only from the command line interface. This isn’t the only way to run Linux bash scripts.

There are various control methods like sending signals to your script, modifying a script’s priority, and switching the run mode while a script is running. This post describes the different ways to control your Linux bash scripts.

Linux signals

There are more than 30 Linux signals that can be generated by the system and applications.

These are the most common Linux system signals:

Signal    Value                     Description

1              SIGHUP               Hangs up the process

2              SIGINT                 Interrupts the process

3              SIGQUIT              Stops the process

9              SIGKILL               Unconditionally terminates the process

15           SIGTERM             Terminates the process if possible

17           SIGSTOP              Unconditionally stops, but doesn’t terminate, the process

18           SIGTSTP              Stops or pauses the process, but doesn’t terminate

19           SIGCONT             Continues a stopped process

If the bash shell receives a SIGHUP signal, such as when you leave an interactive shell. Before you leave it, it passes the SIGHUP signal to any processes started by the shell, including any running shell scripts.

With a SIGINT signal, the shell is just interrupted. The Linux kernel stops providing the shell processing time on the CPU. When this happens, the shell passes the SIGINT signal to any processes started by the shell to notify them.

Your Linux bash scripts don’t control these signals, you can program your bash script to recognize signals and perform commands based on the signal that was sent.

Generating signals

The bash shell allows you to generate two basic Linux signals using key combinations on the keyboard. This feature comes in handy if you need to stop or pause a running bash script.

Stop a process

The Ctrl+C key combination generates a SIGINT signal and sends it to any processes currently running in the shell which simply stops the current process running in the shell.

$ sleep 100

Ctrl+C

stop process

Pause a process

The Ctrl+Z keys generates a SIGTSTP signal to stop any processes running in the shell. Stopping a process is different than terminating the process. Stopping the process leaves the program in memory so you can continue running from where it left off.

$ sleep 100

Ctrl+Z

pause process

The number in the square brackets is the job number assigned by the shell. The shell refers to each process running in the shell as a job. It assigns the first started process job number 1, the second job number 2, and so on.

If you have a stopped job assigned to your shell, the bash warns you if you try to exit the shell.

You can view the stopped jobs using the ps command.

ps l

ps -l

In the S column (process state), it shows the stopped job’s state as T. This indicates that the command is either being traced or stopped.

If you want to terminate a stopped job you can kill its process by using kill command I recommend you to review the basic Linux commands if you need more info about kill command.

kill processID

Trapping signals

The trap command allows you to specify which Linux signals your shell script can watch from the shell. If the script gets a signal listed in the trap command, it stops it from being processed by the shell and instead handles it locally.

So instead of allowing your Linux bash script to leave signals ungoverned, you can use trap command to do that.

You just list the commands you want the shell to execute, along with a space-separated list of signals you want to trap like the following example:

The trap command in this example shows a simple text message each time it detects the SIGINT signal when hitting Ctrl+C.

trap signal

Each time the Ctrl+C key combination is used, the script executes the echo statement specified in the trap command instead of stopping the script. Cool, right?

Trapping the script exit

You can trap the shell script exit, just add the EXIT signal to the trap command:

trap exit

When the bash script exits, the trap is triggered and the shell executes the echo command specified.

Also, the EXIT trap works if you prematurely exit the script.

Modifying or removing a trap

You can reissue the trap command with new options like this:

modify trap

After the signal trap is modified, the bash script manages the signal or signals differently.

You can also remove a trap. Simply add two dashes after the trap command.

If the program receives a signal before the trap is removed, the script processes it per the original trap command.

$ ./myscript

Crtl+C

remove trap

The first Ctrl+C was used to terminate the script. Because the signal was received before the trap was removed, the script executed the echo command specified in the trap. After the script executed the trap removal, then Ctrl+C terminated the bash script.

Running Linux bash scripts in background mode

Sometimes your Linux bash scripts can take a long time to process, and you may not want to tie up the command line interface waiting, you can’t do anything else in your terminal session. Fortunately, there’s a simple solution for this problem.

If you see the output of the ps command, you will see all the running processes in the background and not tied to the terminal.

We can do the same, just place ampersand symbol after the command.

$ ./myscipt &

run in background

Once you’ve done that, the script runs in a separate background process on the system and you can see the process id between the square brackets and when the background process finishes, it displays a message on the terminal that it is done.

Notice that while the background process is running, it still uses your terminal monitor for STDOUT and STDERR messages so if an error occurs, you will see the error message, and normal output also.

run script in background

If you exit the terminal session, the background process also will exit.

So what if you want to continue running even if you close the terminal?

Running Scripts without a Hang-Up

You can run your Linux bash scripts in the background process even if you exit the terminal session.

You can do this using the nohup command.

The nohup command runs another command, blocking any SIGHUP signals that are sent to the process. This blocks the process from exiting when you exit your terminal.

$ nohup ./myscript &

linux bash nohup command

The nohup command disassociates the process from the terminal, the process loses the STDOUT and STDERR output links. To get any output generated by the command, the nohup command automatically redirects STDOUT and STDERR messages to a file called nohup.out.

Note: when running multiple commands from the same directory, all the output is sent to the same nohup.out file.

Viewing jobs

The jobs command enables you to view the current jobs being handled by the shell.

Then run it.

$ ./myscript

Then stop it using the Ctrl+Z combination.

linux bash view jobs

Run the same bash script but in the background using the ampersand symbol and to make it easier, I’m going to make the output of that script is redirected to a file so it doesn’t appear on the screen.

$ ./myscript > outfile &

linux bash list jobs

The jobs command shows both the stopped and the running jobs.

jobs l

-l parameter to view the process ID

Restarting stopped jobs

To restart a job in background mode, use the bg command.

$ ./myscript

Then press Ctrl+Z

Now it is stopped.

$ bg

linux bash restart job

After using bg command, it is now running in background mode.

If you have multiple stopped jobs, you can do the same by specifying the job number to the bg command.

If you want to restart a job in foreground mode, use the fg command.

$ fg 1

Scheduling a job

The Linux system provides a couple of ways to run a bash script at a preselected time: the at command and the cron table.

The at command

This is the format of the command

at [-f filename] time

The at command recognizes lots of different time formats:

  • A standard hour and minute, such as 10:15
  • An AM/PM indicator, such as 10:15PM
  • A specifically named time, such as now, noon, midnight

In addition to specifying the time to run the job, you can also include a specific date, using a few different date formats:

  • A standard date format, such as MMDDYY, MM/DD/YY, or DD.MM.YY
  • A text date, such as Jul 4 or Dec 25, with or without the year
  • Now + 25 minutes
  • 10:15PM tomorrow
  • 10:15 + 7 days

We don’t want to dig deep into the at command, but for now, just make it simple.

$ at -f ./myscript now

linux bash at command

The -M parameter is used to send the output to email if the system has email, and if not, this will suppress the output of the at command.

To list the pending jobs use atq command.

$ atq

linux bash at queue

Removing jobs

You can use the atrm command to remove a pending job by specifying the job number.

$ atrm 18

delete at queue

Scheduling scripts

You can use the at command to schedule a script to run at a specific time, but what if you need that script to run at the same time every day or once a week or once a month.

The Linux system uses the crontab command to allow you to schedule jobs that need to run regularly.

The crontab program runs in the background and checks special tables called cron tables, for jobs that are scheduled to run.

To list an existing cron table, use the -l parameter

$ crontab l

The format for crontab is:

minute,Hour, dayofmonth, month, and dayofweek

So if you want to run a command at 10:30 on every day, you would use this cron table entry:

30 10 * * * command

The wildcard character (*) used in the dayofmonth, month, and dayofweek fields indicates that the cron will execute the command every day of every month at 10:30.

To specify a command to run at 4:30 PM every Monday, you would use the following:

30 16 * * 1 command

The day of the week starts from 0 to 6 where 0 is Sunday and 6 is Saturday.

Here’s another example: to execute a command at 12 noon on the first day of every month, you would use the following format:

00 12 1 * * command

The day of the month is from 1 to 31.

Let’s keep it simple for now and we will discuss the cron in great detail in future posts.

To add entries to your cron table, use the -e parameter like this:

crontab e

Then type your command like the following

 30 10 * * * /home/likegeeks/Desktop/myscript

This will schedule our script to run at 10:30 every day.

Note: sometimes you see error says Resource temporarily unavailable.

All you have to do is this:

$ rm -f /var/run/crond.pid

You should be a root user to do this.

Just that simple!

You can use one of the pre-configured cron script directories. There are four basic directories: hourly, daily, monthly, and weekly.

/etc/cron.hourly

/etc/cron.daily

/etc/cron.weekly

/etc/cron.monthly

Just put your bash script file on any of these directories and it will run periodically.

Starting scripts with a new shell

In the previous posts, we’ve talked about startup files, I recommend you to review the previous.

$HOME/.bash_profile

$HOME/.bash_login

$HOME/.profile

Just place any scripts you want to run at login time in the first file listed $HOME/.bash_profile.

OK, what about running our bash script when the shell opens? Easy.

Type your script on .bashrc file.

And now if you open the shell window, it will execute that command.

I hope you find the post useful. keep coming back.

Thank you.