Cron Job In Linux | Scheduling job with crontab in Linux

Cron job are used to schedule commands to be executed periodically. You can setup commands or scripts, which will repeatedly run at a set time. Cron is one of the most useful tool in Linux or UNIX like operating systems. The cron service (daemon) runs in the background and constantly checks the
/etc/crontab file, and /etc/cron.*/ (cron.d/,cron.daily/,cron.hourly/,cron.monthly/,cron.weekly/) directories. It also checks the /var/spool/cron/ directory. Each user can have their own crontab file.

Other than crontab there are 2 more services to schedule the job the "at" command and "batch" command. The "at" command is used to schedule a one-time task at a specific time. The batch command is used to schedule a one-time task to be executed when the systems load average drops below 0.8.

Install or create or edit my own cron jobs:

To edit your crontab file, type the following command at the UNIX / Linux shell prompt:

$ crontab -e

Syntax of crontab (field description)

The syntax is:

    1 2 3 4 5 /path/to/command arg1 arg2

OR

    1 2 3 4 5 /root/backup-script.sh

Where,

1: Minute (0-59)
2: Hours (0-23)
3: Day (0-31)
4: Month (0-12 [12 == December])
5: Day of the week(0-7 [7 or 0 == sunday])

/path/to/command - Script or command name to schedule

Easy to remember format:

Examples of Run backup cron job script:
If you wish to run a backup-script.sh daily at 2.00 AM then first install your cronjob by running the following command as following .
$ crontab -e
append the following entry at the end

0 2 * * * /root/backup-script.sh

save and close the crontab file . The backup-script.sh will run every day at 2.00 AM.

More examples:
To run /path/to/script.sh every 5 min , Enter following command:

*/5 * * * * /path/to/script.sh

To run /path/to/script.sh five minutes after midnight, every day, Enter following:

5 0 * * * /path/to/script.sh

To run /path/to/script.sh at 3:15 PM on the first of every month, Enter following:

15 14 1 * * /path/to/script.sh

To run /path/script.sh at 10 PM on weekdays(Mon,Tue,Wed,Thu,Fri), Enter following:

0 22 * * 1-5 /scripts/script.sh

To run /path/myscripts/script.pl at 23 minutes after midnight, 2am, 4am ..., everyday, Enter:

23 0-23/2 * * * /path/myscripts/script.pl

How do I use operators?

An operator allows you to specifying multiple values in a field. There are four operators:

The asterisk (*) :

This operator specifies all possible values for a field. For example, an asterisk in the hour time field would be equivalent to every hour or an asterisk in the month field would be equivalent to every month.

The comma (,) :

This operator specifies a list of values, for example: "1,5,10,15,20, 25".

The dash (-) :

This operator specifies a range of values, for example: "5-15" days , which is equivalent to typing "5,6,7,8,9,....,13,14,15" using the comma operator.

The separator (/) :

This operator specifies a step value, for example: "0-23/" can be used in the hours field to specify command execution every other hour. Steps are also permitted after an asterisk, so if you want to say every two hours, just use */2.

How To Send Email From Linux Command Line

In Linux the mail command is an essential in our practical life. There is lot of instance when we want to send mail from Unix/Linux server to our outlook or personal email inbox. For example in Linux server we have generated csv file from database and want to pull this in our window machine. In this case we can use mail command to send file in our mailbox direct from Linux rather using FTP to pull report in local window machine.

Also mail command is useful in our automation script in which we want to generate some kind of report and send to its in recipients email box.

Before starting we should make sure we have installed mailx command in our system. If not installed run the following commands.

# For ubuntu/debian

$ sudo apt-get install heirloom-mailx

# For fedora/centos

$ sudo yum install mailx

Usage of mail command:
Once installation done you are ready to use mail command from command line. Following are the couple of examples .

1. Simple mail :
Run the following command and hit enter then you can write message for email body and for new line just hit enter and write message.
Once message complete press Ctrl+D and it would display EOT at the end.

Check the below message in recipient email if you not getting mail in inbox then check in spam/junk folder.

$ mail -s "This is subject" xyz@example.com
Hi there
This is a simple email body
bye!
EOT

2. Redirect message into mail from a file:

We can send email body message from a file by redirecting file as following.

$ mail -s "This is subject" xyz@example.com < /home/mukesh/message/message-body.txt

3. Message body in echo :

We can write the body message with echo as following.

$ echo -e "This is message body \nHere is second line." | mail -s "This is subject" xyz@example.com

4. Send mail to multiple recipients:

To send email to multiple recipient just separate the email id's  by comma.

echo -e "This is message body \nHere is second line." | mail -s "This is subject" xyz@example.com,abc@example.com

5. Using CC and BCC option:

Use -c to add CC and -b to add BCC

$ echo -e "This is message body" | mail -s "This is subject" -c ccrecipient@example.com xyz@example.com

$ echo -e "This is message body" | mail -s "This is subject" -c ccrecipient@example.com -b bccrecipient@example.com xyz@example.com

6. Specifying from Email:
If you wish to put sender email for recipient use -r option as following.

$ echo "This is message body" | mail -s "This is Subject" -r "Mukesh<fromemail@example.com>" recipient@example.com

7.Email with attachment ( Important ):

To attach  file use -a option as following.

echo "This is Message Body" | mail -s "This is Subject" -a /home/mukesh/message/sampleFile.csv recipient@example.com

echo "This is Message Body" | mail -s "This is Subject" -r "Mukesh<fromemail@example.com>" -a /home/mukesh/message/sampleFile.csv recipient@example.com

**** End****

How to find the length of a string variable in UNIX.

You may need to find the length of string or size of string variable in your shell script program. Here are the 5 best ways through which you can acheive it.

Above picture may clear you whole story if not please go in details as following.


Consider we have a variable name VAR and it stores a string "Hello" which length is 5.
VAR="Hello"

1) echo : In the bash we can use echo command as following .

VAR="Hello"
echo ${#VAR}
5

2) echo with wc : Second method is echo with wc with -c option as following.

VAR="Hello"
echo -n $VAR | wc -m
5

or

echo -n $VAR | wc -c
5

Note:
Where -m print the character counts and -c print the byte counts .

3) printf with wc : 3rd method is to use printf with wc as following.

printf $VAR | wc -c
5

or

printf $VAR | wc -m
5

4) echo with awk : 4th method is echo with awk as following.

echo $VAR | awk '{print length ;}'
5

5) expr : 5th method to use expr as following.

expr length $VAR
5

or

expr $VAR : '.*'
5

Watch video in detail