How to run schedule task in Linux using crontab
cron is the time-based job scheduler in Unix-like computer operating systems. cron enables users to schedule jobs (commands or shell scripts) to run periodically at certain times or dates. It is commonly used to automate system maintenance or administration, though its general-purpose nature means that it can be used for other purposes as well. cron checks for scheduled jobs every minute. Using cron you can’t schedule a job that needs to run say every 5 seconds.
Cronjobs are written in the following format:
* * * * * command or shell script to be executed ┬ ┬ ┬ ┬ ┬ │ │ │ │ │ │ │ │ │ │ │ │ │ │ └───── day of week (0 - 6) (0 is Sunday, or use names) │ │ │ └────────── month (1 - 12) │ │ └─────────────── day of month (1 - 31) │ └──────────────────── hour (0 - 23) └───────────────────────── min (0 - 59)
Each user can have their own crontab file. Signon as root to see system wide schedule tasks. To check for existing entries
$ crontab -l
To edit the crontab file
$ crontab -e
If you keep all the 5 values to asterisks that means the job will run every minute. For example
* * * * * /usr/local/myscript.sh
Now if you don’t want to specify the full path you can set the PATH in the crontab file
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin: /usr/local * * * * * myscript.sh
How to send the crontab output to a file for logging
* * * * * /usr/local/myscript.sh &>> /usr/local/crontablog.txt
Scheduling a job to run everyday at 5AM
0 5 * * * /usr/local/myscript.sh
Scheduling a job to run every Tuesday at 5AM
0 5 * * 2 /usr/local/myscript.sh
Scheduling a job to run only on weekdays at 5AM
0 5 * * 1-5 /usr/local/myscript.sh
Scheduling a job to run everyday at 5AM and 4PM
0 5,16 * * * /usr/local/myscript.sh
Scheduling a job to run only on weekdays between 9AM and 5PM
0 9-17 * * 1-5 /usr/local/myscript.sh
Scheduling a job to run every 5 minutes
*/5 * * * * /usr/local/myscript.sh
Special crontab keywords
@reboot Run once, at startup @yearly Run once a year "0 0 1 1 *" @monthly Run once a month "0 0 1 * *" @weekly Run once a week "0 0 * * 0" @daily Run once a day "0 0 * * *" @hourly Run once an hour "0 * * * *
For example:
@daily /usr/local/myscript.sh
Linux send STDOUT to a file
$ ls -l > report.txt
OR
$ ls -l 1> report.txt
Output in the file report.txt
total 4 -rwxr-xr-x 1 luser luser 78 2012-05-10 19:08 mySample -rw-rw-r-- 1 luser luser 0 2012-05-13 23:42 report.txt
Linux send STDERR to a file
abcd 2> report.txt
Output in the file report.txt
No command 'abcd' found, did you mean: Command 'lbcd' from package 'lbcd' (universe) Command 'ascd' from package 'ascd' (universe) Command 'abcde' from package 'abcde' (universe) Command 'bcd' from package 'bsdgames' (universe) abcd: command not found
Linux send both STDERR and STDOUT to a file
abcd &> report.txt
Output in the file report.txt
No command 'abcd' found, did you mean: Command 'lbcd' from package 'lbcd' (universe) Command 'ascd' from package 'ascd' (universe) Command 'abcde' from package 'abcde' (universe) Command 'bcd' from package 'bsdgames' (universe) abcd: command not found
How to find files in Linux with specific permissions
Find all files that have 777 permission
$ find / -type f -perm 0777
Find all files in the current directory with 644 permission
$ find . -type f -perm 0644
How to display file permissions in Linux using Shell command
You need to use ls command with -l option. The first character tell whether its a file or a directory and after that are the permissions. Here is an example
$ ls -l /var/lib/tomcat6
If you don’t specify the path it will list files and directories in the current path.
The response to the above command is something like this
total 20 drwxr-xr-x 3 tomcat6 tomcat6 4096 2012-05-08 20:43 common lrwxrwxrwx 1 root root 12 2012-01-26 08:08 conf -> /etc/tomcat6 lrwxrwxrwx 1 root root 17 2012-01-26 08:08 logs -> ../../log/tomcat6 -rw-r--r-- 1 root root 20 2012-05-12 16:01 myFile.txt drwxr-xr-x 3 tomcat6 tomcat6 4096 2012-05-08 20:43 server drwxr-xr-x 3 tomcat6 tomcat6 4096 2012-05-08 20:43 shared drwxrwxr-x 3 tomcat6 tomcat6 4096 2012-05-08 20:43 webapps lrwxrwxrwx 1 root root 19 2012-01-26 08:08 work -> ../../cache/tomcat6
The very first column shows the file type and permissions. The second column shows the number of links, the third one shows the owner of the file, and the fourth one shows the group the file belongs to. The other columns show the file’s size in bytes, date and time of last modification, and the filename.
The first character
- d = directory
- - = file
- l = symbolic link
- s = socket
- p = named pipe
- c= character (unbuffered) device file special
- b=block (buffered) device file special ;
The file permissions
- r = Read permission
- w = Write permission
- x = Execute permission
- - = No permission
The file permissions are defined for the owner level, group level, and the public level (meaning anyone else). The first set of 3 characters are for the user level security basically for the owner of the file. The second set of 3 characters are for those users that belong to the same group as the file and the last 3 characters are for anyone else who has no affiliation to the file.
How to list only directories?
$ ls -d */ -l
How to find files and then print permissions?
$ find . -type f -exec ls -l {} \;
How to set permissions for all files and directories recursively using chmod
In the current directory
find . -exec chmod 755 {} \;
– OR —
chmod -R 755
Find files starting with a specific string and then change permissions
find /usr/local -name “abc*” -exec chmod 755 {} \;
How to find empty files and directories in linux
find /usr/local -empty
How to delete ?
find /usr/local -empty -delete
How to find only empty files in linux
find /usr/local -type f -empty
How to delete ?
find /usr/local -type f -empty -delete
How to find only empty directories in linux
find /usr/local -type d -empty
How to delete ?
find /usr/local -type d -empty -delete
How to find a file in Linux but skip any mounted file systems
find / -xdev -name “abc.txt”
This will help you skip mounted network drives or filesystems that you do not want searched.
How to find a file in linux anywhere in your file system
find / -name “abc.txt”
This will start the search from the root and drill into every sub-directory to find the file with name abc.txt
If you want to start from a specific folder then change / to that one. For example
find /usr/local -name “httpd.conf”
This will try to find the file in /usr/local and any sub-directories within it.
How to do case insensitive file name search in Linux
find . -iname “vm*”
-name is case sensitive and -iname is case insensitive
the DOT means current directory, you can specify a path instead. For example
find /usr/lib -iname “vm*”