Tag Archives: unix

Automatically start Tomcat on startup in Ubuntu

Apache Tomcat is not configured with autostart by default in Ubuntu. So, custom init script is required to configure Tomcat for autostart on startup.

Create the init script in /etc/init.d/tomcat8 with the contents as per below.

Init script contents:

Note: Please change {tomcat_root} with your Tomcat installation folder path.

Change its permissions and add the correct symlinks automatically:

And from now on it will be automatically started and shut down upon entering the appropriate run levels.

It could be also controlled with justservice tomcat8 <stop|start|restart> like Apache control commands.

Using above process any server script can be created and configured to start on startup.

Add Jobs to cronjob in UNIX

Cron jobs allows Unix users to run commands or scripts at a given date and time as per scheduled period of time. Cron is one of the most useful tool in a UNIX like operating systems. In Windows, same thing will be achieved by Scheduled Tasks. It is usually used for sysadmin jobs such as periodic backups, scripts to be executed on period of time or do any periodic activities and more. The cron service (daemon) runs in the background and constantly checks the/etc/crontab file, /etc/cron.*/ directories and /var/spool/cron/directory.

crontab command

crontab command is used to edit/create, install, deinstall or list the cron jobs in Cron. Every user can have their own crontab file, and though crontab files in /var/spool/cron/crontabs are not intended to be edited directly. Each user need to use crontab command for editing or setting up their own cron jobs.

Types of cron configuration files

There are different types of configuration files:

  1. The UNIX system crontab : Usually, used by system services and critical jobs that requires root like privileges. The sixth field (see below for field description) is the name of a user for the command to run as. This gives the system crontab the ability to run commands as any user.
  2. The user crontabs: User can install their own cron jobs using the crontab command. The sixth field is the command to run, and all commands run as the user who created the crontab

Note: This faq features cron implementations written by Paul Vixie and included in many Linux distributions and Unix like systems such as in the popular 4th BSD edition. The syntax is compatible with various implementations of crond service.

Install or create or edit cron jobs

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

Cron will examine the modification time on all crontab files and reload those which have changed. Thus cron need not be restarted whenever a crontab file is modified.

Syntax of crontab (field description)

The syntax is:

OR

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:

Your cron job looks as follows for system jobs:

OR

Example: Run backup cron job script

If you wished to have a script named /root/backup.sh run every day at 3am, your crontab entry would look like as follows. First, install your cronjob by running the following command:

Append the following entry:

Save and close the file.

More examples

To run /path/to/command five minutes after midnight, every day, enter:

Run /path/to/script.sh at 2:15pm on the first of every month, enter:

Run /scripts/phpscript.php at 10 pm on weekdays, enter:

Run /root/scripts/perl/perlscript.pl at 23 minutes after midnight, 2am, 4am …, everyday, enter:

Run /path/to/unixcommand at 5 after 4 every Sunday, enter:

How do I use operators?

An operator allows can be used to specifying multiple values in a field. There are three operators:

  1. 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.
  2. The comma (,) : This operator specifies a list of values, for example: “1,5,10,15,20, 25”.
  3. 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.
  4. 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 do I disable email output?

By default the output of a command or a script (if any produced), will be email to provided email account. To stop receiving email output from crontab, append >/dev/null 2>&1. For example:

To mail output to particular email account let us say abc@example.com, define MAILTO variable as follows:

Task: List all your cron jobs

Type the following command:

To remove or erase all crontab jobs use the following command:

Use special string to save time

Instead of the first five fields, use any one of eight special strings. It will not just save time but also improve readability.

Special string Meaning
@reboot Run once, at startup.
@yearly Run once a year, “0 0 1 1 *”.
@annually (same as @yearly)
@monthly Run once a month, “0 0 1 * *”.
@weekly Run once a week, “0 0 * * 0”.
@daily Run once a day, “0 0 * * *”.
@midnight (same as @daily)
@hourly Run once an hour, “0 * * * *”.

Examples

Run ntpdate command every hour:

Make a backup everyday:

More about /etc/crontab file and /etc/cron.d/* directories

/etc/crontab is system crontabs file. Usually only used by root user or daemons to configure system wide jobs. All individual user must use crontab command to install and edit their jobs as described above. /var/spool/cron/ or /var/cron/tabs/ is directory for personal user crontab files. It must be backup with users home directory.

Understanding Default /etc/crontab

Typical /etc/crontab file entries:

First, the environment must be defined. If the shell line is omitted, cron will use the default, which is sh.

If the PATH variable is omitted, no default will be used and file locations will need to be absolute.

If HOME is omitted, cron will use the invoking users home directory.

Additionally, cron reads the files in /etc/cron.d/ directory. Usually system daemon such as sa-update or sysstat places their cronjob here and used as the root user or superuser to configure cron jobs. Users can directly drop your scripts here. The run-parts command run scripts or programs in a directory via /etc/crontab file:

Directory Description
/etc/cron.d/ Put all scripts here and call them from /etc/crontab file.
/etc/cron.daily/ Run all scripts once a day
/etc/cron.hourly/ Run all scripts once an hour
/etc/cron.monthly/ Run all scripts once a month
/etc/cron.weekly/ Run all scripts once a week

Examples

Here is a sample shell script called clean.cache. This script is created to clean up cached files every 10 days. This script is directly created at /etc/cron.daliy/ directory. In other words create a text file called /etc/cron.daily/clean.cache as follows.

Save and close the file. Set the permissions:

Backup installed cron job entries

Simply type the following command to backup cronjobs to a nas server mounted at /nas01/backup/cron/users.root.bakup directory:

See also

vi Editor: UNIX

vi is a command-line text editor originally created for the Unix operating system.

The name vi is derived from the shortest unambiguous abbreviation for the command visual; the command in question switches the line editor ex to visual mode.

Most of the network administrators are familiar with this little editor in Unix, because they use it regularly. But, for first timers, it’s most difficult editor. First timers have to remember all commands and keys to edit a simple file.

vi has two modes, Insert mode and Command mode. In insert mode, you can add/edit the texts in file. And in command mode, you can navigate and command the editor like save, exit, copy, paste, etc.

These are the commands and keys for those who want to get familiar with vi editor.

Command to open the vi editor:

vi filename

This command creates a new file if filename is not available in current directory. By default, vi begins in command mode.

To start the insert mode, you can use following keys:

Insert text at beginning of line:

I

Insert text at cursor:

i

append text after cursor:

a

Append text at line end:

A

Open line above cursor:

O

Open line below cursor:

o

To switching back, and start the Command mode, press [ESC]

Most commands execute as soon as typed except for “colon” commands which execute when you press the return key.

For cursor movement in command mode, you can use following commands/keys:

Go to beginning of line

0

Go to end of line

$

Go to line number ##

:##

Go to line n

nG

Go to last line

G

Left 6 chars

6h

Move left, down, up, right

h j k l

Move left, down, up, right

← ↓ ↑ →

Scroll Backward 1 screen

[ctrl] b

Scroll Forward 1 screen

[ctrl] f

Scroll by sentence forward/backward

( )

Scroll by word forward/backward

w b

Scroll by paragraph forward/backward

{ }

Scroll Up 1/2 screen

[ctrl] u

Scroll Down 1/2 screen

[ctrl] d

For deleting/changing text/character in command mode, you can use following commands/keys:

Change word

cw

Replace one character

r

Delete word

dw

Delete text at cursor

x

Delete entire line (to buffer)

dd

Delete (backspace) text at cursor

X

Delete 5 lines (to buffer)

5dd

Delete current to end of line

D

Delete lines 5-10

:5,10d

For editing content in command mode, you can use following commands/keys:

Copy line

yy

Copy n lines

nyy

Copy lines1-2 /paste after 3

:1,2t3

Move lines 4-5/paste after 6

:4,5m6

Paste above current line

P

Paste below current line

p

Undo all changes to line

U

Undo previous command

u

Join previous line

J

Find next string occurrence

n

Search backward for string

?string

Search forward forstring

/string

% (entire file) s (search and replace) /old text with new/ c (confirm) g (global – all)

:%s/oldstring/newstring/cg

Ignore case during search

:set ic

Repeat last command

.

For saving and quiting in command mode, you can use following commands/keys:

Save changes to buffer

:w

Save changes and quit vi

zz or :wq

Save file to new file

:w file

Quit without saving

:q!

Save lines to new file

:10,15w file

In all of the above commands, a number n will tell vi to repeat that command n times.

:syntax on Turn on syntax highlighting
:syntax off Turn off syntax highlighting
:set number Turn on Line numbering (shorthand :set nu)
:set nonumber Turn off Line numbering (shorthand :set nonu)

:set ignorecase Ignore case sensitivity when searching
:set noignorecase Restore case sensitivity (default)