crontab

CRON Scheduler

cron is a unix, solaris utility that allows tasks to be automatically run in the background at regular intervals by the cron daemon. These tasks are often termed as cron jobs in unix , solaris.  Crontab (CRON TABle) is a file which contains the schedule of cron entries to be run and at specified times

Crontab Restrictions

You can execute crontab if your name appears in the file /usr/lib/cron/cron.allow. If that file does not exist, you can use
crontab if your name does not appear in the file /usr/lib/cron/cron.deny.
If only cron.deny exists and is empty, all users can use crontab. If neither file exists, only the root user can use crontab. The allow/deny files consist of one user name per line.

Crontab Commands

export EDITOR=vi ;to specify a editor to open crontab file.

crontab -e    Edit your crontab file, or create one if it doesn’t already exist.
crontab -l      Display your crontab file.
crontab -r      Remove your crontab file.
crontab -v      Display the last time you edited your crontab file. (This option is only available on a few systems.)

Crontab file

Crontab syntax:
A crontab file has five fields for specifying day, date and time followed by the command to be run at that interval.

Several special predefined values can substitute in the cron expression. Note that in some uses of the cron format there is also a seconds field at the beginning of the pattern (e.g. Quartz).

Entry	Description	Equivalent to
@yearly (or @annually)	Run once a year at midnight in the morning of January 1	0 0 1 1 *
@monthly	Run once a month at midnight in the morning of the first day of the month	0 0 1 * *
@weekly	Run once a week at midnight in the morning of Sunday	0 0 * * 0
@daily	Run once a day at midnight	0 0 * * *
@hourly	Run once an hour at the beginning of the hour	0 * * * *
@reboot	Run at startup	@reboot

*     *     *   *    *        command to be executed -     -     -   -    - |     |     |   |    | |     |     |   |    +----- day of week (0 - 6) (Sunday=0) |     |     |   +------- month (1 - 12) |     |     +--------- day of the month (1 - 31) |     +----------- hour (0 - 23) +------------- min (0 - 59)

Sunday               -0

Monday             -1

Tuesday             -2

Wednesday       -3

Thursday           -4

Friday                -5

Saturday            -6

* In the value field above means all legal values as in braces for that column.
The value column can have a * or a list of elements separated by commas. An element is either a number in the ranges shown above or two numbers in the range separated by a hyphen (meaning an inclusive range).

Notes
A.) Repeat pattern like /2 for every 2 minutes or /10 for every 10 minutes is not supported by all operating systems. If you try to use it and crontab complains it is probably not supported.

B.) The specification of days can be made in two fields: month day and weekday. If both are specified in an entry, they are cumulative meaning both of the entries will get executed.

Crontab Example

A line in crontab file like below removes the tmp files from /home/someuser/tmp each day at 6:30 PM.

30     18     *     *     *         rm /home/someuser/tmp/*

Changing the parameter values as below will cause this command to run at different time schedule below:

min	hour	day/month	month	day/week	Execution time
30	0	1	1,6,12	*	– 00:30 Hrs  on 1st of Jan, June & Dec.
0	20	*	10	1-5	–8.00 PM every weekday (Mon-Fri) only in Oct.
0	0	1,10,15	*	*	– midnight on 1st ,10th & 15th of month
5,10	0	10	*	1	– At 12.05,12.10 every Monday & on 10th of every month

Note: If you inadvertently enter the crontab command with no argument(s), do not attempt to get out with Control-d. This removes all entries in your crontab file. Instead, exit with Control-c.

Crontab Environment

cron invokes the command from the user’s HOME directory with the shell, (/usr/bin/sh).
cron supplies a default environment for every shell, defining:
HOME=user’s-home-directory
LOGNAME=user’s-login-id
PATH=/usr/bin:/usr/sbin:.
SHELL=/usr/bin/sh

Users who desire to have their .profile executed must explicitly do so in the crontab entry or in a script called by the entry.

Disable Email

By default cron jobs sends a email to the user account executing the cronjob. If this is not needed put the following command at the end of the cron job line.

>/dev/null 2>&1

Generate log file

To collect the cron execution log in a file:

30 18 * * * rm /home/someuser/tmp/* > /home/someuser/cronlogs/clean_tmp_dir.log

vi editor

Vi EDITOR

General Startup

                To use vi: vi filename

                To exit vi and save changes: ZZ   or  :wq

                To exit vi without saving changes: :q!

                To enter vi command mode: [esc]

Counts

        A number preceding any vi command tells vi to repeat that command that many times.

Cursor Movement

h	move left (backspace)
j	move down
k	move up
l	move right. (spacebar)[return]   move to the beginning of the next line
$	last column on the current line
0	move cursor to the first column on the current line
^	move cursor to first nonblank column on the current line
w	move to the beginning of the next word or punctuation mark
W	move past the next space
b	move to the beginning of the previous word or punctuation mark
B	move to the beginning of the previous word, ignores punctuation
e	end of next word or punctuation mark
E	end of next word, ignoring punctuation
H	move cursor to the top of the screen
M	move cursor to the middle of the screen
L	move cursor to the bottom of the screen

Screen Movement

G	move to the last line in the file
xG	move to line x
z+	move current line to top of screen
z	move current line to the middle of screen
z-	move current line to the bottom of screen
^F	move forward one screen
^B	move backward one line
^D	move forward one half screen
^U	move backward one half screen
^R	redraw screen ( does not work with VT100 type terminals )
^L	redraw screen ( does not work with Televideo terminals )

Inserting

r	replace character under cursor with next character typed
R	keep replacing character until [esc] is hit
i	insert before cursor
a	append after cursor
A	append at end of line
O	open line above cursor and enter append mode

Deleting

x	delete character under cursor
dd	delete line under cursor
dw	delete word under cursor
db	delete word before cursor

Copying Code

yy      (yank)'copies' line which may then be put by the p(put) command. Precede with a count for multiple lines.

Put Command

        Brings back previous deletion or yank of lines, words, or characters

P	bring back before cursor
p	bring back after cursor

Find Commands

?	finds a word going backwards
/	finds a word going forwards
f	finds a character on the line under the cursor going forward
F	finds a character on the line under the cursor going backwards
t	find a character on the current line going forward and stop one character before it
T	find a character on the current line going backward and stop one character before it
;	repeat last f, F, t, T

Miscellaneous Commands

.	repeat last command
u	undoes last command issued
U	undoes all commands on one line
xp	deletes first character and inserts after second (swap)
J	join current line with the next line
^G	display current line number
%	if at one parenthesis, will jump to its mate
mx	mark current line with character x
'x	find line marked with character x

               

NOTE: Marks are internal and not written to the file.

Line Editor Mode

                Any commands form the line editor ex can be issued upon entering line mode.

                To enter: type ':'

                To exit: press [return] or [esc]

READING FILES

                Copies (reads) filename after cursor in file

                Currently editing

                :r filename

WRITE FILE

                :w           saves the current file without quitting

MOVING

                :#            move to line #

                :$            move to last line of file

SHELL ESCAPE

                Executes 'cmd' as a shell command.

                :!'cmd'