Burn ISO to DVD

Published / by MasonCloud / Leave a Comment
$ diskutil list
/dev/disk0 (internal, physical):
 0: GUID_partition_scheme *251.0 GB disk0
 1: EFI EFI 209.7 MB disk0s1
 2: Apple_CoreStorage www.MasonCloud.com 250.1 GB disk0s2
 3: Apple_Boot Recovery HD 650.0 MB disk0s3

/dev/disk1 (internal, virtual):
 0: hddrive +249.8 GB disk1
 Logical Volume on disk0s2
 Unlocked Encrypted

/dev/disk2 (external, physical):
 0: FDisk_partition_scheme *32.4 GB disk2
 1: Windows_NTFS RedHat_v7.2 32.4 GB disk2s1

$ sudo dd if=/path-to-iso/rhel-server-7.2-x86_64-dvd.iso of=/dev/disk2 bs=1m

# Press CNTR+T inorder to show below output:
load: 1.90 cmd: dd 4045 running 0.00u 14.54s
1931+0 records in
1930+0 records out
2023751680 bytes transferred in 117.854438 secs (17171620 bytes/sec)
3856+0 records in
3856+0 records out
4043309056 bytes transferred in 232.334790 secs (17402943 bytes/sec)

Syntax of crontab

Published / by MasonCloud / Leave a Comment
Syntax of crontab (field description)

The syntax is:

1 2 3 4 5 /path/to/command arg1 arg2
1 2 3 4 5 /root/backup.sh


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:

* * * * * command to be executed
- - - - -
| | | | |
| | | | ----- Day of week (0 - 7) (Sunday=0 or 7)
| | | ------- Month (1 - 12)
| | --------- Day of month (1 - 31)
| ----------- Hour (0 - 23)
------------- Minute (0 - 59)

Your cron job looks as follows for system jobs:

1 2 3 4 5 USERNAME /path/to/command arg1 arg2
1 2 3 4 5 USERNAME /path/to/script.sh

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:
# crontab -e

Append the following entry:
0 3 * * * /root/backup.sh

Save and close the file.

More examples
To run /path/to/command five minutes after midnight, every day, enter:
5 0 * * * /path/to/command

Run /path/to/script.sh at 2:15pm on the first of every month, enter:
15 14 1 * * /path/to/script.sh

Run /scripts/phpscript.php at 10 pm on weekdays, enter:
0 22 * * 1-5 /scripts/phpscript.php

Run /root/scripts/perl/perlscript.pl at 23 minutes after midnight, 2am, 4am ..., everyday, enter:
23 0-23/2 * * * /root/scripts/perl/perlscript.pl

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

How do I use operators?
An operator allows you to specifying multiple values in a field. There are three 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 do I disable email output?
By default the output of a command or a script (if any produced), will be email to your local email account. To stop receiving email output from crontab you need to append >/dev/null 2>&1. For example:
0 3 * * * /root/backup.sh >/dev/null 2>&1

To mail output to particular email account let us say vivek@nixcraft.in you need to define MAILTO variable as follows:
0 3 * * * /root/backup.sh >/dev/null 2>&1

See "Disable The Mail Alert By Crontab Command" for more information.

Task: List all your cron jobs
Type the following command:
# crontab -l
# crontab -u username -l

To remove or erase all crontab jobs use the following command:
# Delete the current cron jobs #
crontab -r

## Delete job for specific user. Must be run as root user ##
crontab -r -u username

Use special string to save time

Instead of the first five fields, you can use any one of eight special strings. It will not just save your time but it will 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 * * * *".
Run ntpdate command every hour:
@hourly /path/to/ntpdate

Make a backup everyday:
@daily /path/to/backup/script.sh

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 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:

# run-parts
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly
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. As a root user or superuser you can use following directories to configure cron jobs. You 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
How do I use above directories to put my own scripts or jobs?
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.

# A sample shell script to clean cached file from lighttpd web server

# Clean files every $DAYS

# Web server username and group name

# Okay, let us start cleaning as per $DAYS
/usr/bin/find ${CROOT} -type f -mtime +${DAYS} | xargs -r /bin/rm

# Failsafe
# if directory deleted by some other script just get it back
if [ ! -d $CROOT ]
 /bin/mkdir -p $CROOT
 /bin/chown ${LUSER}:${LGROUP} ${CROOT}
Save and close the file. Set the permissions:
# chmod +x /etc/cron.daily/clean.cache

How do I backup installed cron jobs entries?

Simply type the following command to backup your cronjobs to a nas server mounted at /nas01/backup/cron/users.root.bakup directory:
# crontab -l > /nas01/backup/cron/users.root.bakup
# crontab -u userName -l > /nas01/backup/cron/users.userName.bakup