Crontab Entries

Crontab Entries

Below is the syntax of crontab entry

 Minute   Hour   Day of Month       Month          Day of Week        Command
 (0-59)     (0-23)       (1-31)       (1-12/Jan-Dec)  (0-6 or Sun-Sat)          ----


for Example
00 20 * * * date > /root/date

The above command will execute the date command and save output in /root/date on 20:00 P.M. everyday.

Zipping a file or folder using ZIP command

The "zip" command can be used to zip a file or folder.

syntax

zip <<filename.zip>> <<files_to_zip>>


zip <<filename.zip>> <<folders_to_zip>>

example

zip try.zip file1.txt file2.txt

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Linux Boot process

---> ---> 3 stage Linux Boot Process in short
1. Bios executing  a short program in MBR (master boot record) of first physical drive. The program is in assembly language of 512 bytes. This is called Boot loader stage. The stage 1 boot resides in MBR and contain sector number of the stage 2 boot located at /boot/grub/grub.conf

2. Stage 2 boot loader presents boot menu based in /boot/grub/grub.conf. It loads the linux kernel. The boot loader loads the kernel, as the kernel boots up it scans the hardware, loads the loadable modules it needs and starts process number 1 i.e. /sbin/init

3. Stage 3 process parses the request from /sbin/init to /etc/inittab file. The inittab file holds the default run level by which it starts loading. There are 7 different run levels from 0 to 6. All this run level use the following file /etc/rc.d/rc[0-6]/ directory. Depending on the prefix to a file service is started or killed. S means started and K means killed.