Commands_Part_2

Head

Syntax: command                  -args                                    source

Head displays the first 10 lines of any file in the terminal.

# head  /etc/passwd	Displays the first 10 lines of /etc/passwd on the terminal
# head  -n  20  /etc/passwd	Displays the first 20 lines of /etc/passwd on the terminal

Tail

Syntax: command                  -args                                    source

Head displays the last 10 lines of any file in the terminal.

# tail  /etc/passwd	Displays the last 10 lines of /etc/passwd on the terminal
# tail  -n  20  /etc/passwd	Displays the last 20 lines of /etc/passwd on the terminal
# tail –f  /var/log/messages   OR # tailf  /var/log/messages	Dynamically displays the last portion of the log file /var/log/messages

Echo

Syntax: command                  destination

The string after the command echo would be displayed at the terminal.

# echo  hello. how are you?	Displays the text ‘hello. how are you?’ on the terminal
# echo  this is easy	Displays the text ‘this is easy’ in the terminal

Date

Time in any Linux system can be changed in the following methods.

# system-config-date  OR # system-config-time	Change the time/date in GUI
# date MMddhhmmyyyy Example:# date 051117372011	This is a universal command that would work on all UNIX systems. Here MM = Month (1~12), dd = date (1~31), hh = hour (00~23), mm = minute (00~59), yyyy = year
# hwclock  --systohc	Required if time is changed using the command ‘date’. This command synchronizes hardware clock with operating system clock.

History

Whenever a user executes a command, it is saved to history. Each user has a separate history as a reference of the commands that were executed. History is stored in the file .bash_history which is located in the home directory of the user i.e. /root/.bash_history, /home/sarmed/.bash_history.

# history	Displays history
# history –c	Clears history temporarily.

To delete history permanently, open the file .bash_history and delete its contents.