Mount

mount command examples

To mount a file system, you should first create a directory and mount it as shown below.

# mkdir /u01

# mount /dev/sdb1 /u01

You can also add this to the fstab for automatic mounting. i.e Anytime system is restarted, the filesystem will be mounted.

/dev/sdb1 /u01 ext2 defaults 0 2

CAT

cat command examples

You can view multiple files at the same time. Following example prints the content of file1 followed by file2 to stdout.

$ cat file1 file2

While displaying the file, following cat -n command will prepend the line number to each line of the output.

$ cat -n /etc/logrotate.conf
    1	/var/log/btmp {
    2	    missingok
    3	    monthly
    4	    create 0660 root utmp
    5	    rotate 1
    6	}

Service

service command examples

Service command is used to run the system V init scripts. i.e Instead of calling the scripts located in the /etc/init.d/ directory with their full path, you can use the service command.

Check the status of a service:

# service ssh status

Check the status of all the services.

service --status-all

Restart a service.

# service ssh restart

DF

df command examples

Displays the file system disk space usage. By default df -k displays output in bytes.

$ df -k
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sda1             29530400   3233104  24797232  12% /
/dev/sda2            120367992  50171596  64082060  44% /home

df -h displays output in human readable form. i.e size will be displayed in GB’s.

ramesh@ramesh-laptop:~$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1              29G  3.1G   24G  12% /
/dev/sda2             115G   48G   62G  44% /home

Use -T option to display what type of file system.

ramesh@ramesh-laptop:~$ df -T
Filesystem    Type   1K-blocks      Used Available Use% Mounted on
/dev/sda1     ext4    29530400   3233120  24797216  12% /
/dev/sda2     ext4   120367992  50171596  64082060  44% /home

Free

free command examples

This command is used to display the free, used, swap memory available in the system.

Typical free command output. The output is displayed in bytes.

$ free
             total       used       free     shared    buffers     cached
Mem:       3566408    1580220    1986188          0     203988     902960
-/+ buffers/cache:     473272    3093136
Swap:      4000176          0    4000176

If you want to quickly check how many GB of RAM your system has use the -g option. -b option displays in bytes, -k in kilo bytes, -m in mega bytes.

$ free -g
             total       used       free     shared    buffers     cached
Mem:             3          1          1          0          0          0
-/+ buffers/cache:          0          2
Swap:            3          0          3

If you want to see a total memory ( including the swap), use the -t switch, which will display a total line as shown below.

ramesh@ramesh-laptop:~$ free -t
             total       used       free     shared    buffers     cached
Mem:       3566408    1592148    1974260          0     204260     912556
-/+ buffers/cache:     475332    3091076
Swap:      4000176          0    4000176
Total:     7566584    1592148    5974436

Crontab

crontab -a filename
Install filename as your crontab file. On many systems, this command is executed simply as crontab filename (i.e., without the -a  option).
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 available on only a few systems.)
crontab -u user
Used in conjunction with other options, this option allows you to modify or view the crontab file of user. When available, only administrators can use this option.

Field
Value
Description
minute
0-59
The exact minute that the command sequence executes
hour
0-23
The hour of the day that the command sequence executes
day
1-31
The day of the month that the command sequence executes
month
1-12
The month of the year that the command sequence executes
weekday
0-6
The day of the week that the command sequence executes (Sunday = 0, Monday = 1, Tuesday = 2, and so forth)
command
Special
The complete sequence of commands to execute. The command string must conform to Bourne shell syntax. Commands, executables (such as scripts), or combinations are acceptable.

# Example 1:  0,30 8-17 * * 1-5 cmd


Show/Hide Answer (+/-)

Run cmd on the half-hour from 8:00 AM to 5:30 PM, Monday thru Friday



# Example 2:  0 12 1,15 * 5 cmd


Show/Hide Answer (+/-)

Run cmd at noon each Friday AND the first and fifteenth of every month



# Example 3:  17 3 * * 1 cmd


Show/Hide Answer (+/-)

Run cmd at 3:17 AM Monday (a backup program perhaps)



# Example 4:
# This command sends a "popup" message to the user's screen,
# at 3:00 PM Fridays, if they are logged in:
0 15 * * 5 echo "Time for staff meeting" | write $LOGNAME >/dev/null 2">"&1



# Example 5:
# This command sends a multi-line "popup" message to the user's screen,
# at 3:00 PM Fridays, if they are logged in:
0 15 * * 5 write $LOGNAME '>'/dev/null 2'>'&1 %Time for the%staff meeting.

shutdown

shutdown command examples

Shutdown the system and turn the power off immediately.

# shutdown -h now

Shutdown the system after 10 minutes.

# shutdown -h +10

Reboot the system using shutdown command.

# shutdown -r now

Force the filesystem check during reboot.

# shutdown -Fr now