Basic CentOS Linux Commands

Before learning the tools of a CentOS Linux Administrator, it is important to note the philosophy behind the Linux administration command line.

Linux was designed based on the Unix philosophy of “small, precise tools chained together simplifying larger tasks”. Linux, at its root, does not have large single-purpose applications for one specific use a lot of the time. Instead, there are hundreds of basic utilities that when combined offer great power to accomplish big tasks with efficiency.

Examples of the Linux Philosophy

For example, if an administrator wants a listing of all the current users on a system, the following chained commands can be used to get a list of all system users. On execution of the command, the users are on the system are listed in an alphabetical order.

[root@centosLocal centos]# cut /etc/passwd -d":" -f1 | sort 
abrt 
adm 
avahi 
bin 
centos 
chrony 
colord 
daemon 
dbus

It is easy to export this list into a text file using the following command.

[root@localhost /]# cut /etc/passwd -d ":" -f1 > system_users.txt        
[root@localhost /]# cat ./system_users.txt | sort | wc –l 
40       
[root@localhost /]#

It is also possible to compare the user list with an export at a later date.

[root@centosLocal centos]#  cut /etc/passwd -d ":" -f1 > system_users002.txt && 
   cat system_users002.txt | sort | wc -l 
41 
[root@centosLocal centos]# diff ./system_users.txt ./system_users002.txt  
evilBackdoor [root@centosLocal centos]#

With this approach of small tools chained to accomplish bigger tasks, it is simpler to make a script performing these commands, than automatically email results at regular time intervals.

Basic Commands every Linux Administrator should be proficient in are −

• vim
• grep
• more and less
• tail
• head
• wc
• sort
• uniq
• tee
• cat
• cut
• sed
• tr
• paste

In the Linux world, Administrators use filtering commands every day to parse logs, filter command output, and perform actions with interactive shell scripts. As mentioned, the power of these commands come in their ability to modify one another through a process called piping.

The following command shows how many words begin with the letter a from the CentOS main user dictionary.

[root@centosLocal ~]# egrep '^a.*$' /usr/share/dict/words | wc -l 
25192 
[root@centosLocal ~]#