Basic Linux (Red Hat) Commands

The 'cat' command:

We use cat command to create a file with content and to display or modify the contents of a file.

Syntax:

# cat > filename (press enter)
Enter the text you want to put inside the file (press enter)
Control D (to save the file)

Example:
# cat > testfile
This is my test file.
^d

To display the contents of file:
# cat testfile
This is my test file.

To append the data in the already existing file:
# cat >> testfile
I created this for testing purpose.
^d

# cat testfile
This is my test file.
I created this for testing purpose.

The 'touch' command:

By touch command we can create multiple files at the same time.
# touch file{1..3}.txt

Display the files created by touch command:
# ls
file1.txt
file2.txt
file3.txt

The 'mkdir' command:

By using mkdir command we can create a new directory.
# mkdir dir_name
# mkdir dir1
# ls
dir1

Making multiple directories inside a directory:
# mkdir -p dir1/{dir1.1,dir1.2,dir1.3}
# ls
dir1.1  dir1.2  dir1.3

The 'cp' command:

The 'cp' command is used to copy the files from one location to another location.
# cp <source_file> <destination_file>
# cp testfile /tmp/testfile

Copy directories from one location to another location:
# cp -rvfp <dir_name> <destination_name>
# cp -irvf testfile testfile2

The 'mv' command:

We use 'mv' command to move a file from one location to another location. This act is just like cut and paste.
# mv <file_name> <destination_directory>
# mv testfile dir1

Same case in directory:
# mv <directory_name> <destination_location>
# mv dir1 dir2

Renaming a file:
# mv <old_name> <new_name>
# mv testfile mytestfile

Renaming a directory:
# mv <old_name> <new_name>
# mv dir1 dir2

The 'rm' command:

The 'rm' command is used to remove files and directories.
Removing a file:
# rm <filename>

Removing a directory:
# rm –rf <dirname>
Here, r stands for recursive and f stands for forcefully.

Filter commands:

The commands which are used to filter the output are:
1. less
2. more
3. head
4. tail
5. sort
6. cut
7. sed

The 'less/more' command: 

The less and more commands are same and used to see the output line wise or page wise.
e.g. # less /etc/passwd

or

# more /etc/passwd

Press enter to scroll line by line.
Press d to go to the next page.
Press b to go to the previous page.
Press / to search a word in the file.
Press v to go to vi mode to make any change in to the file.

The 'head' command: 

The 'head' command is used to display the top 10 lines of a file.

e.g. # head /etc/passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
operator:x:11:0:operator:/root:/sbin/nologin

If we want to see only 5 top lines of this files, we need to use:

# head -5 /etc/passwd

The 'tail' command: 

The 'tail' command is used to display the last 10 lines of a file:

e.g. # tail /etc/passwd
avahi:x:70:70:Avahi mDNS/DNS-SD Stack:/var/run/avahi-daemon:/sbin/nologin
avahi-autoipd:x:170:170:Avahi IPv4LL Stack:/var/lib/avahi-autoipd:/sbin/nologin
postfix:x:89:89::/var/spool/postfix:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
divakar:x:1000:1000:divakar rastogi:/home/divakar:/bin/bash
systemd-network:x:192:192:systemd Network Management:/:/sbin/nologin
tss:x:59:59:Account used by the trousers package to sandbox the tcsd daemon:/dev/null:/sbin/nologin
neha:x:1001:1001::/home/neha:/bin/bash
u1:x:1002:1002::/home/u1:/bin/bash
u2:x:1003:1003::/home/u2:/bin/bash

In this case also, we can use -n to get desired number of lines as output (where n can be any number).

The 'sort' command: 

The 'sort' command is used to get output in numeric or alphabetic order.
# sort <filename>

To remove the duplicate entries in output we can use:
# sort -u <filename>

The 'cut' command:

The 'cut' command is used to pick the given expression (in columns).
# cut -d: -f<column_number> <filename>

e.g.:
# cut -d: -f1 /etc/passwd
root
bin
daemon
adm
lp
sync
shutdown
halt
mail

The 'sed' command:

The 'sed' command is used to search a word in the file and replace that with the desired word. This command only make changes in output, the original file remains the same.’
e.g.:
# cat test
My file name is test.
# sed 's/test/testfile/g' test
My file name is testfile.

I/O Redirection:

We use redirection to copy the output of any command or file into a new file. There are two ways (> and >>) by which we can use redirection.

Syntax:
# command > new file

e.g.:
# cat testfile >> file1.txt

Find command:

We use find command to find files or directories path.

Syntax:
# find / (under root) -options filename

We can use below-mentioned options with find command:
-name: for searching a file with its name
-type: for searching a particular type of file
-inum: for searching a file with particular inode number
-user: for files whose owner is a particular user
-group: for files belonging to particular group

pwd: To display the current location.

cd: To change the directory.

mkfs: To provide filesystem OR to format

w: Who is login on which time and what they are working

who: To display the information about all currently logged In users

whoami: Tells us by which user we are login

logname: To display the real user who logged in initially

id: To displlay a user’s UID (User Identification), username, GID (Group identification), groupname
and all secondary groups that the user is a member

groups: To list all groups that a user is member of

uname: To display basic information about the system

hostname: To display the system name

clear: To clear the screen

date: To display current system date and time

cal: To display the calander

ping: To test connectivity

get: Retrieves one file

mget: Retrieves multiple files

put: Upload one file from your computer to remote host

mput: Upload many files from your computer to remote host

which: To show the absolute path of the command

whereis: To display the binary name and full path

mkdir: To create a directory

rmdir: To remove a directory

touch: To create a file

rm: To remove a file

useradd: To add a user

userdel: To delete a user

groupadd: To add a group

groupdel: To delet a group

chmod: To change the permission of a file or directory

crontab: To change cron jobs

palimpsest: To view full disk use in graphic mode

Create a user with UID 5001:
# useradd -u 5001 username          //if we want to create a new user
# usermod -u 5001 username          //if we want to make modification in existing user