Unix Important Commands and Shell Scripting Programs
Unix Important Commands and Shell Scripting Programs
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- Unix Important Commands
- Shell Scripting Programs
The above Two Topics are going to discuss majorly here…
Before that, You should know the basics of Unix…
What is an UNIX?
- UNIX is an computer operating system like windows.
- UNIX is a stable, multi-user, multi-tasking system for servers, desktops and laptops.
- UNIX is not an acronym, therefore UNIX doesn’t have a full expansion.
- UNIX systems also have a graphical user interface (GUI) like Microsoft Windows. However, the knowledge of UNIX is required for operations. For example still we are using command prompt for telnet sessions in windows.
If you are willing to learn the Unix/Linux basic commands and shell script but you do not have a setup for the same, then do not worry. We are providing screenshots of all functions with examples.
UNIX Types
There are many different versions of UNIX, although they share common similarities. In that Sun Solaris, GNU/Linux, and MacOS X are most popular types of UNIX.
Parts of UNIX
The kernel, the shell and the programs are the three parts of UNIX.
Kernel: it allocates time and memory to programs and handles the filestore and communications in response to system calls.
Shell: As an illustration of the way that the shell and the kernel work together, suppose a user types rm myfile (which has the effect of removing the file myfile). The shell searches the filestore for the file containing the program rm, and then requests the kernel, through system calls, to execute the program rm on myfile. When the process rm myfile has finished running, the shell then returns the UNIX prompt % to the user, indicating that it is waiting for further commands.
Advantages of UNIX
- Many peoples can able to access UNIX computer at the same time. So its called as Multi user system
- A user can also run multiple programs at the same time. hence UNIX is called multitasking.
1. Unix Important Commands
| UNIX Command | Description |
| cal | To view calendar |
| passwd | To change your password |
| ls | lists the contents of your current working directory |
| ls -a | To view hidden files |
| whoami | To view the current user of the system |
| users | List out the current users of UNIX operating system |
| who | To view who is logged on UNIX operating system and what they are doing? |
| w | To view who is logged on UNIX operating system |
| logout | To logging out the system |
| mkdir | To create new directory |
| cd files | Change Current Directory to files or enter into files directory |
| vi filename | To create new file or view existing file |
| cat | To display the content of file |
| cat -b | To display the content of file with Line numbers |
| cp source_file destination_file | Copying the files – Its create copy of source file in the name of destination_file |
| mv | Renames a file
mv ram ragu – Here this command renames ram file into ragu |
| rm | To remove the files.
rm ram – This command removes the ram file in current directory |
System shutdown Commands
| halt | Brings the system down immediately. |
| init 0 | Powers off the system using predefined scripts to synchronize and clean up the system prior to shutdown |
| init 6 | Reboots the system by shutting it down completely and then bringing it completely back up |
| poweroff | Shuts down the system by powering off. |
| reboot | Reboots the system. |
| shutdown | Shuts down the system. |
2. Shell Scripting Programs
How to check Shells installed in unix operating system?
$cat /etc/shells [Use this command to view Shells installed in unix operating system]
/bin/sh
/bin/bash
/sbin/nologin
/bin/tcsh
/bin/csh
/bin/zsh
/bin/ksh
bash shell is here most powerfull shell.
cat /etc/passwd – you can view all the users
How to change the shell
chsh [Type this command to change the shell and give respective login when login]
whoami – It displays somebody who logged in right now.
who – It displays all the users that are currently logged on to the system
users all the users that are currently logged in a little bit of different format comparing who command
uname – It shows You are logged on to a linux machine
uname -r – It shows your kernell release
echo “Hi There” Command Dispalys Hi There
clear the screen using clear command
bash script file ends with .sh extension
A very first line of your script is the name of your shell with #!
#!/bin/bash which is going to act as middle lane between shell script and kernel
Write any comment or description followed by #
# this is my first script
Shell script is a list of unix commands written in a text file
ls -l – It shows permissions, owner, last modification date and time and file name of the file
Use chmod command to give permission to the script file
chmod u+x scriptfile.sh
it will give the excute permission to the owner of the file.
#!/bin/bash
clear
echo “————”
echo “First script”
echo “————”
Output:
————
First script
————
Variables dont have Data types and length. We dont need to declare the variables.
a=’raj,
b=’kumar’
echo $a when executing this you will get raj
echo $b when executing this you will get kumar
How to concatenate the both?
c=$a$b
echo $c when executing this you will get rajkumar
If you want spaces between the name use double quotes and give space between them like c=”$a $b”
echo $c execute this command you will get raj kumar
What is read command in unix shell scripting?
read -p ‘What is your name’ nm
If you execute above command you will get output as What is your name. If you type your name to answer for the question for example rajkumar, Then the answer will be assigned to nm.
echo $nm will give the output as rajkumar
#!/bin/bash
#This is a shell script to ask user his/her input
clear
read -p “What is your name : ” nm
echo “Hello $nm , How are you?”
The above script asks you What is your name, if you answers as raj. It will ask again Hello raj , How are you?
How to run the script without giving permission?
Use bash script.sh – It will work
How do u do mathematical calculation in shell scripting?
a=”10″
b=”20″
echo $a – you will get 10
echo $b – you will get 20
c=a+b
echo $c – It will give a+b
c=$a+$b
echo $c – It will give 10+20
c=$(( $a + $b )) —use parenthesis
echo $c – It will give 30
A simple calculator shell scripting
#!/bin/bash
#A simple calculator
clear
echo “—————————————-”
readp -p “Please enter the first number : ” a
readp -p “Please enter the second number : ” b
sum=$(( $a + $b ))
sum=$(( $a – $b ))
echo ” $a + $b = $sum”
echo ” $a – $b = $sub”
echo “—————————————-”
Save above script and execute. Probabily It will ask first and second number. Give 100 and 50.
Your output displays like this…
—————————————-
Please enter the first number : 100
Please enter the second number : 50
100 + 50 = 150
100 – 50 = 50
—————————————-
What is User defined variables?
We will declare the variables like a=2 and b=4 These User defined variables.
System Defined Variables?
System Defined Variables are the variables already defined by the system. Also valuses are already assigned to that variables that we are using for our own benefits.
How to see the System Defined Variables?
Type env, you will get like this and more…
PWD=/home/raj
HOME=/home/raj
TMP=/tmp
SHELL=/bin/bash
USER=raj
#!/bin/bash
# Some basic system defined variables
clear
echo “Value of variable HOME = $HOME”
echo “Value of variable SHELL = $SHELL”
echo “Value of variable USER = $USER”
echo “Value of variable PWD = $PWD”
echo “Value of variable PATH= $PATH”
Run this above script, you can get the system defined variables
Here we learned about Types of shells, Variables(User, System), Mathematical calculation using shell scripting and some basic shell scripts.
What is the difference between the single quote and double quotes in shell scripts?
a=’raj’ – It is Literal (Whatever you write in single quotes is literal)
b=’$a’
echo $b we can get $a not raj.
use double quotes you will get actual value.
b=”$b”
echo $b You can get raj now
Double quotes:
Whenever you want any variable to be evaluated before getting the sign to the another variable or before getting printed to the another variable then you should use double quotes.
Single Quotes:
If you want the stings to be literally as it is to be assigned to another variable or printed, you should use single quotes
Inverted Quotes:
a=`whoami`
echo $a
output= raj
k=`hostname`
echo $k
sql.example.com
p=`users`
echo $p
raj rajkumar
It displays the users
#!/bin/bash
a=`whoami’
if [ $a = ‘root’ ]; then
echo ” You are authorized to execute this script”
else
echo ” You are not authorized to execute this script”
exit
fi
echo “Hello there”
date
cal
If we execute above script in root user login we can able to view date and calender
other wise it throws You are not authorized to execute this script
#!/bin/bash
a=`whoami’
if [ $a = ‘root’ ]; then
echo ” You are authorized to execute this script”
elif [ $a = ‘raj’ ]; then
echo ” You are also authorized to execute this script”
else
echo ” You are not authorized to execute this script”
exit
fi
echo “Hello there”
date
cal
If either root or user raj were able to execute the command and they can able to see the result of date and calender command.
Number comparision in shell scripting
= -eq —equal
!= -ne —not equal
< -lt —less than
> -gt —greater than
>= -ge —greater than or equal
<= -le —less than or equal
How to check file or directory exists or not in shell scripting?
#!/bin/bash
read -p “Enter the name of the file ” file
if [ -f $file ]; then
echo “File named #file exists”
else
echo “File named $file does not exist”
fi
If you give the file name it will tell whether the file file is exists or not.
-d ===> To check the directory is exists or not
-x ===> To check Whether the file executable or not
-r ===> To check Whether the file readable or not
-w ===> To check Whether the file writable or not
-l ===> To check Whether the file link or not
#!/bin/bash
# Example of a for loop
for i in 1 2 3
do
echo “Value of variable i = $i”
done
Output :
Value of variable i = 1
Value of variable i = 2
Value of variable i = 3
What is seq command?
seq 1 2 10
Output:
1
3
5
7
9
Here 1 is starting point 2 is increment 10 is ending point
seq 1 1 5
Output:
1
2
3
4
5
Here 1 is starting point 1 is increment 5 is ending point
#!/bin/bash
# Example of a for loop
num=`seq 1 1 100`
for i in $num
do
echo “Value of variable i = $i”
done
This will replaces i by 1 to 100 like below…
Value of variable i = 1
Value of variable i = 2
.
.
.
.
.
Value of variable i = 100
===============
#!/bin/bash
# Example of a for loop
for i in 1 A 3
do
echo “Value of variable i = $i”
done
If we change the 2nd iteration as A, we will get the output below
Output :
Value of variable i = 1
Value of variable i = A
Value of variable i = 3
================
#!/bin/bash
# Example of a for loop
files=`ls /etc`
for i in $files
do
echo “Value of variable i = $i”
done
It displays entire files in /etc
output:
Value of variable i = file1
Value of variable i = file2
Value of variable i = file3
=========================
#!/bin/bash
# Example of a for loop
users=`cut -d’:” -f1 /etc/passwd`
for i in $users
do
echo “Value of variable i = $i”
done
Cut the entire line into fields
delimiter -d which separates by colon[ : ]here
-f1 ===> displays field 1
output:
Value of variable i = raj
Value of variable i = root
================
#!/bin/bash
# Example of a for loop
users=`cut -d’:” -f1 /etc/passwd`
for i in $users
do
if [ $i != ‘root’ ]; then
echo “Please wait. Changing the password for user $i”
echo “1234” | passwd –stdin $i
done
It changes the passwords for all users except root
==================
grep pmon ==>to view database currently running or not
#!/bin/bash
# how to logon to oracle database
export ORCALE_SID=focus1
sqlplus raj/raj <<EOF
show user
select sysdate from dual;
EOF
oracle data base name = focus1
Use ! or EOF
Output:
SQL> USER is ‘raj’
SQL>
SYSDATE
——–
23-DEC-13
============
Output Redirection in Shell scripting
One is >
Second is >>
whoami > a.txt
cat a.txt
output :
raj
if we use >> it will append to the existing file
whoami >> a.txt
cat a.txt
output :
raj
raj
What is standard error and how to capture standard error output?
Use the command 2> or 2>> or &>
ls -z 2> a.txt
cat a.txt
ls: invalid option — ‘z’
try ‘ls –help’ for more information.
ls -z 2>> a.txt
cat a.txt
ls: invalid option — ‘z’
try ‘ls –help’ for more information.
ls: invalid option — ‘z’
try ‘ls –help’ for more information.
clear
ls -z &> a.txt
cat a.txt
ls: invalid option — ‘z’
try ‘ls –help’ for more information.
$ amberson symbol
=================
wc – < /etc/passwd ===> std in
731
<< (here document)
< (stdin)
&>> (both stdout and stderr)
Detaily Updated Soon…