ShellScripting

Basics of shell scripting

Agenda

• Variables
• Mathematical operations
• Conditions / Loops
• File Handling
• User input
• Text Manipulation

Working with commands

Lets go through some random commands and its definitions

man - shows up the manual of each command in linux
which - display an executables location
head - displays first 10 lines of a file (more # of lines can be displayed if needed)
tail - displays last 10 lines of a file (more # of lines can be displayed if needed)
sort - sort the lines of text
uniq - report or omit repeated lines
wc - print newline, word and byte counts for each file
echo - print some messages
file - prints the type of file 
touch - creates a file
					

Files and permissions

We can see a file permission using ls -l , stat commands. Modification of permission can be done via chmod,chown,chgrp commands

Environment variables

There are two types of variables in the bash shell :

Global Variables

eg:- $HOME,$LOGNAME 

Local Variables

eg:- $Abcd,$Test_1 

The BackTick

The backtick allows you to assign the output of a shell command to a variable.

				    	
#!/bin/bash
# using the backtick character
testing=`date`
echo "The date and time are: " $testing
				    	
					

Output redirection

The most basic type of redirection is sending output from a command to a file. The bash shell uses the greater-than symbol for this.

command > outputfile 

Anything that would appear on the screen as an output of a command, is now getting stored in the outputfile specified

				    	
$date > test6
$ ls -l test
$ cat test6
Tue Sep 24 17:56:58 EDT 2007
				    	
					

Performing Math

We use expr command to do math in bash

				    	
$ expr 1 + 5
6

$ expr 5 \* 2
10
				    	
				    

				    	
#!/bin/bash
# An example of using the expr command
var1=10
var2=20
var3=`expr $var2 / $var1`
echo The result is $var3
				    	
				    

Using brackets

In bash, when assigning a mathematical value to a variable, you can enclose the mathematical equation using a dollar sign and square brackets $[ operation ]

				    	
$ var1=$[1 + 5]
$ echo $var1
6
$ var2 = $[$var1 * 2]
$ echo $var2
12
$ cat test7
#!/bin/bash
var1=100
var2=50
var3=45
var4=$[$var1 * ($var2 - $var3)]
echo The final result is $var4
				    	
				    

Every command that runs in the shell uses an exit status to indicate to the shell that it’s done processing. The exit status is an integer value between 0 and 255 that’s passed by the command to the shell when the command finishes running. Linux provides the $? special variable that holds the exit status value from the last command that executed

				    	
$ date
Sat Sep 29 10:01:30 EDT 2007
$ echo $?
0
$
$ asdfg
-bash: asdfg: command not found
$ echo $?
127
$
$ cat test13
#!/bin/bash
# testing the exit status
var1=10
var2=30
var3=$[ $var1 + $var2 ]
echo The answer is $var3
exit 5
$
$ chmod u+x test13
$ ./test13
The answer is 40
$ echo $?
5
				    	
				    

if command
then
commands
else
commands
fi
				    

If the command in the if statement line returns with an exit status code of zero, the commands listed in the then section are executed, just as in a normal if-then statement. If the command in the if statement line returns a non-zero exit status code, the bash shell executes the commands in the else section.

if command1
then
commands
elif command2
then
more commands
fi
				    

n1 -eq n2	 Check if n1 is equal to n2.
n1 -ge n2	 Check if n1 is greater than or equal to n2.
n1 -gt n2 	 Check if n1 is greater than n2.
n1 -le n2	 Check if n1 is less than or equal to n2.
n1 -lt n2	 Check if n1 is less than n2.
n1 -ne n2	 Check if n1 is not equal to n2.
				    

str1 = str2 	     Check if str1 is the same as string str2.
str1 != str2 	     Check if str1 is not the same as str2.
str1 < str2 	     Check if str1 is less than str2.
str1 > str2 	     Check if str1 is greater than str2.
-n str1 	     Check if str1 has a length greater than zero.
-z str1 	     Check if str1 has a length of zero.
				    

-d file 	Check if file exists and is a directory.
-e file 	Checks if file exists.
-f file 	Checks if file exists and is a file.
-r file 	Checks if file exists and is readable.
-s file 	Checks if file exists and is not empty.
-w file 	Checks if file exists and is writable.
-x file 	Checks if file exists and is executable.
-O file 	Checks if file exists and is owned by the current user.
-G file	Checks if file exists and the default group is the same as the currentuser.
file1 -nt file2 	Checks if file1 is newer than file2.
file1 -ot file2 	Checks if file1 is older than file2.
				    

for var in list
do
commands
done
				    

$ cat test1
#!/bin/bash
# basic for command
for test in Alabama Alaska Arizona Arkansas California Colorado
do
echo The next state is $test
done
$ ./test1
The next state is Alabama
The next state is Alaska
The next state is Arizona
The next state is Arkansas
The next state is California
The next state is Colorado
				    

for (( variable assignment ; condition ; iteration process ))

for (( a = 1; a ‹ 10; a++ ))
				    

while test command
do
other commands
done
				    

until test commands
do
other commands
done
				    

By using for loop, each lines can be obtained for further processing

#!/bin/bash
for entry in `cat /etc/passwd`
do
        echo "Values in $entry -"
        for value in $entry
        do
                echo " $value"
        done
done
				    

The bash shell assigns special variables, called positional parameters, to all of the parameters entered in a command line. This even includes the name of the program the shell executes. The positional parameter variables are standard numbers, with $0 being the name of the program, $1 being the first parameter, $2 being the second parameter, and so on, up to $9 for the ninth parameter

$ cat test1
#!/bin/bash
# using one command line parameter
factorial=1
for (( number = 1; number ‹= $1 ; number++ ))
do
factorial=$[ $factorial * $number ]
done
echo The factorial of $1 is $factorial

$ ./test1 5
The factorial of 5 is 120
				    

The read command accepts input from the standard input (the keyboard), or from another file descriptor. After receiving the input, the read command places the data into a standard variable. Here’s the read command at its simplest:

$ cat test21
#!/bin/bash
# testing the read command
echo -n "Enter your name: "
read name
echo "Hello $name, welcome to my program.“

$ ./test21
Enter your name: Test
Hello Test, welcome to my program.


$ cat test27
#!/bin/bash
# hiding input data from the screen
read -s -p "Enter your password: " pass
echo
echo "Is your password really $pass?“

$ ./test27
Enter your password:
Is your password really T3st1ng?
				    

It reads one line of data at a time from the input, matches that data with the supplied editor commands, changes data in the stream as specified in the commands, then outputs the new data to STDOUT. After the stream editor matches all of the commands against a line of data, it reads the next line of data and repeats the process. After the stream editor processes all of the lines of data in the stream, it terminates.

$ echo "This is a test" | sed ’s/test/big test/’
This is a big test
				    

Let's try an example

$ cat data1
The quick brown fox jumps over the lazy dog.
The quick brown fox jumps over the lazy dog.
The quick brown fox jumps over the lazy dog.
$ sed ’s/dog/cat/’ data1
The quick brown fox jumps over the lazy cat.
The quick brown fox jumps over the lazy cat.
The quick brown fox jumps over the lazy cat.
$
				    

gawk is a more advanced tool for manipulating data in a file. Within this programming language you can:

• Define variables to store data.
• Use arithmetic and string operators to operate on data.
• Use structured programming concepts, such as if-then statements and loops, to add logic to your data processing.
• Generate formatted reports by extracting data elements within the data file and repositioning them in another order or format.

$echo "This is windows" | gawk '{$3="Linux"; print $0}' 
This is Linux