What is ShellScripting ?
- Shell scripting is used to automate certain tasks, make processes easy for user such as installation, monitor system resources, etc.
- Default linux shell is BASH -> Bourne Again Shell replacement for Bourne Shell. Written by Brian Fox.
- It is basically an interpretter, which interprete our commands and convert it into binary for CPU to understand.
- Bash also can read commands from file which is also called as Script.
- Script is nothing but a sequence of commands in a simple ASCII text
How to write bash script ?
Shebang Line:
#!/bin/bash –> Shebang line
Every shell script starts with this Shebang line.
#!<interpreter> <options>
When I write a script abc.sh. In that I have mentioned shebang line as #!/bin/csh
Then when I execute that script by “./abc.sh”, actually it will get executed like:
/bin/csh abc.sh
Please note that # is used to comment line in Shell scripting. If you want to any line just for information or not to execute then you should put ‘#’ in the beginning of that line.
eg. test.sh
#!/bin/bash –> Shebang line
echo “Hello World”
And to execute it, Please go into the directory where that shellscript is stored and run
./test.sh
Variables & echo statement
Variables are container and when we call it with $ that means we are opening the container and getting what is inside.
define varialbles by <variable>=value eg. var=123
getting variable value by using $ sign: echo $var
Reading keyboard inputs
echo -e “What is your name: c”
read <variable_name> eg. read myname
To get value of “myname” use echo $myname
File Descripters and redirecting
File Descriptors : 0 –> stdin Def:keyboard
: 1 –> stdout Def:screen
: 2 –> stderr Def:screen
: /dev/null – Bit Bucket
To redirect output of command to a file
command > /path/to/file
To redirect errors of command to a file
command 2> /path/to/file
To redirect both output and errors to a file
command > /path/to/file 2>&1
To hide output and error of command
command > /dev/null 2>&1
Executing commands inside script using backtick
val=`uname -a`
echo $val —> you will get output of ‘uname -a’ command.
“If” statement
if [ $var <operator> value ]
then
<command>
else
<command>
fi
operators:
-lt or < –> less than
-gt or > –> greater than
-le or <= –> less than and equal to
-ge or >= –> greater than and equal to
-eq or == –> equal
Please remember comparing variable with numerical value for maximum compatibility use -lt,-gt instead of symbolic operators.
“case” statement
case $val in
value1)
commands
;;
value2)
commands
;;
*)
default commands
;;
esac
Function
Function is a set of commands which are repeatedly require in same or different scripts
To define function
function_name() {
command1
command2
….
}
To call function in same script
function_name
To call functions in other shellscript
Write all required functions in one file
Call that file inside shellscript
source </path/to/script name> OR
. </path/to/script_name>
and call function name directly.
What is Retrun code $?
Whenever we execute any command, it stores an exit value to bash. That value is called return code.
if return code is 0 then command executed successfully or else command failed.
To get return code, give “echo $?” immidiate after executing command.
Eg. To test return code using if statement
var=`echo $?` —> Enter value of return code in Variable
if [ $var == 0 ]
then
command
else
command
fi
Passing arugements to script .. $0 $1 $2
<script_name> argument1 argument2
| | |
| | |
$0 $1 $2
So if I want argument1 should be used inside the script then
In script I should get the value of argument1 by using “echo $1” command.
Conditional expressions in if statement
-a file –> True if file exists.
-b file –> True if file exists and is a block special file.
-c file –> True if file exists and is a character special file.
-d file –> True if file exists and is a directory.
-e file –> True if file exists.
-f file –> True if file exists and is a regular file.
-g file –> True if file exists and its set-group-id bit is set.
-h file –> True if file exists and is a symbolic link.
-k file –> True if file exists and its “sticky” bit is set.
-p file –> True if file exists and is a named pipe (FIFO).
-r file –> True if file exists and is readable.
-s file –> True if file exists and has a size greater than zero.
-t fd –> True if file descriptor fd is open and refers to a terminal.
-u file –> True if file exists and its set-user-id bit is set.
-w file –> True if file exists and is writable.
-x file –> True if file exists and is executable.
-O file –> True if file exists and is owned by the effective user id.
-G file –> True if file exists and is owned by the effective group id.
-L file –> True if file exists and is a symbolic link.
-S file –> True if file exists and is a socket.
-N file –> True if file exists and has been modified since it was last read.
file1 -nt file2 –> True if file1 is newer (according to modification date) than file2, or if file1 exists and file2 does not.
file1 -ot file2 –> True if file1 is older than file2, or if file2 exists and file1 does not.
file1 -ef file2 –> True if file1 and file2 refer to the same device and inode numbers.
-o optname –> True if shell option optname is enabled. The list of options appears in the description of the -o option to the set builtin (see The Set Builtin).
-z string –> True if the length of string is zero.
example 1 ++++ Checking file ++++:
if [ -f /var/log/messages ]
then
echo -e “Yes messages file is there”
else
echo -e “No messages file is not there”
fi
example 2 ++++ checking strings +++++
val=`df -h|grep neelesh`
if [ -z $val ]
then
echo “Neelesh partition is not mounted”
else
echo “Neelesh partition is mounted”
fi
“for” loop
for varname in list
do
command1
command2
done
Here for, in, do, done are keywords.
list contain list of values
varname is any bash variable name
example 1: ++++
for i in 1 2 3 4 5
do
echo “Welcome $i times”
done
example 2: ++++
for i in {1..5}
do
echo “Welcome $i times”
done
example 3: ++++
for i in {0..10..2}
do
echo “Welcome $i times”
done
example 4: ++++
for i in $(seq 1 2 20)
do
echo “Welcome $i times”
done
example 5: ++++
for (( EXP1; EXP2; EXP3 ))
do
command1
command2
command3
done
——————
for (( i=1; i<=7; i++))
do
echo “Welcome $i times”
done
“while” loop
while [ condition ]
do
command1
command2
command3
done
example 1: ++++
x=1
while [ $x -le 5 ]
do
echo “Welcome $x times”
x=$(( $x + 1 ))
done
example 2: ++++
Script.sh
#!/bin/bash
counter=$1
factorial=1
while [ $counter -gt 0 ]
do
factorial=$(( $factorial * $counter ))
counter=$(( $counter – 1 ))
done
echo $factorial
Basic regular expressions
A regular expression is a pattern that describes a set of strings.
Regular expressions are constructed analogously to arithmetic expressions by using various operators to combine smaller expressions.
Most characters, including all letters and digits, are regular expressions that match themselves. Any metacharacter with special meaning may be quoted by preceding it with a backslash.
Use of regular expressions provides the ability to perform multiple tasks at one time,
regular expressions save time and resources on a server. Instead of performing many passes
over the same text in order to achieve one goal at a time, you can achieve many goals using regular expressions.
* – matches 0 or more times
? – matches 0 or one times
+ – matches 1 or more times
[] – define character classes of one character [Bb]
() – group characters and to provide alternation (mail | db), group text
^ – anchor text at start of line
$ – anchor text at end of line
| – matches alternate words
. – match only one character
– escapes following character
{} – qualifier for max or min, max {1,6}
b. Regular Characters include: A-Z, a-z, 0-9 and “_”.
example 1.
tail me*.[1-3]
example 2.
ps aux | grep -E 10[1-9][1-9]
example 3.
ps aux | grep -E 10[1-9]{2}
ps aux | grep -E 10[1-9]{4}
example 4.
ls ?[[:digit:]][[:digit:]][[:digit:]]?
POSIX Character Classes. [:class:]
This is an alternate method of specifying a range of characters to match.
[:alnum:] matches alphabetic or numeric characters. This is equivalent to A-Za-z0-9.
[:alpha:] matches alphabetic characters. This is equivalent to A-Za-z.
[:blank:] matches a space or a tab.
[:cntrl:] matches control characters.
[:digit:] matches (decimal) digits. This is equivalent to 0-9.
[:graph:] (graphic printable characters). Matches characters in the range of ASCII 33 – 126. This is the same as [:print:], below, but excluding the space character.
[:lower:] matches lowercase alphabetic characters. This is equivalent to a-z.
[:print:] (printable characters). Matches characters in the range of ASCII 32 – 126. This is the same as [:graph:], above, but adding the space character.
[:space:] matches whitespace characters (space and horizontal tab).
[:upper:] matches uppercase alphabetic characters. This is equivalent to A-Z.
[:xdigit:] matches hexadecimal digits. This is equivalent to 0-9A-Fa-f.
Here document
It is a form of I/O redirction to feed a list of commands to interactive program or command such as ftp, usersadd, etc.
example:
#!/bin/bash
echo -e “Enter username to add: c”
read var1
echo -e “Enter desired password: c”
read var2
useradd $var1
passwd $var1 <<EOF ###### Here document
$var2
$var2
EOF
