Introduction
Linux Shell
Computer understand the language of 0's and 1's called binary language. Shell is a special program which accepts instruction or commands in English and if its a valid command, it passes the command to the kernel. Shell is not part of system kernel, but uses the system kernel to execute programs, create files etc.
Shell Script
Normally shells are interactive. It means shell accept command from you (via keyboard) and execute them. But if you use command one by one (sequence of 'n' number of commands) , the you can store this sequence of command to text file and tell the shell to execute this text file instead of entering the commands. This is know as shell script.
How to write shell script
- Write shell script with a text example (for example vi).
- set execute permission: chmod +x your-script-name
- Execute script:bash your-script-name or ./your-script-name
Example
$vi first.sh
clear
echo "first shell script"
$chmod +x first.sh
$./first.sh
Basics
Variables
In Linux (Shell), there are two types of variable:
- System variables - Created and maintained by Linux itself. This type of variable defined in CAPITAL LETTERS.
- User defined variables (UDV) - Created and maintained by user. This type of variable defined in lower letters.
#!/bin/bash
x=10
X=20
$y=
echo "x = $x"
echo "X = $X"
echo "y = $y"
Comments
#!/bin/bash
echo "A comment will follow."
echo "The # here does not begin a comment."
echo 'The # here does not begin a comment.'
echo The \
echo The
echo ${PATH
echo $(( 2
Quotes
- Double Quotes: " " -Anything enclose in double quotes removed meaning of that characters (except \ and $).
- Single quotes: ' ' - Enclosed in single quotes remains unchanged.
- Back quote: ` ` - To execute command
Shell Built in Variables
| ~ Shell Built in Variables | ~ Meaning |
| $# | Number of command line arguments. |
| $? | Exit Status |
| $* | string that contains all arguments to shell |
| $@ | Same as above, except when quoted. |
| $- | Option supplied to shell |
| $$ | PID of shell |
| $! | PID of last started background process (started with &) |
Input - Output redirection
Redirection symbols
- >: To output Linux-commands result to file.
- »: To output Linux-commands result to END of file.
- <: To take input to Linux-command from file instead of keyboard.
Example 1
#!/bin/bash
clear
echo "ls > file_list"
ls > file_list
echo "ls -la >> file_list"
ls -la >> file_list
echo "cat < file_list"
cat < file_list
Example 2
$cat > sname
vivek
ashish
zebra
babu
Press CTRL + D to save.
$ sort < sname > sorted_names
$ cat sorted_names
Example 3
$ tr "[a-z]" "[A-Z]" < sname > cap_names
$ cat cap_names
Example 4
$ sort > new_sorted_names < sname
$ cat new_sorted_names
Pipes
A pipe is a way to connect the output of one program to the input of another program without any temporary file.
Example
$ ls |
$ who | sort
$ who | sort > user_list
$ who | wc -l
$ ls -l | wc -l
$ who | grep chrys
Filter
If a Linux command accepts its input from the standard input and produces its output on standard output is know as a filter. A filter performs some kind of process on the input and gives output.
Example
Suppose you have file called 'hotel.txt' with 100 lines data, And from 'hotel.txt' you would like to print contains from line number 20 to line number 30 and store this result to file called 'hlist' then give command:
$ tail +20 < hotel.txt | head -n30 >hlist
Processes
A process is program (command given by user) to perform specific Job. In Linux when you start process, it gives a number to process (called PID or process-id), PID starts from 0 to 65535.
Language Constructs
if condition
Example 1: Mathematical Operators
#!/bin/bash
if test $1 -gt 0
then
echo "$1 > 0"
fi
if test $1 -ge 0
then
echo "$1 >= 0"
fi
if test $1 -eq 0
then
echo "$1 == 0"
fi
if test $1 -ne 0
then
echo "$1 != 0"
fi
if test $1 -lt 0
then
echo "$1 < 0"
fi
if test $1 -le 0
then
echo "$1 <= 0"
fi
Example 2: Logical Operators
if test $1 -lt 0
then
echo "$1 < 0"
fi
if test $1 -le 0
then
echo "$1 <= 0"
fi
Example 3: String Operators
string_null=""
string1="string1"
if [ $string_null -n ]
then
echo "not null string"
else
echo "null string"
fi
if [ $string_null -z ]
then
echo "null string"
else
echo "not null string"
fi
if [ "$string_null" == "$string1" ]
then
echo "strings equal"
else
echo "strings not equal"
fi
if [ "$string_null" != "$string1" ]
then
echo "strings not equal"
else
echo "strings equal"
fi
Example 4: Test for files and directories
#!/bin/bash
if test -s $1
then
echo "$1 not empty file"
fi
if test -f $1
then
echo "$1 normal file. Not a directory"
fi
if test -e $1
then
echo "$1 exists"
fi
if test -d $1
then
echo "$1 is directory and not a file"
fi
if test -r $1
then
echo "$1 is read-only file"
fi
if test -x $1
then
echo "$1 is executable"
fi
if…else…fi
If given condition is true then command1 is executed otherwise command2 is executed.
#!/bin/sh
if [ $
then
echo "$0 : You must give/supply one integers"
exit 1
fi
if test $1 -gt 0
then
echo "$1 number is positive"
else
echo "$1 number is negative"
fi
Multilevel if-then-else
#!/bin/sh
if [ $1 -gt 0 ]; then
echo "$1 is positive"
elif [ $1 -lt 0 ]
then
echo "$1 is negative"
elif [ $1 -eq 0 ]
then
echo "$1 is zero"
else
echo "Opps! $1 is not number, give number"
fi
Loops
for loops
Example 1
$ cat > testfor
for i in 1 2 3 4 5
do
echo "Welcome $i times"
done
Example 2
#!/bin/sh
if [ $
then
echo "Error - Number missing form command line argument"
echo "Syntax : $0 number"
echo "Use to print multiplication table for given number"
exit 1
fi
n=$1
for i in 1 2 3 4 5 6 7 8 9 10
do
echo "$n * $i = `expr $i \* $n`"
done
while loops
#!/bin/sh
if [ $
then
echo "Error - Number missing form command line argument"
echo "Syntax : $0 number"
echo " Use to print multiplication table for given number"
exit 1
fi
n=$1
i=1
while [ $i -le 10 ]
do
echo "$n * $i = `expr $i \* $n`"
i=`expr $i + 1`
done
case
if [ -z $1 ]
then
rental="*** Unknown vehicle ***"
elif [ -n $1 ]
then
rental=$1
fi
case $rental in
"car") echo "For $rental Rs.20 per k/m";;
"van") echo "For $rental Rs.10 per k/m";;
"jeep") echo "For $rental Rs.5 per k/m";;
"bicycle") echo "For $rental 20 paisa per k/m";;
*) echo "Sorry, I can not gat a $rental for you";;
esac
Debugging Shell Scripts
-v Print shell input lines as they are read.
-x After expanding each simple-command, bash displays the expanded value of PS4 system variable, followed by the command and its expanded arguments.
Advanced Features
Local and Global Shell variables
Local variable can be used in same shell only.
Global variables or environment variables are available in all shells. Commands env or printenv can be used to display environment variables.
Functions
Function is series of instruction/commands. Function performs particular activity in shell i.e. it had specific work to do or simply say task.
sum()
{
if [ -z "$2" ]; then
echo $1
else
a=$1;
shift;
b=`sum $@`
echo `expr $a + $b`
fi
}
Reading from the shell
Example 1
echo "Name?"
read name
echo "Age?"
read age
echo "Hello $name, you are $age years old"
Example 2
while :
do
clear
echo "-------------------------------------"
echo " Main Menu "
echo "-------------------------------------"
echo "[1] Show Todays date/time"
echo "[2] Show files in current directory"
echo "[3] Show calendar"
echo "[4] Start editor to write letters"
echo "[5] Exit/Stop"
echo "======================="
echo -n "Enter your menu choice [1-5]: "
read yourch
case $yourch in
1) echo "Today is `date` , press a key. . ." ; read ;;
2) echo "Files in `pwd`" ; ls -l ; echo "Press a key. . ." ; read ;;
3) cal ; echo "Press a key. . ." ; read ;;
4) vi ;;
5) exit 0 ;;
*) echo "Opps!!! Please select choice 1,2,3,4, or 5";
echo "Press a key. . ." ; read ;;
esac
done
