Introduction
| If a group of commands or command list are used frequently, such list of commands can be stored in a file so that they are analyzed and executed by a shell as a single entity.
A file containing such command names is called a shell script. Shell scripts are text files and can be created with an editors like vi, emacs, pico etc. |
Creating & Executing
A Shell can be executed in two forms:
- sh scriptname
- scriptname (only when the execute permission of the shellscript is enabled)
$ ls -l
-rwxr-xr-x 1 shelluser shellgroup Dec 10 01:22 shellscript
Example:
Creating and executing a shell script named script1 that executes pwd and ls commands
$ pico script1
pwd
ls Quit the editor with Ctrl-X
Now run the script as:
$ sh script1
/home/kundan — Result of pwd execution
TEST Maildir mbox — Result of ls execution
If the execute permission of the script is enabled then the script can also be run merely by specifying its pathname.
You can enable the execute permission by issuing the chmod command:
$ chmod +x script1
After enabling the execute bit of the file, the script can be run as:
$ ./script1
Note: Shell scripts may contain comment lines. To code a comment line, place a # symbol before the comment line.
#Shell Script to display the pwd and ls command
pwd #Displays the current directory
ls #Displays the list of directories and files in the current directory
Parameters
Shell scripts can contain parameters that are equivalent to variables in a program.
Types of Parameters:
- Positional Parameters
- Shell Variable
- Special Parameter
By using these types of parameters, the user can create and run program-equivalent files without the use of programming language.
Positional Parameters
The user can specify arguments in a shell script at run time as in ordinary commands. The shell script uses positional parameters as parameters for receiving those arguments.
$ sh scriptname [arg1 arg2 ………….. argn]
$0 $1 $2 ……….. $n
A positional parameter is represented by a $ symbol followed by a number (0-9) that identifies the ordinal position of the parameter.
Example:
Running shell script script2 that examines the contents of the parameters specifying two arguments
Contents of script2
echo $0
echo $1
echo $2
Execute the shellscript script2
$ sh script2 param1 param2
script2
param1
param2 Execution Result
Shell Variables
Shell variables are similar to variables in a program. They are used to define and change values in shell scripts.
a) Shell variable naming conventions
- A shell variable must begin with a letter or underscore
- A shell variable must be a combination of letters, digits and underscores
b) Assigning a value to a shell variable
shellvariable = value
c) The value of a shell variable is referenced by prepending a $ symbol to the variable name.
$shellvariable
Example:
Defining and displaying the value of variable admin in a shell script to string administrator
Contents of script3
admin=administrator
echo This training is for system $admin
Run the Script
$ sh script3
This training is for system administrator
Special Parameters
Special Parameters are automatically passed by the system for the user to use them to establish his or her execution environment.
Major Special Parameters:
a) HOME
Holds the path name of the home directory of currently logged user
b) PATH
Holds a list of path names of directories that shell programs frequently searches for commands. Since commands are stored in various directories, their path names are separated by colons. Eg: /bin:/usr/bin:/usr/local/bin
c) PS1
Holds the primary prompt symbol. The default value of PS1 is a $ symbol and a space.
Escape Symbols
If the user wants to handle characters that have special meaning to a shell as ordinary characters in a string to be given as a command argument, it is necessary to escape them.
‘(single quotation): String enclosed in single quotation marks are escaped except the quotation mark.
“(double quotation): String enclosed in double quotation marks are escaped except parameter values and the quotation mark.
\single-character: All single-characters following a backslash (\) are escaped.
The above symbols escape the characters that are handled by a shell as special characters. The characters that a shell handles as special characters include the following:
; & | ( ) ^ < > ? * [ ] space newline
Example:
* echo ‘?message?’ handling a ? which the shell regards as a special character as an ordinary character
Execution Result: ?message?
* echo ‘e n d’ To include two or more spaces in a message, enclose them with single quotatios
Execution Result: e n d
* echo “*$HOME*” When displaying * as an ordinary character and replacing the variable with its value
Execution Result: */home/kundan*
* echo \\111 To handle a backslash as an ordinary character, specify two backslashes consecutively.
Execution Result: \111
Commands
- echo command
<<General Format>>
echo [arg1 arg2…… argn]
The echo command echoes the string specified in arguments on the standard output as is. It is used to display messages during the execution of shell scripts.
Example:
Displaying messages at the beginning and end of a shell script
Contents of script4
echo –start of a program-
pwd
ls
echo –end of a program-
Run
$ sh script4
-start of a program-
/home/kundan
TEST Maildir mbox
-end of a program-
- 2. read command
<<General Format>>
read shellvariable
The read command enables a value to be assigned to a shell variable not within a shell script but from the standard input at run time.
Example:
Shell script script5 assigns a value read from the standard input to shell variable X after displaying a message and extracts the lines that contain the value of shell variable X as the search string from the file specified in $1.
Contents of script5
echo input value of pattern
read x
grep “$x” $1
Run
$ sh script5 /etc/passwd
input value of pattern
^muk waits for a value from the keyboard
mukesh:x:501:502:/home/mukesh:/bin/bash
mukunda:x:503:502:/home/mukunda:/bin/bash
Note: The user can suppress a newline by using the escape symbol “\c” after displaying a message with an echo command. This escape symbol must be enclosed in single quotes. The similar can be done using “-n” option.
Eg:
echo “Input value of pattern ‘\c’
read X
OR
echo –n “Input value of pattern”
read X
- 3. Control Commands
Some shell command control the execution of shell scripts. Such commands include decision making commands and iteration control commands.
- Decision Making Commands
- I. if command
- II. case command
- Iteration Control Commands
- I. for command
- II. while command
a) Decision Making Commands
- I.
When if commands are nested
if list of conditions
then
list1
elif list of conditions
then
list1
else
list2
fi
if command
<<General Format>>
if list of conditions
then
list1
else
list2
fi
The operators that can be specified in the conditional expression differ for arithmetic comparison and string comparison as summarized below:
- Arithmetic Comparison
| Operators Used | Meaning |
| number1 –gt number2
number1 –ge number2 number1 –lt number2 number1 –le number2 number1 –eq number2 number1 –ne number2 |
number1 > number2
number1 >= number2 number1 < number2 number1 <= number2 number1 = number2 number1 = number2 |
- String Comparison
| Operators Used | Meaning |
| String1 = string2
String1 != string2 String |
String1 = string2
String1 = string2 Whether string has assigned a value |
Example:
Shell Script script6 determines whether the value of argument1 specified at run time is greater that that of argument2
Contents of script6
|
if test $1 –gt $2
then
echo ‘para1 > para2’ Run
else
echo ‘para1 <= para2’
fi
- II. case command
<<General Format>>
case word in
pattern1) list1;;
pattern2) list2;;
.
.
.
patternN) listN;;
esac
The word part of the case command is primarily a parameter. The command compares the parameter value with the patterns sequentially from the top and, if a match occurs, the command executes the list following the matching pattern. Each list must be terminated by two consecutive semicolons (;).
Example:
Shell script script7 displays Linux if the run-time argument1 has the value L, Unix if the value is U, Windows if the value is W, and an error message if the value is none of the above.
Contents of script7
|
Case $1 in
L) echo Linux;;
U) echo Unix;;
W) echo Windows;;
*) echo Error !!
echo Try again;;
esac
Run
$ sh script7 L
Linux
$ sh script7 A
Error !!
Try again
Note: When executing the same pattern for two or more patterns, the user can separate the patterns by vertical bars.
When patterns L and l are specified in the above example: L | l) echo Linux;;
b) Iteration Control Commands
- I. for command
<<General Format>>
for shellvariable [in word1 word2 ….. wordn]
do
list
done
A list of commands to be executed repeatedly are specified in list. The words after the keyword in are ordinary strings. If the in part is specified, the list of commands are executed the number of times equal to the number of words specified in the in part. Firstly, word1 is assigned to the shell variable and the list is executed. Secondly, word2 is assigned to the shell variable and the list is executed, and so on, until the end of word list is reached.
Example:
1. shell script script8 extracts from file /etc/passwd the lines of data of users containing bash or pppd as shell.
Contents of script8
|
for x in bash pppd
do
echo “shell = $x” Run
grep “$x” /etc/passwd
done
- Omitting the in part to assign a string to the shell variable not from the shell script but at runtime.
Contents of script9
|
for x
do
echo “shell = $x” Run
grep “$x” /etc/passwd
done
- II. while command
<<General Command>>
while list for evaluating iteration condition
do
list
done
The while command repeatedly executes a list of commands while the given conditions are met.
Example:
Shell script10 first displays a message and while the character entered from the standard input is not Q, check to determine whether the number entered from the standard input is positive, negative or 0 and repeatedly displays messages accordingly.
Contents of script10
echo –n “please input: “
read val
while [ “$val” != “q” ]
|
do
if [ $val –gt 0 ]
then
echo input value is plus Run
elif [ $val –lt 0 ]
then
echo input value is minus
else
echo input value is zero
fi
echo –n “please input: “
read val
done
File Manipulation
Example Script script11 to display the username and shell of users from /etc/passwd file
Contents of script11
PASSFILE=/etc/passwd
STAT=0
COUNT=1
While [ $STAT = 0 ]
Do
LINE=`cat $PASSFILE | awk –v n=$COUNT ‘NR==n {print}’`
If [ “$LINE” = “” ]
Then
STAT=1
else
$USER=`echo $LINE | awk –F: ‘{print $1}’`
$SHELL=`echo $LINE | awk –F: ‘{print $7}’`
echo “$USER::$SHELL”
let COUNT=COUNT+1
fi
done
Functions
<<General Format>>
Defining a Function Calling a Function
|
|
Example:
Shell script script11 to display the mathematical result of two numbers using a function. Two numbers and the operator are passed as positional parameters.
Contents of script11
#Function to add, subtract and divide
function perform
{
case $3 in
add)
let RESULT=A+B;;
sub)
let RESULT=A-B;;
div)
let RESULT=A/B;;
*)
RESULT=”Error: Unknown Operator, Use add, sub, div”;;
esac
}
#start of main program
A=$1
B=$2
operator=$3
perform
echo $RESULT