Shell Script Constructs

Shell Script Constructs

case word in pattern ) ;;esac
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for i in expression; do #codedone
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for (i = 0; i < 10; i++); do #codedone
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function_name () { #code}
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if [ condition ]; then #codeelif [ condition ]; then #codeelse #codefi
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if [ condition ]; then #codeelse #codefi
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if [ condition ]; then #codefi
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until [ condition ]; do #codedone
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while [ condition ]; do #codedone

In Details:-
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Common environment variables

PATH - Sets the search path for any executable command. Similar to the PATH variable in MSDOS.

HOME - Home directory of the user.
MAIL - Contains the path to the location where mail addressed to the user is stored.

IFS - Contains a string of characters which are used as word seperators in the command line. The string normally consists of the space, tab and the newline characters. To see them you will have to do an octal dump as follows:
$ echo $IFS | od -bc
PS1 and PS2 - Primary and secondary prompts in bash. PS1 is set to $ by default and PS2 is set to '>' . To see the secondary prompt, just run the command :

$ ls |... and press enter.

USER - User login name.

TERM - indicates the terminal type being used. This should be set correctly for editors like vi to work correctly.

SHELL - Determines the type of shell that the user sees on logging in.


Note: To see what are the values held by the above environment variables, just do an echo of the name of the variable preceeded with a $. For example, if I do the following:



$ echo $USERravi... I get the value stored in the environment variable USER.

Some bash shell scripting rules

1) The first line in your script must be

#!/bin/bash ... that is a # (Hash) followed by a ! (ban) followed by the path of the shell. This line lets the environment know the file is a shell script and the location of the shell.



2) Before executing your script, you should make the script executable. You do it by using the following command:

$ chmod ugo+x your_shell_script.sh3) The name of your shell script must end with a .sh . This lets the user know that the file is a shell script. This is not compulsary but is the norm.


Conditional statements

The 'if' Statement - evaluates a condition which accompanies its command line. Those words marked in blue are compulsory. But those marked in red are optional.

syntax:

if condition_is_truethenexecute commandselseexecute commandsfiif condition also permits multiway branching. That is you can evaluate more conditions if the previous condition fails.

if condition_is_truethenexecute commandselif another_condition_is_truethenexecute commandselseexecute commandsfiExample :

if grep "linuxhelp" thisfile.htmlthenecho "Found the word in the file"elseecho "Sorry no luck!"fiif's companion - test

test is an internal feature of the shell. test evaluates the condition placed on its right, and returns either a true or false exit status. For this purpose, test uses certain operators to evaluate the condition. They are as follows:


Relational operators

-eq Equal to-lt Less than-gt Greater than-ge Greater than or equal to-lt Less than-le Less than or equal to File related tests

-f file True if file exists and is a regular file-r file True if file exists and is readable-w file True if file exists and is writable-x file True if file exists and is executable-d file True if file exists and is a directory-s file True if file exists and has a size greater than zero.String tests

-n str True if string str is not a null string-z str True if string str is a null stringstr1 == str2 True if both strings are equalstr1 != str2 True if both strings are unequalstr True if string str is assigned a value and is not null.Test also permits the checking of more than one expression in the same line.

-a Performs the AND function-o Performs the OR functionExample:

test $d -eq 25 ; echo $d... which means, if the value in the variable d is equal to 25, print the value.

test $s -lt 50; do_something

if [ $d -eq 25 ]thenecho $dfiIn the above example, I have used square brackets instead of the keyword test - which is another way of doing the same thing.

if [ $str1 == $str2 ]then do somethingfiif [ -n "$str1" -a -n "$str2" ]then echo 'Both $str1 and $str2 are not null'fi... above, I have checked if both strings are not null then execute the echo command.

Things to remember while using test

If you are using square brackets [] instead of test, then care should be taken to insert a space after the [ and before the ].

Note: test is confined to integer values only. Decimal values are simply truncated.

Do not use wildcards for testing string equality - they are expanded by the shell to match the files in your directory rather than the string.


Case statement
Case statement is the second conditional offered by the shell.
Syntax:

case expression inpattern1) execute commands ;;pattern2) execute commands ;;...esacThe keywords here are in, case and esac. The ';;' is used as option terminators. The construct also uses ')' to delimit the pattern from the action.


Example:

...echo "Enter your option : "read i;case $i in 1) ls -l ;; 2) ps -aux ;; 3) date ;; 4) who ;; 5) exitesacNote: The last case option need not have ;; but you can provide them if you want.

Here is another example:

case `date |cut -d" " -f1` in Mon) commands ;; Tue) commands ;; Wed) commands ;; ...esacCase can also match more than one pattern with each option.You can also use shell wild-cards for matching patterns.

...echo "Do you wish to continue? (y/n)"read anscase $ans in Y|y) ;;[Yy][Ee][Ss]) ;; N|n) exit ;;[Nn][Oo]) exit ;; *) echo "Invalid command"esac
In the above case, if you enter YeS, YES,yEs and any of its combinations, it will be matched.

This brings us to the end of conditional statements.

Looping Statements
while loop
Syntax :

while condition_is_truedo execute commandsdoneExample:

while [ $num -gt 100 ]do sleep 5donewhile :do execute some commandsdoneThe above code implements a infinite loop. You could also write 'while true' instead of 'while :' .

Here I would like to introduce two keywords with respect to looping conditionals. They are break and continue.

break - This keyword causes control to break out of the loop.

continue - This keyword will suspend the execution of all statements following it and switches control to the top of the loop for the next iteration.


until loop

Until complements while construct in the sense that the loop body here is executed repeatedly as long as the condition remains false.

Syntax:

until falsedo execute commandsdoneExample:

...until [ -r myfile ]dosleep 5doneThe above code is executed repeatedly until the file myfile can be read.

for loop
Syntax :

for variable in listdoexecute commandsdoneExample:

...for x in 1 2 3 4 5do echo "The value of x is $x";done
Here the list contains 5 numbers 1 to 5. Here is another example:

for var in $PATH $MAIL $HOMEdo echo $vardoneSuppose you have a directory full of java files and you want to compile those. You can write a script like this:

...for file in *.javado javac $filedoneNote: You can use wildcard expressions in your scripts.

A few special symbols and their meanings w.r.t shell scripts

$* - This denotes all the parameters passed to the scriptat the time of its execution. Which includes $1, $2and so on.$0 - Name of the shell script being executed.$# - Number of arguments specified in the command line.$? - Exit status of the last command.The above symbols are known as positional parameters. Let me explain the positional parameters with the aid of an example. Suppose I have a shell script called my_script.sh . Now I execute this script in the command line as follows :

$ ./my_script.sh linux is a robust OS... as you can see above, I have passed 5 parameters to the script. In this scenario, the values of the positional parameters are as follows:

$* - will contain the values 'linux','is','a','robust','OS'.
$0 - will contain the value my_script.sh - the name of the script being
executed.
$# - contains the value 5 - the total number of parameters.

$$ - contains the process ID of the current shell. You can use this parameter while giving unique names to any temporary files that you create at the time of execution of the shell.


$1 - contains the value 'linux'
$2 - contains the value 'is'
... and so on.

The set and shift statements
set - Lets you associate values with these positional parameters .
For example, try this:

$ set `date`$ echo $1$ echo $*$ echo $#$ echo $2shift - transfers the contents of a positional parameter to its immediate lower numbered one. This goes on as many times it is called.


Example :

$ set `date`$ echo $1 $2 $3$ shift$ echo $1 $2 $3$ shift$ echo $1 $2 $3To see the process Id of the current shell, try this:

$ echo $$2667Validate that it is the same value by executing the following command:

$ ps -f |grep bashread statement

Make your shell script interactive. read will let the user enter values while the script is being executed. When a program encounters the read statement, the program pauses at that point. Input entered through the keyboard id read into the variables following read, and the program execution continues.

Eg:

#!/bin/shecho "Enter your name : "read nameecho "Hello $name , Have a nice day."Exit status of the last command

Every command returns a value after execution. This value is called the exit status or return value of the command. A command is said to be true if it executes successfully, and false if it fails. This can be checked in the script using the $? positional parameter.



Here I have given a concise introduction to the art of bash shell scripting in Linux. But there is more to shell scripting than what I have covered. For one, there are different kinds of shells, bash shell being only one of them. And each shell has a small variation in its syntax. Like the C shell for example, which uses a syntax close to the C language for scripting. But what I have covered above applys to all the shells.