C-shell Scripting

made by B Chari K working in semiconductors domain

Shell ScriptsThe basic concept of a shell script is a list of commands, which are listed in the order of execution.There are conditional tests, loops, variables and files to read and store data.A script can include functions also.

Steps to create Shell script

Specify shell to execute program Script must begin with #! (pronounced “shebang”)

l to identify shell to be executedExamples:#! /bin/sh#! /bin/bash#! /bin/csh#! /usr/bin/tcsh Make the shell program executable

Use the “chmod” command to make the program/script file executable

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Example Script

Variables

Local variables – a variable present with in the current instance in the shell Environment variables – a variable available to any child process of the shell.Usually a shell script defines only those environment variables that are needed by the programs that it runs. Shell variables - A shell variable is a special variable that is set by the shell and is required by the shell in order to function correctly

Shell Logic Structures

Basic logic structures needed for program development:

Sequential logic User input Decision logic Looping logic Case logic

Input to a shell script

Reading user input Providing input as command line arguments Accessing contents of files

Reading User Input

There is a special C shell variable: $<

Reads a line from terminal (stdin) up to, but not including the new line

#! /bin/cshecho "What is your name?"set name = $<echo Greetings to you, $nameecho "See you soon"

Reading User Input

Command Line arguments

Use arguments to modify script behavior command line arguments become

positional parameters to C shell script positional parameters are numbered variables: $1, $2, $3 …

Command line arguments

Meaning$0 -- name of the script$1, $2 -- first and second parameter${10} -- 10th parameter { } prevents “$1” misunderstanding$* -- all positional parameters$#argv -- the number of arguments

Command Line arguments : Example

Decision Logic

if Statement: simplest formsif ( expression ) command

if ( expression ) thencommand(s)endif

if-then-else Statementif ( expression ) thencommand(s)elsecommand(s)endif

Decision Logic

Decision Logic If-then-else if Statement

if ( expression ) thencommand(s)else if ( expression ) thencommand(s)elsecommand(s)endif

Basic Operators in expressions

Meaning( ) grouping! Logical “not”

> >= < <= greater than, less than

== != equal to, not equal to

|| Logical “or”&& Logical “and”

Example

#! /bin/cshif ( $#argv == 0 ) then echo -n "Enter time in minutes: " @ min = $<else @ min = $1endif@ sec = $min * 60echo “$min minutes is $sec seconds”

Example

File Testing Operators

opr Meaningr Read accessw Write accessx Execute accesse Existencez Zero lengthf Ordinary filed directory

Syntax: if ( -opre filename )

Example

if ( -e $1 ) thenecho $1 existsif ( -f $1 ) then

echo $1 is an ordinary file

elseecho $1 is NOT ordinary

fileendif

else echo $1 does NOT existendif

Example : if-else

Looping constructs

predetermined iterations- repeat- foreach

condition-based iterations- while

Fixed Number Iterations

Syntax: repeatrepeat number command

l executes “command” “number” times

Examples:repeat 5 lsrepeat 2 echo “go home”

The Foreach Statement

foreach name ( wordlist ) commands end

wordlist is:list of words, ormulti-valued variable

each time through, foreach assigns the next item in wordlist to the variable

$name

Example : Foreach

foreach word ( one two three )echo $word

end

lor set list = ( one two three ) foreach word ( $list )

echo $word end

Loops with Foreach

Example:#! /bin/csh@ sum = 0foreach file (`ls`) set size = `cat $file | wc -c` echo "Counting: $file ($size)" @ sum = $sum + $sizeendecho Sum: $sum

Example : Foreach

While Statement

while ( expression )commands

end

use when the number of iterations is not known in advanceexecute ‘commands’ when the expression is trueterminates when the expression becomes false

Example : While

#! /bin/csh@ var = 5while ( $var > 0 )

echo $var@ var = $var – 1

end

Loop Control

lbreakends loop, i.e. breaks out of current loop

lcontinueends current iteration of loop, continues

with next iteration

Example : Loop Control

#! /bin/cshwhile (1) echo -n "want more? " set answer = $< if ($answer == "y") echo "fine" if ($answer == "n") break if ($answer == "c") continue echo "now we are at the end"end

Example : Loop Control Example

The Switch Statement

lUse when a variable can take different valueslUse switch statement to process different cases (case statement)lCan replace a long sequence of

if-then-else statements

lUse when a variable can take different valueslUse switch statement to process different cases (case statement)lCan replace a long sequence of

if-then-else statements

Example:switch (string)

case pattern1:command(s)

breakswcase pattern2:

command(s)breaksw default:

command(s)breaksw

endsw

Example : Switch

switch ($var) case one: echo it is 1 breaksw case two: echo it is 2 breaksw default: echo it is $var breakswendsw

Example : Switch#! /bin/csh# Usage: greeting name# examines time of day for greetingset hour=`date`switch ($hour[4]) case 0*: case 1[01]*: set greeting=morning ; breaksw case 1[2-7]*: set greeting=afternoon ; breaksw default: set greeting=eveningendswecho Good $greeting $1

Example : Switch

Quoting Mechanism

mechanism for marking a section of a command for special processing:

command substitution: `...` double quotes: “…“ single quotes: ‘…‘ backslash: \

Double Quotes

Prevents breakup of string into words Turn off the special meaning of most wildcard characters and the single quote

$ character keeps its meaning ! history references keeps its meaning

Examples:echo "* isn't a wildcard inside quotes"echo "my path is $PATH"

Single Quotes

lwildcards, variables and command substitutions are all treated as ordinary textlhistory references are recognized

Examples:echo '*'echo '$cwd' echo '`echo hello`' echo 'hi there !'

Back Slash

lbackslash character \treats following character literally

Examples:echo \$ is a dollar signecho \\ is a backslash

Wild cards

A wild card that can stand for all member s of same class of characters The * wild card

ls list* This will list all files starting with list

ls *list This will list all files ending with list

The ? Wild card ls ?ouse

This will match files like house, mouse, grouse

Regular Expressions Regular expression is a set of characters

that specify a pattern. Search for specific lines of text containing

a particular pattern Usually using for pattern matching A shell meta characters must be quoted

when passed as an expression to the shell

Anchor Characters : ^ and $

Regular Expression Matches^A A at the begining of lineA$ A at the end of the line

^\^ “^” at the beginning of line$\$ “$” at the end of the line^.$ Matches any character with

“.”

Example:Grep '^from:' /home/shastra/Desktop/file2

Searches all the lines starting with pattern ‘from’

Matching words with [ and ]

Regular expression matches[ ] The characters “[]” [0-9] Any number[^0-9] Any character other than

a number[-0-9] Any number or a “-”[]0-9] Any number or “]”[0-9]] Any number followed by

“]”^[0-9] A line starting with any

number[0-9]$ A line ending with any

number

Matching a specific number of sets with \{ and \}

Regular expression matches^AA*B Any line starts with one or

more A s followed by B^\{4,8\}B Any line starting with

4,5,6,7 or 8 A s followed by B

^A\{4,\}B Any line starting with 4 or more "A"'s

\{4,8\} Any line with "{4,8}"A{4,8} Any line with "A{4,8}"

Matching exact words

Regular expression matches\<the\> Matching individual word

“the” only\<[tT]he\> Matches for both t and T

followed by he

Example : Regex grep "^abb" file1

It matches the line that contains “abb” at the very beginning of line

grep "and$" file1 It matches the line that contains “and” at

the end of the line grep “t[wo]o” file1

It matches the line that contains “two” or “too”

Example : C shell Script #!/bin/csh echo This script would

find out the prime numbers from given numbers

echo Enter the numbers: set n = ($<) foreach num ($n) set i = 2 set prime = 1 while ( $i <= `expr $num

/ 2` ) if (`expr $num % $i` ==

0) then

set prime = 0 break endif @ i = $i + 1 end if ($prime == 1) then echo $num is a prime

number else echo $num is not a prime endif end

Example : C shell Scirpt #!/bin/csh echo This script would 'find' the .txt files

and change their permissions set ar = `find / -name "*.txt"` set arr = ($ar[*]) foreach a ($arr) chmod 777 $a ls -l $a end

T h a n k Y o u

made by B Chari K working in semiconductors domain