CSCI 330THE UNIX SYSTEM

C Shell Programming

STEPS TO CREATE SHELL PROGRAMS

Specify shell to execute program Script must begin with #! (pronounced

“shebang”)to identify shell to be executed

Examples:#! /bin/sh (defaults to bash)#! /bin/bash#! /bin/csh#! /usr/bin/tcsh

Make the shell program executable Use the “chmod” command to make the

program/script file executable2

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EXAMPLE: “HELLO” SCRIPT

#! /bin/csh

echo "Hello $USER"

echo "This machine is `uname -n`"

echo "The calendar for this month is:"

cal

echo "You are running these processes:"

ps

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EXAMPLE SCRIPT OUTPUT

% chmod u+x hello

% ./hello

Hello ege!

This machine is turing

The calendar for this month is

February 2008

S M Tu W Th F S

1 2 3 4 5 6 7

8 9 10 11 12 13 14

15 16 17 18 19 20 21

22 23 24 25 26 27 28

You are running these processes:

PID TTY TIME CMD

24861 pts/18 0:00 hello.csh

24430 pts/18 0:00 csh4

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SHELL LOGIC STRUCTURES

Basic logic structures needed for program development:

Sequential logic User input Decision logic Looping logic Case logic

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INPUT TO A C SHELL SCRIPT

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

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READING USER INPUT WITH $<

Use a special C shell variable: $<

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

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EXAMPLE: ACCEPTING USER INPUT

#! /bin/csh

echo "What is your name?"

set name = $<

echo Greetings to you, $name

echo "See you soon"

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EXAMPLE: ACCEPTING USER INPUT

% chmod u+x greetings

% ./greetings

What is your name?

Laura Flowers

Greetings to you, Laura Flowers

See you soon

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User entered Laura Flowers

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 …

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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

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EXAMPLE: COMMAND LINE ARGUMENTS

#! /bin/csh

# Usage: greetings name1 name2

# Input: name1 and name2

echo $0 to you $1 $2

echo Today is `date` $1 $2

echo Good Bye $1 $2

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EXAMPLE: COMMAND LINE ARGUMENTS

% chmod u+x greetings

% ./greetings Mark Flowers

./greetings to you Mark Flowers

Today is Mon Feb 16 14:18:03 CST 2008

Good Bye Mark Flowers

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$0 => greetings$1 => Mark

$2 => Flowers

DECISION LOGIC

if Statement: simplest forms

if ( expression ) command

if ( expression ) then

command(s)

endif

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DECISION LOGIC

if-then-else Statement

if ( expression ) then

command(s)

else

command(s)

endif

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DECISION LOGIC

if-then-else Statement

if ( expression ) then

command(s)

else if ( expression ) then

command(s)

else

command(s)

endif

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BASIC OPERATORS IN EXPRESSIONS

Meaning( ) grouping! Logical “not”

> >= < <= greater than, less than

== != equal to, not equal to

|| Logical “or”&& Logical “and”

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EXPRESSION EXAMPLES

if ( $1 == “next” ) echo $2

if ( $#argv != 0 ) then

…

endif

if ( $#argv > 0 && $#argv < 5) then

…

endif

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EXAMPLE: COMMAND LINE ARGUMENTS

#! /bin/csh

if ( $#argv == 0 ) then

echo -n "Enter time in minutes: "

@ min = $<

else

@ min = $1

endif

@ sec = $min * 60

echo “$min minutes is $sec seconds”

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EXAMPLE: READING FILE CONTENTS

#! /bin/csh# Usage: lookup nameOrNumberset list = "users.txt"if ( $#argv == 0 ) then echo -n "Enter name OR z-id: " set name = $<else set name = $*endifgrep -i "$name" $listif ( $status ) echo "$name not found"

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FILE TESTING OPERATORS

Syntax: if ( -opr filename )

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opr Meaning

r Read access

w Write access

x Execute access

e Existence

z Zero length

f Ordinary file

d directory

EXAMPLE: FILE TESTING

if ( -e $1 ) then

echo $1 exists

if ( -f $1 ) then

echo $1 is an ordinary file

else

echo $1 is NOT ordinary file

endif

else

echo $1 does NOT exist

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C SHELL LOOPING CONSTRUCTS

predetermined iterationsrepeatforeach

condition-based iterationswhile

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FIXED NUMBER ITERATIONS

Syntax:

repeat number command

executes “command” “number” times

Examples:repeat 5 ls

repeat 2 echo “go home”

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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

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EXAMPLE: FOREACH STATEMENT

foreach word ( one two three )

echo $word

end

or

set list = ( one two three )

foreach word ( $list )

echo $word

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LOOPS WITH FOREACH useful to process result of command,

one at a time

Example:#! /bin/csh@ sum = 0foreach file (`ls`) set size = `cat $file | wc -c` echo "Counting: $file ($size)" @ sum = $sum + $sizeendecho Sum: $sum 27

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THE WHILE STATEMENT

while ( expression )

commands

end

use when the number of iterations is not known in advance

execute ‘commands’ when the expression is true

terminates when the expression becomes false

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EXAMPLE: WHILE

#! /bin/csh

@ var = 5

while ( $var > 0 )

echo $var

@ var = $var – 1

end

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EXAMPLE: WHILE

#! /bin/csh

echo -n "Enter directory to list: "

set dirname = $<

while ( ! -d $dirname )

echo "$dirname is not directory"

echo -n "Enter directory to list: "

set dirname = $<

end

ls $dirname

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LOOP CONTROL

break

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

continue

ends current iteration of loop, continues with next iteration

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LOOP CONTROL EXAMPLE

#! /bin/csh

while (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

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LOOP CONTROL EXAMPLE

#! /bin/csh

while ( 1 )

echo -n "Enter directory to list: "

set dirname = $<

if ( -d $dirname ) break

echo "$dirname is not directory"

end

ls $dirname

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THE SWITCH STATEMENT

Use when a variable can take different values Use switch statement to process different

cases (case statement)

Can replace a long sequence of if-then-else statements

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THE SWITCH STATEMENT

switch ( string )

case pattern1:

command(s)

breaksw

case pattern2:

command(s)

breaksw

endsw

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C shell compares ‘string’ to each ‘pattern’ until it finds a match

When a match is found, execute the command(s)

… until breaksw

THE SWITCH STATEMENT

switch (string)case pattern1:

command(s)breakswcase pattern2:

command(s)breaksw default:

command(s)breaksw

endsw36

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When a match is not found, execute the commands following the default label

EXAMPLE: SWITCH

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

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THE SWITCH STATEMENT

if no pattern matches and there is no default, then nothing gets executed

do not omit the breaksw statement !If you omit the breaksw statement, all the

commandsunder the next case pattern are executed until abreaksw or endsw statement is encountered

pattern may contain wildcards:*, ?, []

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EXAMPLE: SWITCH GREETING#! /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 39

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EXAMPLE C SHELL PROGRAM

AVAILABLE OPTIONS

*******************

[1] Display today's date

[2] How many people are logged on

[3] How many user accounts exist

[4] Exit

Enter Your Choice [1-4]:

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USERUTIL SHELL SCRIPT 1 OF 2

#! /bin/csh

# Usage: userutil

while (1)

echo "AVAILABLE OPTIONS"

echo "*******************"

echo "[1] Display today's date"

echo "[2] How many people are logged on"

echo "[3] How many user accounts exist"

echo "[4] Exit"

echo "Enter Your Choice [1-4]:"

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USERUTIL SHELL SCRIPT 2 OF 2 set answer = $<

switch ($answer) case "1": echo `date`; breaksw case "2": echo `users | wc -w` users are logged in breaksw case "3": echo `cat /etc/passwd | wc -l` users exists breaksw case "4": echo "BYE" break breaksw endswend # end of while

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ADVANCED C SHELL PROGRAMMING

Quoting Here Debugging Trapping Signals

Functions ? calling other scripts exec, source, eval

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QUOTING

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

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

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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"

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SINGLE QUOTES

wildcards, variables and command substitutions are all treated as ordinary text

history references are recognized

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

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BACKSLASH

backslash character \treats following character literally

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

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THE HERE COMMAND

Example:ispell -l << DONEI was running along quite nicelywhen I was acosted by the mail manwhio insisted that my name is Raimundbut I did not believe himDONE 48

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Command Syntax Meaning

command << keyword Read lines from input until keyword is encountered at the beginning of a line

DEBUGGING SCRIPTS

% csh –n scriptname parse commands but do not execute them

% csh –v scriptname Display each line of the script before execution

% csh –x scriptname Displays each line of the script after variable

substitutions and before execution

can also be added to shebang line ! 49

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TRAPPING SIGNALS

any Unix process can be interrupted by a signal

common signal: ^C typed via keyboard

causes csh to terminate can be “trapped”, i.e. other behavior specified useful for cleanup upon forced exit

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TRAPPING SIGNAL

Syntax:onintr label

execution continues at label if interrupt signal is received

onintr – ignore interrupt signal

onintr restore previous interrupt signal behavior

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ONINTR EXAMPLE

#! /bin/csh

onintr label

while (1)

echo “.”

sleep 1

end

label:

echo “signal received”

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DIVIDE AND CONQUER

how to modularize a shell script call Unix commands and utilities

call other scripts as subshell sourced in place

evaluate strings to commands

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CALLING OTHER SCRIPTS

as subshell, via: csh scriptname

scriptname

subshell does not see current shell’s variables

subshell sees current environment variables

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EXAMPLE: OUTER

#! /bin/csh

set var = "outer"

setenv VAR "outer"

echo "outer: $var $VAR"

csh inner

echo "outer: $var $VAR"55

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EXAMPLE: INNER

#! /bin/csh

if ( ! $?var ) set var = "unknown"

echo "inner: $var $VAR"

set var = "inner"

setenv VAR "inner"

echo "inner: $var $VAR"

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“SOURCE” OTHER SCRIPT: NO SUBSHELL

#! /bin/csh

set var = "outer"

setenv VAR "outer"

echo "outer: $var $VAR"

source inner

echo "outer: $var $VAR"57

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“EXEC” OTHER SCRIPT: NO RETURN

#! /bin/csh

set var = "outer"

setenv VAR "outer"

echo "outer: $var $VAR"

exec ./inner

echo "outer: $var $VAR"58

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THE “EVAL” COMMAND

“eval” evaluates string executes resulting string

Example:set x = 23

set y = x

eval echo \$$y

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EXAMPLE: THE EVAL COMMAND

#!/bin/csh

set A = 1

set B = 2

set C = 3

foreach i (A B C)

eval echo \$$i

end

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