CSCI 330 The UNIX System

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CSCI 330The UNIX System

• C Shell Programming

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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 executable

CSCI 330 - The UNIX System
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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

CSCI 330 - The UNIX System
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Example script output

• chmod ux 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/18 000 hello.csh
• 24430 pts/18 000 csh

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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 ux greetings
• ./greetings
• What is your name?
• Laura Flowers
• Greetings to you, Laura Flowers
• See you soon

User entered Laura Flowers
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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 ux greetings
• ./greetings Mark Flowers
• ./greetings to you Mark Flowers
• Today is Mon Feb 16 141803 CST 2008
• Good Bye Mark Flowers

0 greetings 1 Mark 2 Flowers
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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
• ! 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
• endif

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Example Command Line Arguments

• ! /bin/csh
• if ( argv 0 ) then
• echo -n "Enter time in minutes "
• _at_ min
• else
• _at_ min 1
• endif
• _at_ sec min 60
• echo min minutes is sec seconds

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Example Reading file contents

• ! /bin/csh
• Usage lookup nameOrNumber
• set list "users.txt"
• if ( argv 0 ) then
• echo -n "Enter name OR z-id "
• set name
• else
• set name
• endif
• grep -i "name" list
• if ( status ) echo "name not found"

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File Testing operators

• Syntax if ( -opr filename )

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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
• endif

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C Shell looping constructs

• predetermined iterations
• repeat
• foreach
• condition-based iterations
• while

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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, or
• multi-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
• end

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Loops with foreach

• useful to process result of command,
• one at a time
• Example
• ! /bin/csh
• _at_ sum 0
• foreach file (ls)
• set size cat file wc -c
• echo "Counting file (size)"
• _at_ sum sum size
• end
• echo Sum sum

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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
• _at_ var 5
• while ( var 0 )
• echo var
• _at_ 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
C shell compares string to each pattern until
it finds a match

• switch ( string )
• case pattern1
• command(s)
• breaksw
• case pattern2
• command(s)
• breaksw
• endsw

When a match is found, execute the command(s)
until breaksw
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The switch Statement

• switch (string)
• case pattern1
• command(s)
• breaksw
• case pattern2
• command(s)
• breaksw
• default
• command(s)
• breaksw
• endsw

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

• switch (var)
• case one
• echo it is 1
• breaksw
• case two
• echo it is 2
• breaksw
• default
• echo it is var
• breaksw
• endsw

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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
  commands
• under the next case pattern are executed until a
• breaksw 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 greeting
• set hourdate
• switch (hour4)
• case 0
• case 101
• set greetingmorning breaksw
• case 12-7
• set greetingafternoon breaksw
• default
• set greetingevening
• endsw
• echo Good greeting 1

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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
• endsw
• end 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 sign
• echo \\ is a backslash

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The here Command

• Example
• ispell -l
• I was running along quite nicely
• when I was acosted by the mail man
• whio insisted that my name is Raimund
• but I did not believe him
• DONE

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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 !

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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 shells 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"

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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"

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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"

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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