More Shell Programming

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Control Flow

The shell allows several control flow statements: if while for

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The if statement works mostly as expected:

$ whoami clinton$ cat test7#!/bin/sh

user=`whoami`if [ $user = "clinton" ]then

echo "Hi Bill!"fi$ test7Hi Bill!

However, the spaces before and after the square brackets [ ] are required.

if

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The if then else statement is similar:

$ cat test7#!/bin/sh

user=`whoami`if [ $user = "clinton" ]then

echo "Hi Bill!"else

echo "Hi $user!"fi$ test7Hi horner!

if … then … else

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You can also handle a list of cases:

$ cat test8#!/bin/sh

users=`who | wc -l`if [ $users -ge 4 ]then

echo "Heavy load"elif [ $users -gt 1 ]then

echo "Medium load"else

echo "Just me!"fi$ test8Heavy load!

if … elif … else

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Boolean Expressions Relational operators:

-eq, -ne, -gt, -ge, -lt, -le

File operators:-f file True if file exists and is not a

directory-d file True if file exists and is a directory-s file True if file exists and has a size > 0

String operators:-z string True if the length of string is zero-n string True if the length of string is nonzeros1 = s2 True if s1 and s2 are the sames1 != s2 True if s1 and s2 are differents1 True if s1 is not the null string

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File Operator Example$ cat test9#!/bin/sh

if [ -f letter1 ]then

echo "We have found the evidence!"cat letter1

elseecho "Keep looking!"

fi$ test9We have found the evidence!

Ms. Lewinski:It is getting late. Please order some pizza and stopby my office. We'll tidy up a few more things beforecalling it a night.Thanks!Bill

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The while statement loops indefinitely, while the condition is true, such as a user-controlled condition:

$ cat test11#!/bin/shresp="no"

while [ $resp != "yes" ]

doecho "Wakeup [yes/no]?"

read resp done $ test11 Wakeup [yes/no]? no Wakeup [yes/no]? y Wakeup [yes/no]? yes $

while (1)

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while can also do normal incrementing loops:$ cat fac#!/bin/shecho "Enter number: "read nfac=1i=1

while [ $i -le $n ]

dofac=`expr $fac \* $i`i=`expr $i + 1`

doneecho "The factorial of $n is $fac"$ facEnter number:5The factorial of 5 is 120

while (2)

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The break command works like in C++, breaking out of the innermost loop : $ cat test12 #!/bin/sh while [ 1 ]doecho "Wakeup [yes/no]?"

read respif [ $resp = "yes" ]then

break;fi

done$ test12Wakeup [yes/no]? noWakeup [yes/no]?yWakeup [yrs/no]?yes$

break

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Keyword Shell Variables

The shell sets keyword shell variables.

You can use (and change) them.HOME The path to your home directory

PATH Directories where the shell looks for executables

USER Your login name

SHELL The name of the shell you are running

PWD The current working directory

PRINTER Can be loaded with your default printer

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Keyword Example

$ cat env#!/bin/shecho "Hi $USER!"echo "Your home is: $HOME"echo "Your path is: $PATH"echo "Your current directory is: $PWD"echo "Your shell is: $SHELL"echo "Your printer is: $PRINTER"$ envHi horner!Your home is: /homes/hornerYour path is:/usr/bin:.:.:/homes/horner/Unix/bin:...Your current directory is: /homes/horner/111Your shell is: /bin/cshYour printer is: csl3

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Readonly Shell Variables

$0 is the name the user typed to invoke the shell script:

$ cat print1#!/bin/shecho "This script is called $0"$ print1

This script is called print1$ ./print1This script is called ./print1$ ~/111/print1This script is called

/homes/horner/111/print1

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Command Line Arguments – (1)

The command line arguments that you call a script with are stored in variables $1, $2, ..., $9.

$ cat args1#!/bin/shecho "The args are $1 $2 $3 $4 $5 $6 $7 $8 $9"$ args1 a1 a2 a3 a4 a5 a6 a7 a8 a9 a10

The args are a1 a2 a3 a4 a5 a6 a7 a8 a9

With more than 9 arguments, they are still stored, but they have to be moved using the shift command before they can be accessed.

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Command Line Arguments – (2) Example: How to write a command to swap two files?

$ cat swap#!/bin/shmv $1 /tmp/$1mv $2 $1mv /tmp/$1 $2

$ cat it1contents of file1$ cat it2contents of file2$ swap it1 it2$ cat it1contents of file2$ cat it2contents of file1$

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Command Line Arguments – (3)

$* lists all the command line args:$ cat args2#!/bin/shecho "The args are $*"$ args2 a1 a2 a3 a4 a5 a6 a7 a8 a9 a10

The args are a1 a2 a3 a4 a5 a6 a7 a8 a9 a10

$# contains the number of args:$ cat args3#!/bin/shecho "The number of args is $#"$ args3 a1 a2 a3 a4 a5 a6 a7 a8 a9 a10

The number of args is 10

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Command Handling Argument - shift (1) The shift command promotes each command line

argument by one (e.g., the value in $2 moves to $1, $3 moves to $2, etc.)

$ cat shiftargs#!/bin/shecho "The args are 0 = $0, 1 = $1, 2 = $2"shiftecho "The args are 0 = $0, 1 = $1, 2 = $2"shiftecho "The args are 0 = $0, 1 = $1, 2 = $2"shift

$ shiftargs arg1 arg2 arg3The args are 0 = shiftarg, 1 = arg1, 2 = arg2The args are 0 = shiftarg, 1 = arg2, 2 = arg3The args are 0 = shiftarg, 1 = arg3, 2 =

The previous $1 becomes inaccessible

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Example: How to write a general version of the swap command for two or more files?

swap f1 f2 f3 ... fn_1 fn

f1 <--- f2f2 <--- f3f3 <--- f4...fn_1 <--- fnfn <--- f1

Command Handling Argument - shift (2)

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shift Example (1)$ cat swap#!/bin/shorig1=$1mv $1 /tmp/$1while [ $2 ]do

mv $2 $1shift

donemv /tmp/$orig1 $1

$ cat it1 it2 it3contents of file1contents of file2contents of file3$ swap it1 it2 it3

$ cat it1 it2 it3contents of file2contents of file3contents of file1

1st Sample Run

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shift Example (2)

$ swap it1 it2 it3$ cat it1 it2 it3

contents of file3contents of file1contents of file2$ swap it1 it2 it3

$ cat it1 it2 it3contents of file1contents of file2contents of file3$ swap it1 it2 $ cat it1 it2contents of file2contents of file1

2nd Sample Run

3rd Sample Run

4th Sample run

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The set command sets the args:$ cat set1#!/bin/shset yat yih saamecho "One is $1, two is $2, three is $3"

$ set1One is yat, two is yih, three is saam

The set command is useful for moving the output of command substitution into the args:

$ dateThu Feb 25 17:06:27 HKT 1999$ cat day#!/bin/shset `date`echo "Today is $3 $2 $6"

$ dayToday is 25 Feb 1999

Command Handling Argument - set

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$$ is the process ID (PID) of the current process (the shell script PID, or the shell PID if interactive).

$ cat pid#!/bin/shecho $$$ pid1154$ pid1156$ pid1157$ echo $$892$ ps

PID TTY TIME CMD 892 pts/0 0:01 csh

Special variable - $$ (1)

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It can be used for temporary file names:

$ cat swap#!/bin/shfile=/tmp/tmp$$mv $1 $filemv $2 $1mv $file $1$ cat it1 it2

contents of file1contents of file2$ swap it1 it2$ cat it1contents of file2contents of file1$

Special variable - $$ (2)

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for The for statement executes a loop

once for each of a list of possibilities:$ cat printall#!/bin/shfor file in *do

if [ -f $file ]then

echo "Print $file [y/n]? "read respif [ $resp = "y" ]then

lpr –Pcll2a $filefi

fidone

$ printallPrint it1 [y/n]?yPrint it2 [y/n]?n

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The in clause of the for statement accepts as many parameters as you wish in many forms:

$ for i in 1 2 3 ; do echo $i ; done123

$ for pid in `ps -a | tail +2 | cut -c1-6 | sort`> do> kill -9 $pid> donekill: permission deniedkill: permission deniedkill: permission denied

(you will then be logout!)

Looping using for

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For example (2)

for index in 1 2 3 4 5 6 7 8 9 10

do

if [ $index –le 3 ]; then

echo continue

continue

fi

echo $index

if [ $index –ge 8 ]; then

echo “break”

break

fi

done

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Catch a signal: builtin trap

Built-in trap Syntax: trap ‘commands’ signal-numbers Shell executes the commands when it catches one of the signals Then resumes executing the script where it left off.

– Just capture the signal, not doing anything with it trap ‘ ‘ signal_number

Often used to clean up temp files Signals

– SIGHUP 1 disconnect line– SIGINT 2 control-c– SIGKILL 9 kill with -9– SIGTERM 15 default kill– SIGSTP 24 control-z– …

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Example

[ruihong@dafinn ~/cs3451]$ cat inter

#!/bin/sh

trap 'echo PROGRAM INTERRUPTED' 2

while true

do

echo "programming running."

sleep 1

done