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More Shell Programming

Learning Objectives:

1. To learn the usage of environment (shell)variables in shell programming

2. To understand the handling of command line

arguments

3. To illustrate more useful commands in shellprogramming using examples


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COMP111Lecture 8 / Slide 2

More Shell Programming

Table of Content Keyword Shell Variables

Keyword Example

Read-only Shell Variable

Command Line Argument Command for handling arguments - shift

Command for handling arguments - set

Special Variable - $$

Looping using for

Looping using forMore Examples

Return Status Example


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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 directoryPRINTER 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/horner

Your 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 $0"$ print1This 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 withare 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 a10The args are a1 a2 a3 a4 a5 a6 a7 a8 a9

With more than 9 arguments, they are still stored, butthey have to be moved using the shift command

before they can be accessed.


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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 it2

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

$


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$* lists all the command line args:$ 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 a10The 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

COMP111


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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


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shift Example (1)

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

mv $2 $1shiftdonemv /tmp/$orig1 $1$ cat it1 it2 it3contents of file1

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

contents of file1

1st Sample Run

COMP111


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shift Example (2)

$ swap it1 it2 it3$ cat it1 it2 it3contents 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

COMP111


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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 ofcommand substitution into the args:

$ date

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

Today is 25 Feb 1999

Command Handling Argument - set

COMP111


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

$ cat pid#!/bin/sh

echo $$$ pid1154$ pid1156$ pid

1157$ echo $$

892

$ ps

PID TTY TIME CMD

892 pts/0 0:01 csh

Special variable - $$ (1)

COMP111


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It can be used for temporary file names:

$ cat swap

#!/bin/sh

file=/tmp/tmp$$mv $1 $file

mv $2 $1

mv $file $1

$ cat it1 it2

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

$

Special variable - $$ (2)

COMP111


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The for statement executes a loop once for each of alist of possibilities:

$ cat printall#!/bin/shfor file in *do

if [ -f $file ]then

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

lpr -Pcll3 $file

fifidone$ printallPrint letter1 [y/n]?yPrint names [y/n]?

n

Looping using for Example 1

COMP111


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If the in ___ part is omitted, it defaults to $*:

$ cat p#!/bin/shfor filedo

if [ -f $file ]then

lpr -Pcll3 $filefi

done$ p letter1 names$


COMP111


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

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

12

3

$ for pid in `ps -a | tail +2 | cut -c1-6 | sort`

> do

> kill -9 $pid> done

kill: permission denied



(you will then be logout!)

Looping using for - Example 3

COMP111


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We can make use of the find command to generate alist of files in a sub-directory tree for processing:

#!/bin/sh

if [ $# = 0 ]then

echo "usage: treeproc []"; exit

fi

pattern="$1"; shift

if [ $1 ] ; then cmd=$*; else cmd=echo; fi

echo "Running \"$cmd\" for $pattern in dir: `pwd`"for file in `find . -name "$pattern" -print`

do

$cmd $file

done $ find . -name letter* -print./secret/letter1

./secret/letter2


COMP111


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$ ls -R.:names letter0 secret/

./secret:

letter1 letter2

$ treeproc

usage: treeproc []$ treeproc "*1"

Running "echo" for *1 in dir: /home/kwchiu

./secret/letter1

$ treeproc "letter*" rm -i

Running "rm -i" for letter* in dir: /home/kwchiu

rm: remove ./letter0 (yes/no)?rm: remove ./secrete/letter1 (yes/no)?

rm: remove ./secrete/letter2 (yes/no)?

$ find . -name "letter*" -exec rm -i {} \;

(same as the above)

Looping using for Example 4 testing

{} substitutes each filename

\ denotes end of the exec command

COMP111


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Return Status $?

In UNIX, after a command (or your own program)finishes execution, it returns an exit code to its parent.The shell can retrieve this code with $?.

$ lsname

$ cat testif

if cat $1 >/dev/null 2>&1

then

echo "OK"

else

echo "Problem: Code" $?

fi

$ testif name

OK

$ testif abc

Problem: Code 2

$? returns the previousexit code

The construct >file 2>&1 is useful

for logging. Both the stdout andstderr are sent to the same file.

/dev/null is a special device dumps everything input to it

COMP111


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