리눅스 : Lecture 6Shell Programming

UNIX is designed so that users can extend the functionality To build new tools easily and efficiently To customize the shell and user interface. To string together a series of Unix commands

to create new functionality. To create custom commands that do exactly

what we want.

Extension of Functionality

Shell

Command Interpreter that turns text that you type (at the command line) in to actions

User Interface: take the command from user

Shell Programming We often want to do a number of commands together And bundle them up into one new command. Just like a batch file in MS-DOS

Shell scripts

Any collection of shell commands can be stored in a file called a shell script. Scripts have variables and flow control statements like other programming languages.

Popular Shells

sh Bourne Shell ksh Korn Shell bash Bourne-Again Shell csh,tcsh C Shell (for this course)

Shell scripts among those shells are slightly different

shell script

Creating a simple shell script A shell script is a file that contains commands that the shell

can execute. Any commands you enter in response to a shell prompt.

A utility A compiled program Another shell script

Control flow commands Run a shell script

Enter the script filename on the command line The shell interprets and execute the commands one after

another Why shell script?

Simply and quickly initiate a complex series of tasks or a repetitive procedure.

Shell script example

#!/bin/csh

echo “Current Time - `date`”

echo I am `whoami`

C Shell

Invoking scripts

There are two ways to launch scripts:1) Direct interpretationcsh scriptfile [args …]

2) Indirect interpretationThe first line of the file must be#!/bin/csh

and the file must be executable (permission).

C Shell

Shell Variables

Environment Variables Used to provide information to programs

(Global) environment variable New programs and shells inherit

environment variables from their parent shell (Local) shell variable

Used only by that shell Not passed to other processes

Environment Variables

“env” or “printenv” command Display current environment variables

DISPLAY The graphical display to use, e.g. nyssa:0.0 EDITOR The path to your default editor, e.g. /usr/bin/vi GROUP Your login group, e.g. staff HOME Path to your home directory, e.g. /home/frank HOST The hostname of your system, e.g. nyssa IFS Internal field separators, usually any white space (defaults to tab,

space and <newline>) LOGNAME The name you login with, e.g. frank PATH Paths to be searched for commands, e.g.

/usr/bin:/usr/ucb:/usr/local/bin PS1 The primary prompt string, Bourne shell only (defaults to $) PS2 The secondary prompt string, Bourne shell only (defaults to >) SHELL The login shell you're using, e.g. /usr/bin/csh TERM Your terminal type, e.g. xterm USER Your username, e.g. frank

Set Shell Variables

Mostly set automatically when log in

setenv $ setenv NAME value # in C Shell

set $ set name = value # in C Shell

Variables

To set variables:set X [= value] # processed as a string

To unset variables : unset X

Variable contents are accessed using ‘$’:echo $PATH

C Shell

ArrayTo create lists:set Y = (abc 1 123)

To set a list element:set Y[2] = 3

To view a list element:echo $Y[2]

To count the number of variable elements:echo $#Y

set fname = prog1

rm ${fname}.c

C Shell

Built-in Variables

$user -- who am I? $path -- my execution path (list of directories to be searched

for executables) $term -- what kind of terminal I am using $status -- a numeric variable, usually used to retun error

codes $prompt -- what I am currently using for a prompt $shell -- which shell am I using (usu. either /bin/csh or

/bin/sh)

% set

Will display the variable lists.

Arithmetic (@) command

C shell provides arithmetic operaters must be used with the arithmetic (@)

command Arithmetic command works only with

integers.

set count = 5

@ count += 2

echo $count

90

Shell Arithmetic

expr op1 math-operator op2

Example% expr 1 + 3

% expr 10 \* 3

% set A = `expr 3 + $B`

Command arguments

A shell script to swap files:#! /bin/csh –f

cp $argv[1] tempfile

cp $argv[2] $argv[1]

cp tempfile $argv[2]

rm –f tempfile

Arguments : $argvThe number of arguments to a script: $#argv

-f option says we want fast startup (no read .cshrc) .

C Shell

if-then-else

if ( expr ) simple-command

if ( expr ) then

commandlist-1

[else

commandlist-2]

endif

C Shell

if-then-else cont’d

An example:if ($#argv != 2) then

echo “we need two parameters!“

else

set name1 = $argv[1]

set name2 = $argv[2]

endif

C Shell

Loops

while ( expr )

commandlist

end

foreach var ( worddlist )

commandlist

end

C Shell

switch

switch ( str )case string1:

commandlist1breaksw

case string2:commandlist2breaksw

defaultcommandlist

endsw

C Shell

goto (Considered harmful!)

To jump unconditionally:goto label

A label is a line such as:label:

The classic paper on why not to use goto:Go To Statement Considered HarmfulEdsger W. Dijkstra, CACM, March 1968

C Shell

shift command

Moves the values in the parameters toward the beginning of the parameter list#!/bin/csh –f

echo “There are” $#argv “parameters\n”

while ($#argv > 0)echo –n “$argv[1] “shift

end

echo “\n”echo “There are now” $#argv “parameters”echo “end of script”

C Shell

Input

Reading Line by Line

% set x = $<

This is a line.

% echo $x

This is a line.

File Operators

-e file : True if file exists-r file : True if file is readable-l file : True if file exists and is a symbolic link-w file : True if file exists and is writable-x file : True if file exists and is executable-o file : True if the user owns it-f file : True if the file exists and is a regular file-d file : True if the file exists and is a directory-s file : True if file exists and has a size greater than zero-z file : True if file length is zero (empty)

Logical operator

! : NEGATE && : logical AND || : logical OR

Ex)if (! -e somefile) then

# does not exist

Debugging

%csh –vx somescript args

-v : vervose-x : echoes the commands after all substitutions are

made-n : syntax check. No execution

example

#!/bin/csh

if (-e $argv[1]) then

echo $argv[1] exists

else

echo $argv[1] does not exist and cannot be opened

endif

# rest of script here

C Shell

example#!/bin/csh

set sum = 0

echo –n “Enter a number: ”

set num = $<

while ($num != “”)

@ sum += $num

echo –n “Enter the next number: ”

set num = $<

end

echo “\nThe sum of the number is : $sum”

C Shell

Guidelines

Shell script is better than C program if the problem can be solved by using UNIX commands

Why script? Easier to create and modify Easy to debug

Good thing to do Use redirection and pipe Do validity check (argument number , type) Check existence of files and directories Display error messages

example

#!/bin/csh

set j = (1 2 3 4 5)

foreach i ($j)

echo $i Hello

end

C Shell

example

#!/bin/csh

set ary = `cat ary.dat`

echo “The whole array : $ary”

echo “The number of elements : $#ary”

echo “The first element: $ary[1]”

echo “The last element: $ary[$#ary]”

C Shell

Numeric validation example

#!/bin/csh

echo $argv[2] > temp

grep ‘^[0-9]*$’ temp > /dev/null

if ($status != 0) then

echo “Month argument is not numeric”

exit 1

endif

if ($argv[2] < 1 || $argv[2] > 12) then

echo “Month argument must be <1…12>”

exit 2

endif

echo “Validation is OK. We continue.”

C Shell

example#! /bin/csh -fforeach name ($argv)

if ( -f $name ) thenecho -n "delete the file '${name}' (y/n/q)?"

elseecho -n "delete the entire dir '${name}' (y/n/q)? "

endifset ans = $< # $< means “read a line”switch ($ans)

case n: continue

case q: exit

case y: rm -rf $namecontinue

endswend:

C Shell

Exercise 1

Write a shell script that displays the number of files and directories in a given directory

format% ./fd_count.csh directory_name

Exercise 2

Write a shell script that removes duplicate words from an input text file.

Format % remove_dup.csh in.txt out.txt

FourTwoOneOneFourTwoTwoThree

Four Two One Three

in.txt

out.txt