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July 17, 2003 Serguei A. Mokhov,[email protected]

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Shells and Shell Scripts

COMP 444/5201

Revision 1.3January 25, 2005

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Content

Shells and Shell Scripts

tcsh, enhanced C-Shell

bash, Bourne-Again Shell

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Shell Commands

Shell commands are interpreted directly by theshell you specify.

The commands are similar to the statement insome programming languages, such as C.

Popular shells include: Enhanced C-shell tchs (csh+)

Bourne-Again Shell, bash (sh+) Korn Shell (ksh)

These notes will focus on the first two shells.

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Shells Features The bash an tcsh shells are similar in the

features the offer. In particular: Pass arguments to your script

Set and reference variables

Use of control flow

Interact with the user (read user input)

Comments

Info on commands a given shell offers can befound in the man pages for that shell.

There are many Linux/UNIX references that givedetailed information and tips.

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Shell Scripts

What are they for?

To automate certain common activities an user

performs routinely.They serve the same purpose as batch files in

DOS/Windows.

Example: rename 1000 files from upper case to lowercase

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What are Shell Scripts

Just text/ASCII files with: a set of standard UNIX/Linux commands (ls, mv, cp,less, cat, etc.) along with

flow of control

some conditional logic and branching (if-then),

loop structures (foreach, for, while), and

I/O facilities (echo, print, set, ...).

They allow use of variables.

They are interpreted by a shell directly.

Some of them (csh, tcsh) share some of C syntax.

DOS/Win equivalent - batch files (.bat)

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Why not use C/C++ for that?

C/C++ programming requires compilation

and linkage, maybe libraries, which may not

be available (production servers). For the typical tasks much faster in

development, debugging, and maintenance

(because they are interpreted and do notrequire compilation).

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Shell Script Invocation

Specify the shell directly: % tcsh myshellscript

% tcsh -v myshellscript(-v = verbose, useful for debugging)

Make the shell an executable first and then run is acommand (set up an execution permission): % chmod u+x myshellscript

Then either this:

% myshellscript(if the path variable has . in it; security issue!)

Or: % ./myshellscript

(should always work)

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Shell Script Invocation (2)

If you get an error:myshellscrip: command not found The probably . is not in your path or theres no

execution bit set. When writing scripts, choose unique names, that

preferably do not match system commands. Bad name would be test for example, since there are

many shells with this internal command. To disambiguate, always precede the shell with

./ or absolute path in case you have to nameyour thing not very creatively.

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Start Writing a Shell Script The very first line, often called 'shebang' (#!) should

precede any other line, to assure that the right shell isinvoked.

Comments start with '#', with the exception of #!, $#,

which are a special character sequences. Everything on a line after # is ignored if # is not a part

of a quoted string or a special character sequence.

#!/bin/tcsh #!/bin/bash

# This is for tcsh # For Bourne-Again Shell

#!/bin/sh

# This is for Bourne Shell

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tchs

Quick Ref

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Variables

Variables start with a $ sign when they are used. $x, $val

There's no $ when a variable is declared. set x = 3 @ y = 1

set input = "$

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if

if( ) then

else if ( ) then

else

endif

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foreach

foreach var ( )

end

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switch

switch ( string )

case str1:

breaksw

...

default:

breakswendsw

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while

while ( )

end

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File Inquiry Operators:

-op filer Read access

w Write access

x Execute access

e Existenceo Ownership

z Zero size

s Non-zero size

f Plain file

d Directory

l Symbolic link

b Block special filec Character special file

p Named pipe (FIFO)

S Socket special file

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Example

See creator and uptolow.

NOTE: run them in a some temporary

directory to do not mess with your ownvaluable files.

The uptolow script:

will convert any uppercase letters in anordinary file name to lowercase.

will leave directories untouched.

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Bourne Shell

Quick Ref

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Quick Note

In no way this going to be a duplication for

the zillions of resources on Bourne Shell,

but more a quick reference/syntax for mostoften used constructs and pointers to

resources where else to find that kind of

stuff. Some of it is a lame reap off the manpage and so on.

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Quick Resource Summary

Manual Pages:

man bash

An Intro to UNIX Shell:

How To Write a Shell Script:

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Bourne Shell Script Constructs

Reference System/Internal Variables

Control Flow (if, for, case)

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Internal Variables$# Will tell you # of command line arguments supplied

$0 Ourselves (i.e. name of the shell script executed

with path)

$1 First argument to the script

$2 Second argument, and so on

$? Exit status of the last command

$$ Our PID

$! PID of the last background process

$- Current shell status

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Internal Variables (2)

Use shift command to shift the arguments oneleft: Assume intput:

./shift.sh 1 2 foo bar

$0 = /shift.sh

$1 = 1

$3 = 2

$4 = foo

$5 = bar

shift:

$1 = 2

$2 = foo

$3 = bar

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Environment

These (and very many others) are available to your

shell:

$PATH - set of directories to look for commands $HOME - home directory

$MAIL

$PWDpersonal working directory

$PS1primary prompt

$PS2input prompt

$IFS - what to treat as blanks

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July 17, 2003 Serguei A. Mokhov,[email protected] 26

Control Flow: if

General Syntax:

can either be a logicalexpression or a command and usually acombo of both.

if[ ]; then

elif

else

fi

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if (2)

Some Logical Operators:-eq --- Equal

-ne --- Not equal

-lt --- Less Than

-gt --- Greater Than

-o --- OR

-a --- AND

File or directory?-f --- file

-d --- directory

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case

Syntax:

case in

|)command1

;;

|)command2;;

esac

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case (2)

case $# in

1) cat >> $1

;;

2) cat >>$2

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for

Syntax:

List can also be a result of a command.

for variable in [;]

docommand1

command2

done

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for (3)

for file in *.txt

do

echo File $file:

echo "======"

cat $file

echo "======"

done

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while

Syntax

while

do

command1

command2

done

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until

Syntax

until

do

command1

command2

done

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July 17 2003 Serguei A Mokhovkh @ di 34

To Be Continued...

13 shells

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