Shells and Shell Scripts comp 444/5201

Shells and Shell Scripts

• COMP 444/5201

• Revision 1.3

• January 25, 2005

Content

• Shells and Shell Scripts

• tcsh, enhanced C-Shell

• bash, Bourne-Again Shell

Shell Commands

• Shell commands are interpreted directly by the shell you specify.

• The commands are similar to the statement in some 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.

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 be found in the man pages for that shell.

• There are many Linux/UNIX references that give detailed information and tips.

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

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)

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 not require compilation).

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 a command (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)

Shell Script Invocation (2)

• If you get an error: “myshellscrip: command not found”

• The probably “.” is not in your path or there’s 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 name your thing not very creatively.

Start Writing a Shell Script

• The very first line, often called 'shebang' (#!) should precede any other line, to assure that the right shell is invoked.

• 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.

tchs Quick Ref

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 = "$<"

• There are some system, predefined variables:

• $0, $1, $3 .... - argument references (arguments themselves)
• $* - all the arguments
• $< - user's input from STDIN
• $# - # of arguments passed to the script

if

• if ( ) then

• else if ( ) then

• else

• endif

foreach

• foreach var ( )

• end

switch

• switch ( string )

• case str1:

• breaksw

• ...

• default:

• breaksw

• endsw

while

• while ( )

• end

File Inquiry Operators: -op file

• r Read access

• w Write access

• x Execute access

• e Existence

• o Ownership

• z Zero size

• s Non-zero size

Example

• See creator and uptolow.

• NOTE: run them in a some temporary directory to do not mess with your own valuable files.

• The uptolow script:

• will convert any uppercase letters in an ordinary file name to lowercase.
• will leave directories untouched.

Bourne Shell Quick Ref

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 most often used constructs and pointers to resources where else to find that kind of stuff. Some of it is a lame reap off the man page and so on.

Quick Resource Summary

• Manual Pages: man bash

• An Intro to UNIX Shell:

• How To Write a Shell Script:

Bourne Shell Script Constructs Reference

• System/Internal Variables

• Control Flow (if, for, case)

Internal Variables

Internal Variables (2)

• Use shift command to shift the arguments one left:

• 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

Environment

• These (and very many others) are available to your shell:

• $PATH - set of directories to look for commands
• $HOME - home directory
• $MAIL
• $PWD – personal working directory
• $PS1 – primary prompt
• $PS2 – input prompt
• $IFS - what to treat as blanks

Control Flow: if

• General Syntax:

• can either be a logical expression or a command and usually a combo of both.

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

case

• Syntax:

case (2)

• case $# in

• 1) cat >> $1

• ;;

• 2) cat >>$2 <$1

• ;;

• 3) case $3 in

• -[abc]) echo "-a -b or -c"

• ;;

• -foo|-bar) echo "-foo or -bar"

• ;;

• esac

• ;;

• *) echo "we accept up to 3 args only."; exit 127

• ;;

• esac

for

• Syntax:

• List can also be a result of a command.

for (3)

• for file in *.txt

• do

• echo File $file:

• echo "======"

• cat $file

• echo "======"

• done

while

• Syntax

until

• Syntax

To Be Continued...