Shells and Shell Scripts

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Shells and Shell Scripts

• COMP 444/5201
• Revision 1.3
• January 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 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.

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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 be
  found in the man pages for that shell.
• There are many Linux/UNIX references that give
  detailed 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 not require 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 a command (set up an execution permission)
• chmod ux 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 errormyshellscrip 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 name
  your 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 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.

!/bin/tcsh !/bin/bash This is
for tcsh For Bourne-Again
Shell !/bin/sh This is for Bourne Shell
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tchsQuick 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
• _at_ y 1
• set input "lt"
• There are some system, predefined variables
• 0, 1, 3 .... - argument references (arguments
  themselves)
• - all the arguments
• lt - user's input from STDIN
• - of arguments passed to the script

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if

• if ( ltexpressiongt ) then
• ltstatementsgt
• else if ( ltanother-expressiongt ) then
• ltstatementsgt
• else
• ltstatementsgt
• endif

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foreach

• foreach var ( ltlist-of-valuesgt )
• ltstatementsgt
• end

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switch

• switch ( string )
• case str1
• ltstatementsgt
• breaksw
• ...
• default
• ltstatementsgt
• breaksw
• endsw

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while

• while ( ltexpressiongt )
• ltstatementsgt
• end

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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

• f Plain file
• d Directory
• l Symbolic link
• b Block special file
• c 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 own valuable files.
• The uptolow script
• will convert any uppercase letters in an ordinary
  file name to lowercase.
• will leave directories untouched.

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Bourne ShellQuick 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 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.

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Quick Resource Summary

• Manual Pagesman bash
• An Intro to UNIX Shelllthttp//steve-parker.org/
  sh/bourne.htmlgt
• How To Write a Shell Scriptlthttp//www.tinker.n
  csu.edu/LEGO/shell_help.htmlgt

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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 one
  left
• Assume intput
• ./shift.sh 1 2 foo bar
• 0 ltdirectory-ofgt/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
• PWD personal working directory
• PS1 primary prompt
• PS2 input prompt
• IFS - what to treat as blanks

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Control Flow if

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

if ltexpressiongt then ltstatementsgt elif
ltstatementsgt else ltstatementsgt 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 ltexpressiongt in ltpatter1gtltvalue1gt)
command1 ltpatter2gtltvalue2gt)
command2 esac
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case (2)

• case in
• 1) cat gtgt 1
• 2) cat gtgt2 lt1
• 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

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for

• Syntax
• List can also be a result of a command.

for variable in ltlist of values/wordsgt do
command1 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 ltexpressiongt do command1 command2
done
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until

• Syntax

until ltexpressiongt do command1 command2
done
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To Be Continued...