Introduction to UNIX Shells

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Introduction to UNIX Shells

• Exclusively for VTU

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Agenda

• Languages Compiled vs Interpreted
• What is a SHELL
• The various Unix Shells
• Which Shell to Use ?
• Shell Basic Concepts
• Shell Scripting
• Resources

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Compiled vs Interpreted
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Compiled vs Interpreted (Contd.)

• Compiled Languages
• FORTRAN
• C/C
• Pascal
• Interpreted Languages
• Perl
• Tcl/Tk
• Unix Shell Scripts

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What is a SHELL ?

• A program that allows User Interaction with OS
• Interacts with Terminals
• Allows user to execute Unix utilities
• Provides an interpreted language
• Scripting of common user tasks
• Much more than a DOS Batch file

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What is a SHELL ? (Contd.)

• Interface between Operating System and User

Interaction
Scripting
Our Friendly Shell
Unix Utilities/Daemons (ls, cp, cron etc.)
System Libraries
KERNEL
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The Various Unix Shells

• Bourne Shell (sh)
• Stephen Bourne at Bell Labs 1977
• Good for portable scripting
• C Shell (csh)
• Bill Joy at Berkeley (SUN cofounder, vi guy)
• C Like Syntax
• Korn Shell (ksh)
• David Korn at ATT Bell Labs
• Major enhancement over Bourne Shell

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More Unix Shells

• Tenex C Shell (tcsh)
• Enhancement over C Shell
• Good for interactive use
• Bourne Again Shell (bash)
• GNU OpenSource Shell
• Has some features of KSH
• Good for interactive use

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Confused! What do I Use ?

• Scripting
• sh Portable Scripting
• ksh Advanced Scripting mostly portable
• Interactive Use
• csh Basic interactive use
• tcsh Enhanced interactive features over csh
• bash Effective for interactive use
• Do NOT use csh/tcsh for scripting!
• Can use bash for scripting but mostly non-portable

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Basic Concepts
bash-3.00 echo SHELL /usr/bin/bash bash-3.00
pwd /opt bash-3.00 ls csw onbld SUNWits
SUNWspro netbeans sfw bash-3.00
PS1"Hello Theregt " Hello Theregt
whoami root Hello Theregt ksh exit Hello
Theregt
The Shell PROMPT
Which SHELL ?
Where am I ? (Directory)
Which files are here ?
Change the boring PROMPT
Which User ?
Lets run Korn Shell
Back to Bash
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Basic Concepts (Contd.)
Which Operating System ?
Hello Theregt uname SunOS Hello Theregt echo
1210 Hello Theregt history 1 echo SHELL 2
pwd 3 ls 4 PS1"Hello Theregt " 5 whoami 6
ksh 7 uname 8 echo 9 history
Current shell's Process ID
List out the previous executed commands
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Other Shell Features

• Piping Send output of one command to input of
  another
• ls sort Gives a sorted list of files
• Shell special variables
• ? Exit status of last command
• ltcmdgt Execute ltcmdgt in background
• Shell builtins
• jobs List all background commands
• env List all environment variables
• CTRLC Terminate current command
• CTRLR Search in the history list

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Scripting For the Shell

• Any collection of shell commands that can be
  stored in a file and executed is called a Shell
  Script.
• Scripts have variables and flow control
  statements like other programming languages.
• Variables set var, typeset var, var...
• Arrays set -A arr, arr1one, arr2two
  ...
• Flow Control if-then-else, while, for, switch
• Functions funcname () ...
• Arithmetic expr, (( ... ))
• Much more powerful than DOS batch files
• ksh provides most features full programming
  language

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Executing Scripts

• There are two ways to execute a shell script
• Direct Interpretation
• ksh scriptfile args ...
• Indirect Interpretation
• First line of the scriptfile must be
• !/usr/bin/ksh
• ...
• Make the scriptfile executable
• chmod x scriptfile
• Execute it
• ./scriptfile

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Variables

• Local Variables
• Specific to a Shell Function
• funcname ()
• typeset var1 var2
• ...
• Global Variables
• Available throughout the Shell Script
• Not available outside the Shell Script
• Declared outside of any function

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Variables (Contd.)

• Environment Variables
• Available outside of shell scripts
• Set systemwide and is available to all programs
  and shell scripts
• Can be called Super-Global Variables
• Set at the shell prompt
• bash-0.3 varnamevalue
• bash-0.3 export varname
• Set in /etc/default/init (on Solaris) for
  systemwide availability.
• Set in /home/username/.login, /home/username/.prof
  ile or .bashrc (rc-file) for per-user
  configuration.

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Variables (Contd.)

• Be Careful!
• No spaces between variable name and sign
• varname value
• Having spaces gives an unhelpful error. Correct
  syntax is
• varnamevalue
• To set per-user values interactively, edit your
  rc-file and execute
• on bash source .profile or .bashrc
• on sh/ksh . ./.profile (Note the dots!)

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Arguments

• Arguments can be passed to a shell script
• Arguments can be passed to a shell function
• Accessed using
• ltnumbergt
• 0 Special, name of function or script
• 1 .. 9 Positional arguments
• - Number of positional arguments
• shift Used to access positional arguments
  beyond 9th position

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Flow Control

• If Statements Syntax
• if ltlogical expressiongt
• then
• ....
• fi
• Logical expression is also called a test
  condition
• -e ltfilenamegt Does file exist ?
• -x ltfilenamegt If file executable ?
• - d ltpathnamegt Is the pathname a directory ?

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Flow Control Example

• !/bin/ksh
• if -lt 2
• then
• echo Must give one parameter!
• else
• if -e 1
• then
• echo File already exists!
• exit
• fi
• touch 1
• chmod x 1
• vim 1
• fi

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Other Flow Controls
while ... do ... done switch ltvargt
ltconst1gt) ... ltconst2gt) ... esac for
var in ltexpressiongt do done
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Example

• !/bin/ksh
• list all files and directories
• for fn in ls
• do
• if -f fn then
• echo f is a file
• elif -f fn then
• echo f is a directory
• fi
• done

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Unix Philosophy

• Make each program do one thing well. To do a new
  job, build afresh rather than complicate old
  programs by adding new features.
• - Doug McIlroy

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Things to keep in mind

• Do NOT Develop as root user
• Log into the system as a normal unprovileged user
• Open a separate terminal window in graphical mode
• Make sure you are in your home directory
• You can use vi, vim, gvim, gedit etc to edit your
  script

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Elements of a Good Script

• Interpreter
• Comments
• Search Path
• Argument Checking
• Acceptable User
• Exit Status and
• Stdin, Stderr, and Stdout.

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The End