UNIX

Advance Shell Scripting

Presentation By

Nihar R Paital

Nihar R Paital

Introduction

Advance features of shell scripting/commandsuch as:

Local and Global Shell variable Customizing User Environment Functions User interface Conditional execution File Descriptors traps Multiple command line args handling

Nihar R Paital

/dev/null

This is special Linux file which is used to send any unwanted output from program/command.Syntax: command > /dev/nullExample:

$ ls > /dev/null

Run the following two commands$ ls > /dev/null$ rm > /dev/null1) Why the output of last command is not redirected to /dev/null

device?

Nihar R Paital

Local Shell variable

$a=20$echo $aOutput : 20$/bin/sh # Entering into New Shell$ echo $aOutput : Empty Line Printed due to a is not defined in new shell$ a=50$ echo $aOutput : 50$ exit #Returned to Old Shell by exiting Old Shell$echo $aOutput : 20$

Local and Global Shell variable

Nihar R Paital

Global Shell Variable

To set global varible you have to use export command.Syntax: export variable1, variable2,.....variableN $a=500 # Create local variable a with value 500$echo $aOutput : 500$export a # a became global variable by export command$/bin/sh # Entering into New Shell$ echo $aOutput : 500 # Value of a is constant from old to new as it is global$ exit$echo $aOutput : 500$

Local and Global Shell variable

Nihar R Paital

Customizing User Environment

The most basic means of customization that the Korn shell provides are

Aliases Synonyms for commands or command strings that you can define for convenience.

Options Controls for various aspects of your environment, which you can turn on and off.

Variables Place-holders for information that tell the shell and other programs how to behave under various circumstances. To customize the environment various built-in shell variables are available.

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Customizing User Environment

>To change the values of variables permanently , define it in .profile file.

The .profile File This is a file of shell commands, also called a shell script, that the Korn

shell reads and runs whenever you log in to your system. Various environment variables can be defined in this file

Alias can be defined in .profile file

Customizing User Environment

Nihar R Paital

Aliases

Alias is a synonym for a command or command string Syntax: alias new=original Ex:-

alias search=grepalias cdnew=‘cd /xyz/x1/x2’

>Quotes are necessary if the string being aliased consists of more than one word

>it is possible to alias an alias, aliases are recursive

Ex:- alias c=cdnew

Type alias without any arguments, to get a list of all the aliases you have defined as well as several that are built-in.

The command unalias name removes any alias definition for its argument

Customizing User Environment

Nihar R Paital

set command.

set command

– Used for display all the environment variables.– Shows the current values of system variables.– Also allows conversion of arguments into positional

parameters. – Syntax : set

Customizing User Environment

Nihar R Paital

Set Options

Options let you change the shell's behaviour A shell option is a setting that is either "on" or "off." The basic commands that relate to options are set -o optionnames and set +o optionnameswhere optionnames is a list of option names separated by blanksThe - turns the named option on, while the + turns it offOption Descriptionemacs Enter emacs editing modeignoreeof Don't allow use of [CTRL-D] to log off; require the exit commandnoclobber Don't allow output redirection (>) to clobber an existing filenoglob Don't expand filename wildcards like * and ? (wildcard

expansion is sometimes called globbing)nounset Indicate an error when trying to use a variable that is undefinedvi Enter vi editing modextrace traces shell scriptingnoexec finds syntax error without executing script To check the status of an option, type set -o

Customizing User Environment

Nihar R Paital

Shell Variables

Shell variables can specify everything from your prompt string to how often the shell checks for new mail

built-in variables have names in all capital letters The syntax for defining variables is

$ varname=value if the value is more than one word, it must be surrounded by quotes To delete a variable type the command

$ unset varnameEx:

$ a=20$ echo $aOutput: 20$ unset a$ echo $aOutput: Empty Line

Customizing User Environment

Nihar R Paital

Print Command

To check value of a variable print built-in command can be used

Print command is strongly recommended over echo because its options are the same on all UNIX systems, whereas echo's options differ between BSD-derived and System V-derived UNIX versions.

Ex:- print “$x”

Customizing User Environment

Nihar R Paital

System Variables or Built-in Variables

PATH– Search path referred by Unix for any command. – echo $PATH

HOME– Indicates the home directory for the user.– echo $HOME

In the bash shell, command history is controlled by which group of thefollowing environment variables.HISTCMD, HISTFILE, HISTSIZE, HISTFILESIZE

HISTFILE - Name of history file, on which the editing modes operate.

HISTSIZE– Number of lines kept in history file

Customizing User Environment

Nihar R Paital

System Variables (Contd).

FCEDIT– Pathname of editor to use with the fc command.

PS1– Used for displaying & changing the primary prompt. – echo $PS1

PS2– Used for changing the secondary prompt.

MAIL– Name of file to check for incoming mail (i.e., your mail file)

MAILCHECK– How often, in seconds, to check for new mail (default 600

seconds, or 10 minutes)

Customizing User Environment

Nihar R Paital

System Variables (Contd).

SHELL– Pathname of the shell you are running

PWD– Current directory

HOME– Users home directory

Customizing User Environment

Nihar R Paital

Environment Variables

Environment Variables are known to all kinds of subprocesses Any variable can become an environment variable. First it must be

defined as usual; then it must be exported with the command$ export varnames

To find out environment variables and their values ,type$ export

Customizing User Environment

Nihar R Paital

The Environment File

Although environment variables will always be known to subprocesses,

the shell must define which other variables, options, aliases, etc., are to communicated to subprocesses.The way to do this is to put all such definitions in a special file called the environment file instead of your .profile.

1. Decide which definitions in your .profile you want to propagate to subprocesses. Remove them from .profile and put them in a file you will designate as your environment file.

2. Put a line in your .profile that tells the shell where your environment file is:ENV=envfilename

3 . For the changes to take effect, type either . .profile or login. In either case, your environment file will be run when the shell encounters the ENV= statement.

Customizing User Environment

Nihar R Paital