Advance Shell Scripting

1. UNIX
Advance Shell Scripting
Presentation By
Nihar R Paital

2. Introduction
Advance features of shell scripting/command
such 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

3. /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/null
1) Why the output of last command is not redirected to /dev/null
device?
Nihar R Paital

4. Local and Global Shell variable
Local Shell variable
$a=20
$echo $a
Output : 20
$/bin/sh # Entering into New Shell
$ echo $a
Output :
Empty Line Printed due to a is not defined in new shell
$ a=50
$ echo $a
Output : 50
$ exit #Returned to Old Shell by exiting Old Shell
$echo $a
Output : 20
$
Nihar R Paital

5. Local and Global Shell variable
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 $a
Output : 500
$export a # a became global variable by export command
$/bin/sh # Entering into New Shell
$ echo $a
Output : 500 # Value of a is constant from old to new as it is global
$ exit
$echo $a
Output : 500
$
Nihar R Paital

6. 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.
Nihar R Paital

7. Customizing User Environment
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
Nihar R Paital

8. Customizing User Environment
Aliases
 Alias is a synonym for a command or command string
 Syntax:
alias new=original
Ex:-
alias search=grep
alias 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
Nihar R Paital

9. Customizing User Environment
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
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10. Customizing User Environment
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 optionnames
where optionnames is a list of option names separated by blanks
The - turns the named option on, while the + turns it off
Option Description
emacs Enter emacs editing mode
ignoreeof Don't allow use of [CTRL-D] to log off; require the exit
command
noclobber Don't allow output redirection (>) to clobber an existing file
noglob 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
undefined
vi Enter vi editing mode
xtrace traces shell scripting
noexec finds syntax error without executing script
 To check the status of an option, type
set -o Nihar R Paital

11. Customizing User Environment
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 varname
Ex:
$ a=20
$ echo $a
Output: 20
$ unset a
$ echo $a
Output: Empty Line
Nihar R Paital

12. Customizing User Environment
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”
Nihar R Paital

13. Customizing User Environment
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 the
following 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
Nihar R Paital

14. Customizing User Environment
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) Nihar R Paital

15. Customizing User Environment
System Variables (Contd).
 SHELL
– Pathname of the shell you are running
 PWD
– Current directory
 HOME
– Users home directory
Nihar R Paital

16. Customizing User Environment
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
Nihar R Paital

17. Customizing User Environment
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.
Nihar R Paital

18. Nihar R Paital