Unit2_Unix_OS31.pdf

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UnixProgramming with the Shell.

Welcome to this course on Unix Operating System, Day 2.


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2ER/CORP/CRS/OS31/003

Version No: 2.00Copyright 2004, Infosys

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Road Map

Recap of Day 1

To understand the types of shell.

To understand Shell variables.

To understand the programming constructs.

To understand the System Variables.

To customize the shell.

To understand Commands related to File Handling.

Shell Programming.

To understand

Types of Shells.

Shell Variables.

Programming Constructs.

System Variables

Customizing the shell

To understand Commands related to File Handling.


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Shell Types in Unix

Bourne Shell.

Bourne Again Shell (bash).

C Shell (c-shell).

Korn Shell (k-shell).

TC Shell (tcsh)

Bourne shell (/bin/sh)

This is the original Unix shell written by Steve Bourne of Bell Labs. It is available on allUNIX systems.

This shell does not have the interactive facilities provided by modern shells such asthe C shell and Korn shell.

The Bourne shell does provide an easy to use language with which you can writescripts.

Bourne again shell (/bin/bash)

written by the Free Software Foundation.

This shell is widely used within the academic community.

bash provides all the interactive features of the C shell (csh) and the Korn shell (ksh).Its programming language is compatible with the Bourne shell (sh).


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Shell Types in Unix (Contd)

Please see the notes page for an explanation of the earlier slide

C shell (/bin/csh)

This shell was written at the University of California, Berkeley. It provides a C-likelanguage with which to write shell scripts.

Korn shell, /bin/ksh, David Korn (Bell Labs)

This shell was written by David Korn of Bell labs. It is now provided as the standardshell on Unix systems.

It provides all the features of the C and TC shells together with a shell programminglanguage similar to that of the original Bourne shell.

TC Shell (tcsh)

It provides all the features of the C shell together with emacs style editing of thecommand line.

References

http://unixhelp.ed.ac.uk/


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The Shell.

A program that interprets users requests to run programs.

Provides environment for user programs.

A command line interpreter that

Reads user inputs.

Executes commands.

Ask the audience to come out with the different functions and uses of shell, covered inChapter 1.

Shell acts as an interpreter for users requests.


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The Shell.

Shell allows three types of commands:

An internal command.

An executable file that contains a sequence of shell command lines.

An executable file that contains object code produced by compilation.

Three types of commands are allowed in Unix:

1) An internal command refers to the vast l ibrary of in-built commands.

Example : cal, cat, etc.

2) An executable file could be a file created by a user for a particular application, whichcontains various commands.

It is always advisable to have an extension of .sh for the user-program.

A program in Unix created by the user is called as Shell Script.

3) An executable file whose source code could be statement written in languages likeC, etc.


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Shell Features.

Interactive and background processing.

Input/output redirection

Pipes.

Wild card matching.

Programmable.

Shell Variables.

Programming language constructs.

Shell Scripts.

Shell allows execution of tasks in the background by putting an & sign.

Example

$> sleep 100

Output : For a period of 100 seconds, the terminal would be idle and user cannot work.

$> sleep 100 & # & is used for placing the process in

the background.[1] 32389 # PID of the process

which has been put in the background.

Output : The command is executed in the background. The user could work on anotherjob.

Shell allows the user to work with pipes, wherein the user can redirect commands.

Shell allows the user to work with Wild Cards, wherein the user can select multiplefiles.

The user could write programs based on the users application.


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Shell Variables.

Positional Parameters.

Special Parameters.

Named variables

In shell programming, we deal with three basic types of variables .

Positional parameters

Special parameters

Named variables


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Positional Parameters.

Acquire values from the position of arguments in command line.

$1, $2, $3,..$9

sh file1 10 20 30

$1 $2 $3

Acquire values from the position of arguments in command line.

Example:

$ cat>data1

echo Your marks in Sem1 is : $1



{ctrl - d}

$ sh data1 10 20 30

Your marks in Sem1 is : 10




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Special Parameters.

Shell assigns the value for this parameter.

$$ - PID number.

$# - Number of Command Line Arguments.

$0 Command Name.

$* - Displays all the command line arguments.

$? Exit Status.

$- - Shell options

$! - Process number of the last background command

$@ - Same as $*, except when enclosed in double quotes.

Shell assigns the value for the special parameter.

symbol meaning

$$ The process number(PID) of the current shell

$# The number ofpositional parameters while invoking the shell script

$0 The name of thecommand being executed

$* The list of positionalparameters

$? Exit status of the lastexecuted command

$- Shell options

$! The process numberof the last background command

$@ Same as $*, exceptwhen enclosed in double quotes

Example

$> echo $$

32344 #Displays the PID ofthe users shell.


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Named Variables.

User-defined variable that can be assigned a value.

Used extensively in shell-scripts.

Used for reading data, storing and displaying it.

Named variables

A named variable is a user-defined variable that can be assigned a value within aShell program.

The value of a named variable can be retrieved by preceding the variable name with a$ sign

Example:$ x1=10

$ echo $x1

10


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Accepting Data.

read.

Accepts input from the user.

Syntax : read variable_name.

Example : read sname

VariableName

In shell scripts , read is used for accepting data from the user and storing it for otherprocessing.


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Display Data.

echo

Used to display a message or any data as required by the user.

echo [Message, Variable]

Example:

echo Infosys Technologies Ltd.

echo $sname

VariableName

Echo is used for displaying data.

This data could be either a data inside the variable or any message.


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Program Execution.

sh

Command to execute program in Unix.

Syntax : sh

Example : sh file1

$> cat>file1

echo "Enter your name"

read sname

echo "Your name is "$sname

{ctrl-d}

$> sh file1

Enter your name

Ajit

Your name is Ajit

Here the value of name is stored in the variable sname.

If a file does not have an executable permission, then the file could be executed usingthe . Operator.

Example

$> . file1.sh #Executes the file named file1.sh

If a file has an executable permission, then the file could be executed only by calling itsname.

Example

$> file1.sh #Executes the file named file1.sh


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test command.

Used extensively for evaluating shell script conditions.

It evaluates the condition on its right and returns a true or false exit status.

The return value is used by the construct for further execution.

In place of writing test explicitly, the user could also use [ ].


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test command (Contd).

Operators used with test for evaluating numeral data are:

-eq Equal To

-lt Less than

-le Less than

-gt Greater than

-ge Greater than or equal to

-le Less than or equal to

$> a1=10

$> a2=20

$> test $a1 -eq $a2

$> echo $?

1

Instead of test command, we could also use [ ]

$> a1=10

$> a2=20

$> [ $a1 -eq $a2 ]

$> echo $?

1

Note : A space is mandatory after [ and before ].


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Operators used with test for evaluating string data are:

str1 = str2 True if both equals

str1 != str2 True if not equals

-n str1 True if str1 is not a null string

-z str1 True if str1 is a null string

$> str1="Infosys"

$> str2="Infy"

$> test $str1 = $str2

$> echo $?

1 # indicates both thestrings are different.


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Operators used with test for evaluating file data are:

-f file1 True if file1 exists and is a regular file.

-d file1 True if file1 exists and is directory.

-s file1 True if file1 exists and has size greater than 0

-r file1 True if file1 exists and is readable.

-w file1 True if file1 exists and is writable.

-x file1 True if file1 exists and is executable.

$> test -f grt1

$> echo $?

0 #indicates the file by name grt1 exists and isregular file.


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Logical Operators.

Logical Operators used with test are:

! Negates the expression.

-a Binary and operator.

-o Binary or operator.

Logical operations

Examples:

$> cat>pr1.sh

x=10;

if test $x -lt 15 -o $x -gt 5 #Checks for value of x greater than 5 or lessthan 15.

then

echo "Correct"

else

echo "Wrong"

fi

{ctrl-d}

Output

Correct.


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Shell Quotes-Meanings.

(single forward quotes)

'....' take .... Literally

` ` (single backward quotes)

`` to be interpreted by shell

" " (double quotes)

"...." take .... literally after $.

Examples:

$> x=10

$> echo 'echo $x

echo $x #Output when single quote isused.

$> echo `echo $x`

10 #Output when single backwardquote is used.

$> echo "echo $x"

echo 10 #Output when double quote isused.


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expr command.

Used for evaluating shell expressions.

Used for arithmetic and string operations.

Example : expr 7 + 3

would give an output 10.

When used with variables, back quotes need to be used.

Operator has to be preceded and followed by a space.

$> x=10

$> y=20

$> z=`expr $x + $y`

$> echo $z

30


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Conditional Execution.

&&

The second command is executed only when first is successful.

command1 && command2

||

The second command is executed only when the first is unsuccessful.

command1 || command2

$> cat file2

8000

9000

7000

$> grep 9000 file2 && echo "Correct"

9000

Correct

$> grep 2000 file2 || echo "Wrong"

Wrong


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Program Constructs

if

for

while

until

case


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if statement.

Syntax.

if control command

then

else

fi

Example:

echo "Enter a number"

read num

if test $num -le 30

then

echo "Number is less than or equal to 30"

else

echo "Number is greater than 30"

fi

Output:

If the user gives a number less than or equal to 30 , then the output would be Numberis less than or equal to 30, else the output would be Number is greater than 30.


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for statement.

Syntax.

for variable-name in value1 value2 ....

do

done

$> cat grt1

for x in 1 2 3 6

do

echo "$x"

done

{ctrl-d}

$> sh grt1

1

2

3

6


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while statement.

Syntax.

while control command

do

done

Example:

$> cat > grt2

x=1

while test $x -le 6

do

echo "$x"

x=` expr $x + 1 `

done

{ctrl-d}

$> sh grt2

1

2

3

45

6


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until statement.

Syntax.

untilcontrol command

do

done

$> cat>grt3

x=1

until test $x -ge 6

do

echo "$x"

x=` expr $x + 1 `

done

{ctrl-d}

$> sh grt3

1

2

3

4

5


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case statement.

Syntax.

case value in

choice1) commands ;;

choice2) commands ;;

....

....

esac

The symbols ;; are used as option terminators.

Example:

$> cat>grt5

echo "Enter the data"

read data

case $data in

1) echo "One";;

2) echo "Two";;

3) echo "Three";;

*) echo "Greater than Three"

esac

{ctrl-d}

Output:

If the user enters any number less than 4, it displays the number in words, else itdisplays Greater than Three.


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Useful Shell Scripting commands.

break

To come out of a loop.

continue

To jump to the start of loop.

exit

To prematurely terminate a program.

: or #

To interpret the rest of line as comments.


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Trap command.

trap

To alter the effects of certain events that generate signals, which generally tends to

halt a process.

Syntax:

trap command signal list

UNIX provides the trapcommand which allows the user to alter the effects of certain events thatgenerate signals, which generally tend to halt a process:

The hang-upsignal is denoted by code number 1.

The interrupt (Ctrl-C) signal is denoted by code number 2.

The terminate (by kill command) is denoted by code number 15.

The general form of the trapcommandThe trap command, once it has been read in, lies in wait for those signals mentioned in thesignal list. When such a signal shows up, the command is performed instead of theconsequence of the signal that would have been to halt the process.

Example

$> cat>unix_prog1.sh

trap 'echo "You cannot exit without entering data"' 2 #If user presses ^C key, it displaysthe message.

echo "Enter your data"

read data

{ctrl-d}

$> sh unix_prog1.shEnter your data

#Data is requested from the user.

You cannot exit without entering data #User presses ^Cwhen requested for data.

Infosys#User enters the data.

Note : The signal 9 cannot be trapped.


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export command.

export

To make a variable a part of environment and also be accessible to the child shell.

export

A Shell variable can be exportedso that it becomes part of theenvironment. A process canthen access and use any of the variables in its environment.

Note :An exported variable becomes part of the environment of the current shell and of allprocesses descendingfrom it. Also, it is important that to make a variable accessible to allsubshells, it should be defined and exported from the login shell itself.

A user variable if not exported will not be visible to the subshells.

Example:

$> a=10

$> echo $$ #PID of the shell

6034

$> echo $a

10

$> sh #Creates a child shell

$> echo $$ #PID of child shell

9072

$>echo $a

After using export

$> export a

$> sh #Creates a child shell

$>echo $a

10


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System Variables.

PATH

Search path referred by Unix for any command.

echo $PATH

HOME

Indicates the home directory for the user.

echo $HOME

$> echo $PATH #To display the PATH

/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/home/ajit:.

$> PATH=$PATH:/home #To append to the existing path.

$> echo $PATH

/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/home/ajit:.:/home

HOME

$> echo $HOME #To display the home directory.

/home/ajit


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System Variables (Contd).

PS1

Used for displaying & changing the primary prompt.

echo $PS1

PS2

Used for changing the secondary prompt.

PS1

Example

$> PS1=Infosys> #To change theprimary prompt.

Infosys>

#The prompt changes to Infosys.

PS2

Example

$> PS2=* #The secondary prompt changes to *.


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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

Example

$> set 12 23 34 #The data 12, 23 and 34 are arguments forset command.

$> echo $1

12

$> echo $2

23

$> echo $334

$> set # set command, usedwithout any arguments, will display the environmental variables.

BASH=/bin/bash

BASH_VERSINFO=([0]="2" [1]="05b" [2]="0" [3]="1" [4]="release" [5]="i386-redhat-

linux-gnu")

BASH_VERSION='2.05b.0(1)-release'

COLORS=/etc/DIR_COLORS

COLUMNS=80

DIRSTACK=()

EUID=538

GROUPS=()

G_BROKEN_FILENAMES=1

PATH=/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/home/ajit/bin:/home/ajit:.PIPESTATUS=([0]="0")

PPID=331

PS1='[\u@\h \W]\$ '

PS2='> '

PS4='+ '

PVM_ROOT=/usr/share/pvm3

PVM_RSH=/usr/bin/rsh

PWD=/home/ajit


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Unix.File Handling Commands.


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File Handling Commands.

ulimit

Prints the value of resource limits.

ulimit [OPTIONS] [N]

du

Prints the disk usage.

du [OPTIONS] [FILE]

ulimit - Prints the value of resource limits.

Print the value of one or more resource limits, or, ifn is specified, set a resource limit ton.

Resource limits can be either hard (-H) or soft (-S). By default, ulimit sets both limits or printsthe soft limit.

Options:

-H Hard limit. Anyone can lower the hardlimit; only privileged users can raise it.

-S Soft limit. Must be lower than the hardlimit.

-a Print all limits

-c Maximum block size of core files

-n Maximum file descriptor plus 1

du - Print disk usage.

Prints the number of blocks used by each named directory and its subdirectories. Default is thecurrent directory.

Options:

-a Print usage for all files, not just subdirectories

-r Print cannot open message if a file or directory isinaccesible

-s Print only the grand total for each named directory

Example:

$> du s /home/data

To receive the result for the entire data directory as a summary


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File Handling Commands (Contd).

find

Search for files.

find [PATH] [EXPRESSION]

file

File Type determination.

file [OPTIONS] FILES

find - Search for files.

Most commonly used expressions:

-mount Do not descend directories on the file systems

-print Print the current file name

-exec command Execute command

-user uname File is owned by uname

-type t File has a type of t, such as d for directory and ffor file

- name patternLook for the file name of pattern-newer file File was modified more recently than file

Example:

$> find /home name data

Output:

This searches the path /home looking for the name data.

$> find /home name data1 exec rm {}\;

Output:

This searches the path /home looking for the name data1. It removes all the occurences of the file data1.

file - File Type determination.

classify the named files according to the type of data they contain.

Options:

-c check the format of the magic file (files argument is invalid with c)-flist Run file on the filenames in list

-h Dont follow symbolic links

Example:

$> file prog1.c

Output:

Describes the file command as a C Program text.


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File Handling Commands (Contd).

df

Prints disk free blocks.

DF [OPTIONS] [NAME]

touch.

Updates the access and modification times.

touch [-amc] [mmddhhmm[yy]] files

df - Print disk free blocks.

name can be a device name (eg., /dev/dsk/0s9, the directory name of a mountingpoint (e.g., /usr), a directory name, or a remote resource name (e.g., an RFS/ NFSresource).

Options:

-b Print only the number of freeKilobytes

-e Print only the number of free files

-f Report free blocks but not freeinodes

-l Report only on local file systems

-t Report total allocated space as wellas free space

touch - updates access and modification times of a file[mm month, dd date, hh hour, mm minutes, yy year]

If no time is specified, the current time is used.

Option -a updates only the access time.Option -m updates only the modification time.

Option -c prevents creating the file if it did not exist previously.

Example:

$ touch -a test.c

$ touch -m 1221101395 test.c


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Summary

Types of Shells

Shell Variables

Program Constructs

System Variables.

File Handling Commands.


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Thank You!

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