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Sheridan College - Victor Ralevich 1of 50UNIX/LINUX BASH Programming 1

UNIX

BASH Programming (Part 1)


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UNIX

BASH Programming

Topics:

Shells Running a BASH Script Shell Variables and Related Commands Passing Arguments to Shell Scripts Program Control Flow Commands (if)

Arithmetic Operators


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Shell Scripts

The shell scriptis a file that contains series of commandsto be executed by the shell.

There are three ways to execute Bash shell script:

oBy making the shell script file an executable file usingchmodcommand,

oBy running the /bin/bash command with the script file asits parameter (/bin/bash script_file), or

oBegin a shell script with #!/bin/bash.


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Command: echo

echo[flags][file-list]

Purpose Print-to-screen functionFlags\b\c

\f

\n\t

Moves already printed text back one space.Forces the following command to appear on the sameline as the current command.

Forces the following command to appear on the nextline at a specified horizontal location.Forces the following command to appear on a new line.Indents the following command output by one tab.


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Example:

$cat whoson#!/bin/bash

dateecho Users Currently Logged Inwho

$chmod u+x whoson$whoson

Fri Jun 17 10:59:40 PDT 1994Users Currently Logged In

alex tty1 Jun 17 08:26Jenny tty02 Jun 17 10:04


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The dotcommand

The dotcommand ( . ) is a built-in shell command thatlets you execute a program in the current shell and

prevents the shell from creating the child process.

There is a space between the dot command and itsargument.

$ . whoson

If your login shell is not set up to search for executablefiles in the working directory, try command:

$./whoson


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ShellVariables and Related Commands

Shell environment variablesare used to customize theenvironment in which the shell runs.

A copy of environment variables as passed to everycommand that executes in the shell as its child.

Most of these variables are initialized when/etc/profileexecutes at login time.

It is possible to customize these variables in users~/.profile statrtup file, or the ~/.bashrc,~/.bash_login, and ~/.bash_profilefiles.


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Some Important Writable Bash Environment VariablesBASH Full pathname to bash.CDPATH Contains directory names that are searched.EDITOR Default editor.ENV Path to look for configuration files.

HISTFILE Pathname of the history file.HOME Name of the home directory.PATH Users search path.PPID Parents process ID.

PS1 Primary shell prompt on command line.PS2 Secondary shell prompt.PWD Name of the currently working directory.TERM Type of users console terminal.


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Read-Only Bash Environment Variables$0 Name of program.$1 - $9 Values of command-line arguments.$* Values of all command line arguments.$@ Values of all command line arguments; each argument

is argument individually quoted if $@ is enclosed inquotes.

$# Total number of command-line arguments.$$ PID of current process.$? Exit status of the most recent command.$! PID of the most recent background process.

Commands set, declareor envwithout arguments

are used to see the list of values of the Bash environmentvariables.


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The shiftcommand - promotes each of the commandline arguments. The second $2becomes the first $1....Successive shift commands make additional argumentsavailable. There is no unshift command to bring back

arguments that are no longer available.

$cat demo_shiftecho arg1=$1 arg2=$2 arg3=$3

shiftecho arg1=$1 arg2=$2 arg3=$3

$demo_shift alice helen jenny

arg1=alice arg2=helen arg3=jennyarg1=helen arg2=jenny arg3=


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The setcommand. When you call setcommand with oneor more arguments, it sets the values of the command lineargument variables ($1- $9) to its arguments.

The setcommand may be used to cause it to use thestandard output of another command as its arguments.


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$dateFri Jun 17 23:04:09 PDT 1996$cat datesetset `date`

echo $*echo Argument 1: $1echo Argument 2: $2echo Argument 4: $4

echo $2 $3, $6$datasetFri Jun 17 23:04:09 PDT 1996Argument 1: Fri

Argument 2: JunArgument 4: 23:04:13Jun 17, 1996


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Without any arguments, set displays a list of all variablesthat are set. It displays user-created variables as well asshell keyword variables.

The shell stores the PID number of the process that isexecuting it in the $$variable. The shell substitutes thevalue of $$before it forks a new process to run acommand. The example below demonstrates that the shell

creates a new shell process when it runs a shell script.


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$cat id1echo $0 PID = $$$echo $$14137

$id1id1 PID = 15253$echo $$14137

Number 14137 is the PID of the login shell, and 15253 is thePID of the subshell.


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Shell stores the exit status of the last command in the $?variable. Nonzero exit means that the command failed.Exit status may be specified using an exit command,followed by a number, to terminate the script.

$cat esecho Hi!exit 7

$esHi!$echo $?7

$echo $?0


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Example:

$ set One Two Three$ echo $3 $2 $1

Three Two One

$ set `date`$ echo $1 $2 $3

Wed Nov 29


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Prompt characters\H Domain name of the host\T Time in 12 hours hh:mm:ssformat\d The date in weekday month date format\h Host name up to the first dot

\s The shell name\t Time in 24 hour hh:mm:ssformat\u User name of the current user\v Version of the Bash shell

\w Current working directory

Examples:

PS1=\w$PS1=\ht


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User-defined variables

Name of the variable may be any sequence of letters anddigits with letter as the first character.

When you assign a value to a variable, you must notprecede or follow the equal sign with a Space or Tab.

Bash does not require the type declaration but declareand typeset commands can be used to declare variablestype.

All Bash variable is string by default, but can be declaredto be an integer.


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Use echoto display their values.

The shell substitutes the value of a variable when youprecede the name of the variable with a dollar sign ($).

$person=alex $echo $person$person

$echo person

person $echo \$person$person

$echo $personalex

$echo $personalex


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Double quotations marks are used to assign or display avalue that contains Space or Tab to a variable.

$person=alex and jenny

$echo $personalex and jenny

The special characters such as * and ? should also be

quoted.

$memo=alex*$echo $memo

alex*


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$echo $memoalex.report alex.summary

$ls

alex.reportalex.summary

To remove the value of a variable, set it to null:

$person=

or use unsetcommand:

$unset person


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The readonlycommand is used to ensure that thevalue of a variable cannot be changed. Assign a value toa variable before you declare it to be readonly.

$readonly person

The readonlycommand without an argument, displaysa list of all user-created readonly variables.

In case of use of exportcommand with a variable nameas an argument, the shell places the value of the variablein the calling environment of child process.


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$cat test1feline=lionecho test1 1: $felinesubtestecho test1 2: $feline$cat subtestecho subtest 1:$felinefeline=tiger

echo subtest 2:$feline$test1test1 1: lion

subtest 1:subtest 2: tigertest1 2: lion

$cat test2export felinefeline=lionecho test2 1: $felinesubtestecho test2 2: $feline$test2test2 1: lion

subtest 1: lionsubtest 2: tigertest2 2: lion


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The readcommand reads one line from the standardinput and assigns the line to one or more variables.

If you enter more words than readhas variables, read

assigns one word to each variable, with all the left-overwords going to the last variable.

If input contains special characters, and you want them to

be just values of variables, use double quotes.

If you want the shell to use the special meanings of thespecial characters, do not use quotation marks.


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Example:

$cat readxecho Enter a command: \c

read command$commandecho Thanks

$readxEnter a command: whoalex tty11 Jun 17 07:09scott tty7 Jun 17 08:23

Thanks


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Example:

$cat readc

echo Enter something: \cread word1 word2echo Word 1 is: $word1echo Word 2 is: $word2

$readcEnter something: Monty Pythons FlyingCircus

Word 1 is: MontyWord 2 is: Pythons Flying Circus


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When you enclose a command between two backquotes(`), the shell replaces the command with the output of thecommand (command substitution).

$cat direcho You are using the `pwd` directory.

$dir

You are using the /home/jenny directory.


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Control-Flow Commands

The control-flow commands alter the order of execution ofcommands within a shell script.

if ... then

iftest conditionthen

commandsfi

The test condition is relational expression that might haveone out of two values: trueor false.


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There are two different formats for the test expression:

testexpression

[expression]

The expressioncontains one or more criteria evaluatedby test:

-a separating two criteria is a logical AND operator,-o is a logical OR operator,! is NOT operator.

Operator -atakes precedence over -o.


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Operators for the testCommand String Testing

string True if stringis not null string.-nstring True if stringhas a length greater than zero.

-zstring True if stringhas length zero.

string1=string2 True if string1and string2are the same.

string1!=string2True if string1and string2are not the same.

Operators for the testCommand Integer Testing

int1relop int2 The relop is relational operator:

-gt greater than-ge greater than or equal-eq equal to-ne not equal to

-le less than or equal to-lt less than


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Operators for the testCommand File Testing

-dfile True if fileexists and is directory.

-ffile True if fileexists and is an ordinary file.

-Lfile True if fileexists and is a symbolic link.

-rfile True if fileexists and you have read access permission.-sfile True if fileexists and contains information.

-wfile True if fileexists and you have write access permission.

-xfile True if fileexists and you have execute access permission.


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Example:#!/bin/bashecho Enter filename: \cread filename

if [-r $filename -a -s $filename]thenecho File $filename exists and is not empty.echo You have read access permission to the file.

fi


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if ... then ... else

iftest-conditionthen

commandselsecommands

fi

If the test-command returns a truevalue, if structureexecutes the commands between thenand elsestatements and then passes control to fi, and the shell

continues with the next command in the script. If the test-command returns a falsestatus, it executes thecommands following the elsestatement.


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Example:

$cat AreYouOK#!/bin/bash

echo Are you OK? #User promptecho input Y for yes and N for no:\cread answerif test $answer = Y

thenecho Glad to hear that.else

echo Hope youll get better soon.

fi


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if ... then ... elif

iftest-statementthen

commandseliftest-command

then

commands

elsecommands

fi


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The elifstatement combines elseand ifstatementsand allows construction of a nested set of if ... then...elsestructures.

Example:

#!/bin/bashecho word 1: \c

read word1echo word 2: \cread word2echo word 3: \c

read word3


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if [$word1=$word2 -a $word2=$word3]then

echo Match: words 1, 2 & 3elif [$word1 = $word2]

thenecho Match: words 1 & 2elif [$word1 = $word3]

then

echo Match: words 1 & 3elif [$word2 = word3]then

echo Match: words 2 & 3

elseecho No matchfi


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Arithmetic Operations in BASH

The values are stored in Bash as strings. The Bourneshell does not include a simple, built-in operator for

arithmetic and logic operations.

Example:

$x=20$echo $x20$x=$x+1

$echo $x20+1


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The shell did not add number 1to the value of x. Itconcatenated the string +1to the string value of x.

There are three ways to perform arithmetic on numeric

data in Bash:

oBy using the letcommand,oBy using the shell expression $((expression)), and

oBy using the exprcommand.

The expression evaluation is performed in long integersand no overflow check is performed.


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The Bash letcommand is used to evaluate thearithmetic expressions by specifying them as arguments.

letexpression-list

If expression contains space characters it has to besurrounded by double quotes.

$let x = 2 y = 3$let z=x**y$echo The values of z is $z

The value of z is 8


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In $((expression))

the value of expressionis calculated and the result is

returned.

$x=2 y=3$echo The value of z=x*y is $((x*y))

The value of z=x*y is 8

$let i=5$let i=i+1

$echo $i6


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$x=3$((x=x-2))$echo $x1

Commandexprargs

provides arithmetic operations capability and evaluateseither numeric or nonnumeric character strings.

The exprcommand takes the arguments as

expressions, evaluates them, and displays the results onthe standard output device.


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Arguments may be arithmetic, relational, logical, andstring manipulation related expressions.

Example:

$expr 7 12-5

$expr 2.5 1.2expr: nonnumeric argument

Both Bourne Shell and BASH support only integer

arithmetic.


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Because the * (multiplication) and % (reminder)characters have special meaning to the shell, they mustbe preceded by a backslash (\) for the shell to ignoretheir special meanings.

$expr 15/35$expr 12\*336

$expr 10\%31

The exprutility returns exit status of 0 if the expression

is neither a null string nor the number 0, a status of 1 ifthe expression is null or 0, and status of 2 if theexpression is invalid.


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The single back quotations ( ` ) surrounding thecommand cause the output of the command to exprtobe substituted.

$x=10$x=`expr $x+2`$echo $x12

The exprcommand provides relational operators thatwork on both numeric and nonnumeric arguments.


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If both arguments are numeric, the comparison isnumeric. If one or both arguments are nonnumeric, thecomparison is nonnumeric and uses the ASCII values.

The relational operators are: =, !=, =

The exprcommand displays 1 when the comparison istrue, and 0 when comparison is false.

Because the > (greater than) and < (less than) charactershave special meaning to the shell, they must beproceeded by a backslash ( \ ) for the shell to ignore their

special meaning.


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Example:

$ expr 10\200

Variables can be declared as integers with the

declare i

command. If you attempt to assign any string value, Bash

assigns 0 to the variable.


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Numbers can also be represented in different bases,binary, octal, hex using expression:

variable=base#number

Examples:

$declare i x=017 #octal number

$echo $x15

$x=2#101

$echo $x5


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$x=16#b$echo $x11

$ declare i num$ num=hello$ echo $num0

$num=5+5$echo $num10


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$num=4 * 5$echo $num20

$num=4*5$echo $num20

$num=4 * 5$echo $numbash: *: command not found

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