Chapter 10:BASH Shell Scripting

Fun with fi

In this chapter …• Control structures

• File descriptors

• Variables

Control structure tests• Control structures depend on a test that

equates either true or false

• The test builtin in bash allows logical, relational, and file property-based tests

• Syntax:test expression OR [ expression ]

test expressions• If expression is true, test returns 0; if false, it

returns not 0 (usually 1)

• Comparing text stringsstring1 = string2

string1 != string2

• Comparing numbersnum1 –OP num2– Where OP can be eq, ne, lt, gt, le, ge

test expressions con’t• File tests

-option filename

where option can be:

d : file is a directory

y : file exists

f : file is a regular file

Plus many more (check man bash)

Other test-commands• Instead of test and [ ] you can use other bash

contructs

• ((expression)) can be used for integer comparisons

• [[expression]] can be used for logical expressions and string comparisons

• See pages 505-506 for complete list

if … then structure• Syntax:

if test-command

then

commands

fi

• test-command must evaluate true or false

• commands can be zero or more lines

if … then … else structure• Syntax:

if test-command

then

commands

else

commands

fi

• Same guidelines as if…then

if … then … elif structure• Syntax:

if test-commandthen

commandselif test-command

thencommands

…else

commandsfi

if … then … elif con’t• You can have one or more elif blocks

• Remember, each elif line is following by a then statement

• Rather than multiple elif’s, might try a case statement instead

case structure• Syntax:

case test-string in

pattern-1)

commands

;;

pattern-2)

commands

;;

…

esac

case structure con’t• test-string is any string – usually we want to

check the contents of a variable, so we’d use something like $myvar

• The patterns are similar to ambiguous file references – so the shell special characters apply ([ ], ?, *, |)

• If the last pattern is *, it’s a catch all or default

for … in structure• Syntax:

for loop-index in argument-list

do

commands

done

• loop-index is a variable name – does not have to be previously declared

• argument-list is a space-delimited list

for structure• Syntax:

for loop-index

do

commands

done

• Similar to for … in except values of loop-index are populated with the script’s command line arguments

while structure• Syntax:

while test-command

do

commands

done

• commands will continue to be run until test-command becomes false

until structure• Syntax:

until test-command

do

commands

done

• commands will continue to be run until test-command becomes true

break and continue• break exits a loop structure – jumps down to

after done statement

• continue exits current loop iteration – jumps down to the done statement, and begins next loop iteration test

• Used to short circuit loops

select structure• Syntax:

select varname [in arg1 arg2 …]do

commandsdone

• Similarly to a for loop, varname need not be declared prior

• If in args omitted, command line arguments used

select structure con’t• select structure displays a numbered menu

allowing user to select an arg

• After displaying the menu, select displays the PS3 prompt – by default it’s #?

• Set PS3 to customize the prompt to something more intelligible

• The user’s selection (what was actually typed) is stored in REPLY

File descriptors• Recall 0<, 1>, 2> … now let’s make more

• Syntax:exec n> outfile AND exec m< infile

• exec associates streams with files

• Then can treat those streams just like the standard ones

• To close:exec n>&- AND exec m<&-

Array Variables• Recall we declare bash variables with the

format varname=value• To declare an array, use:

arrayname=(elements …)• Array is zero based and referenced via [ ]• [*] returns all the elements in the array, IFS

delimited• [@] returns all the elements in the array, for

the purpose of copying entire arrays

Variable Scope• By default, bash variables have local scope

• To make global, you must use export (or declare/typeset with –x)

• Variables used in a shell script are local to the script, unless exported

Special Parameters• $$ -- the PID of the process running

• $? -- the exit status of the last process

• $# -- the number of command line arguments

• $0 -- the name of the calling program

• $n -- the nth command line argument– ${n} must be used for n > 9– the shift builtin rotates through the arguments

Null and unset variables• ${varname:-default} : if varname is not set or

is null, substitutes for default

• ${varname:=default} : if varname is not set or null, substitues for default and sets varname

• ${varname:?message} : if varname is not set, displays an error

Functions• Syntax:function name () {…}

• Note on scope – functions have same scope as calling script/shell … which means you can access (or step on!) existing variables

Here document• Allows you to do standard input redirection

within a script• Denoted by << followed by a sentinel• Ex:sort <<MyListdogcatbirdMyList

type• Provides info about a command/builtin

• Syntax: type command• Basically, what is being run?

– Path to executable– Shell builtin– Shell alias– Hashed reference

read• Syntax:

read [options] [varname]

• Reads input from standard in

• If varname not supplied, input goes in REPLY

• Gets everything the user types in before hitting RETURN

read con’t• Options

– a array – sticks each word into an element of array

– d delimiter – use a delimiter other than NEWLINE

– n num – read n characters– p prompt – displays prompt to user– u number – grabs from given file descriptor

getopts• Easy way to make your script use classic

option syntax

• Syntax:getopts optstring varname [args …]– optstring is a list of options (characters)– Options followed by : denote required args– If optstring starts with : getopts handles errors– varname used to hold each argument

getopts con’t• Usually placed in a loop to read options in

one at a time for processing

• Keyword variable OPTIND contains an index of what option you’re processing

• Keyword variable OPTARG contains the argument for the given option

getopts con’t• Ex:while getopts :ab:c myvardo

case $arg ina) do stuff ;;b) do other stuff, with arg ;;c) do something ;;:) display error for missing arg ;;\?) display error for wrong opt ;;

esacdone

Misc• More builtins

• Arithmetic/Logical Evaluation

• Operators

• Recursion