linux crash course chapter 10 3

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 • Synt

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 strings

string1 = string2

string1 != string2

• Comparing numbers

num1 –OP num2

– Where OP can be eq, ne, lt, gt, le, ge

test expressions con’t

f : file is a regular file

Plus many more (check man bash)

• [[expression]] can be used for logical

expressions and string comparisons

• See pages 505-506 for complete list

• test-command must evaluate true or false

• commands can be zero or more lines

if … then … else structure

if … then … elif structure

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 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

• loop-index is a variable name – does not

have to be previously declared

• argument-list is a space-delimited list

test-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 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

• 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

• Note on scope – functions have same scope

as calling script/shell … which means you

can access (or step on!) existing variables

• Provides info about a command/builtin

• Syntax: type command

• Basically, what is being run?

– Path to executable

– Shell builtin

– Shell alias

– Hashed reference

• 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

– n num – read n characters

– p prompt – displays prompt to user

– u number – grabs from given file descriptor

– 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

:) display error for missing arg ;;

\?) display error for wrong opt ;;

esac done