Chapter 10 BASH Shell Scripting Fun with fi

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 string 1 = string 2 string 1 != string 2 • Comparing numbers num 1 –OP num 2 – 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-command then commands elif test-command then commands … else commands fi

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 loopindex 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 testcommand becomes false

until structure • Syntax: until test-command do commands done • commands will continue to be run until testcommand 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 arg 1 arg 2 …] do commands done • 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 PS 3 prompt – by default it’s #? • Set PS 3 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 <<My. List dog cat bird My. List

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: getoptstring 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 myvar do case $arg in a) do stuff ; ; b) do other stuff, with arg ; ; c) do something ; ; : ) display error for missing arg ; ; ? ) display error for wrong opt ; ; esac done

Misc • • More builtins Arithmetic/Logical Evaluation Operators Recursion