1. Chapter 10: BASH Shell Scripting Fun with fi

2. In this chapter … Control structures File descriptors Variables

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 Syntax: test expression OR [ expression ]

4. 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

5. 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)

6. 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

7. if … then structure Syntax: if test-command then commands fi test-command must evaluate true or false commands can be zero or more lines

8. if … then … else structure Syntax: if test-command then commands else commands fi Same guidelines as if…then

9. if … then … elif structure Syntax: if test-command then commands elif test-command then commands … else commands fi

10. 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

11. case structure Syntax: case test-string in pattern-1) commands ;; pattern-2) commands ;; … esac

12. 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

13. 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

14. 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

15. while structure Syntax: while test-command do commands done commands will continue to be run until test- command becomes false

16. until structure Syntax: until test-command do commands done commands will continue to be run until test- command becomes true

17. 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

18. select structure Syntax: select varname [in arg1 arg2 …] do commands done Similarly to a for loop, varname need not be declared prior If in args omitted, command line arguments used

19. 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

20. File descriptors Recall 0, 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<&-

21. 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

22. 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

23. 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

24. 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

25. 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

26. Here document Allows you to do standard input redirection within a script Denoted by << followed by a sentinel Ex: sort <<MyList dog cat bird MyList

27. type Provides info about a command/builtin Syntax: type command Basically, what is being run? –Path to executable –Shell builtin –Shell alias –Hashed reference

28. 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

29. 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

30. 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

31. 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

32. 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

33. Misc More builtins Arithmetic/Logical Evaluation Operators Recursion