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Fun with Control Structures in Shell Scripting

This chapter covers control structures in BASH shell scripting, including file descriptors, variables, tests, logical expressions, syntax examples, and loops. Learn about if-then, if-then-else, if-then-elif structures, case statements, for loops, while and until structures, break, continue, and select statements. Also, explore file descriptors, array variables, variable scope, special parameters, and functions.

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Fun with Control Structures in Shell Scripting

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  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 -optionfilename 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<, 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<&-

  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

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