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Published byBeryl Marsh Modified over 9 years ago
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Chapter 10: BASH Shell Scripting Fun with fi
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In this chapter … Control structures File descriptors Variables
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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 ]
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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
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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)
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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
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if … then structure Syntax: if test-command then commands fi test-command must evaluate true or false commands can be zero or more lines
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if … then … else structure Syntax: if test-command then commands else commands fi Same guidelines as if…then
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if … then … elif structure Syntax: if test-command then commands elif test-command then commands … else commands fi
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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
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case structure Syntax: case test-string in pattern-1) commands ;; pattern-2) commands ;; … esac
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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
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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
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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
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while structure Syntax: while test-command do commands done commands will continue to be run until test- command becomes false
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until structure Syntax: until test-command do commands done commands will continue to be run until test- command becomes true
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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
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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
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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
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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<&-
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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
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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
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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
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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
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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
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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
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type Provides info about a command/builtin Syntax: type command Basically, what is being run? –Path to executable –Shell builtin –Shell alias –Hashed reference
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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
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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
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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
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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
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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
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Misc More builtins Arithmetic/Logical Evaluation Operators Recursion
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