Bash-Beginners-Guide

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-a Variable is an array. -f Use function names only. -i The variable is to be treated as an integer; arithmetic evaluation is performed when the variable is assigned a value (see Section 3.4.6). -p Display the attributes and values of each variable. When -p is used, additional options are ignored. -r Make variables read-only. These variables cannot then be assigned values by subsequent assignment statements, nor can they be unset. -t Give each variable the trace attribute. Bash Guide for Beginners -x Mark each variable for export to subsequent commands via the environment. Using + instead of - turns off the attribute instead. When used in a function, declare creates local variables. The following example shows how assignment of a type to a variable influences the value. [bob in ~] declare -i VARIABLE=12 [bob in ~] VARIABLE=string [bob in ~] echo $VARIABLE 0 [bob in ~] declare -p VARIABLE declare -i VARIABLE="0" Note that Bash has an option to declare a numeric value, but none for declaring string values. This is because, by default, if no specifications are given, a variable can hold any type of data: [bob in ~] OTHERVAR=blah [bob in ~] declare -p OTHERVAR declare -- OTHERVAR="blah" As soon as you restrict assignment of values to a variable, it can only hold that type of data. Possible restrictions are either integer, constant or array. See the Bash info pages for information on return status. 10.1.3. Constants In Bash, constants are created by making a variable read-only. The readonly built-in marks each specified variable as unchangeable. The syntax is: readonly OPTION VARIABLE(s) The values of these variables can then no longer be changed by subsequent assignment. If the -f option is given, each variable refers to a shell function; see Chapter 11. If -a is specified, each variable refers to an array of variables. If no arguments are given, or if -p is supplied, a list of all read-only variables is displayed. Using the -p option, the output can be reused as input. The return status is zero, unless an invalid option was specified, one of the variables or functions does not exist, or -f was supplied for a variable name instead of for a function name. [bob in ~] readonly TUX=penguinpower Chapter 10. More on variables 122
Bash Guide for Beginners [bob in ~] TUX=Mickeysoft bash: TUX: readonly variable 10.2. Array variables 10.2.1. Creating arrays An array is a variable containing multiple values. Any variable may be used as an array. There is no maximum limit to the size of an array, nor any requirement that member variables be indexed or assigned contiguously. Arrays are zero-based: the first element is indexed with the number 0. Indirect declaration is done using the following syntax to declare a variable: ARRAY[INDEXNR]=value The INDEXNR is treated as an arithmetic expression that must evaluate to a positive number. Explicit declaration of an array is done using the declare built-in: declare -a ARRAYNAME A declaration with an index number will also be accepted, but the index number will be ignored. Attributes to the array may be specified using the declare and readonly built-ins. Attributes apply to all variables in the array; you can't have mixed arrays. Array variables may also be created using compound assignments in this format: ARRAY=(value1 value2 ... valueN) Each value is then in the form of [indexnumber=]string. The index number is optional. If it is supplied, that index is assigned to it; otherwise the index of the element assigned is the number of the last index that was assigned, plus one. This format is accepted by declare as well. If no index numbers are supplied, indexing starts at zero. Adding missing or extra members in an array is done using the syntax: ARRAYNAME[indexnumber]=value Remember that the read built-in provides the -a option, which allows for reading and assigning values for member variables of an array. 10.2.2. Dereferencing the variables in an array In order to refer to the content of an item in an array, use curly braces. This is necessary, as you can see from the following example, to bypass the shell interpretation of expansion operators. If the index number is @ or *, all members of an array are referenced. [bob in ~] ARRAY=(one two three) [bob in ~] echo ${ARRAY[*]} Chapter 10. More on variables 123
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Bash Guide for Beginners Table of C
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Introduction 1. Why this guide? The
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Revision 1.6 2005-03-01 Revised by:
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Bash Guide for Beginners See Chapte
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Bash Guide for Beginners When invok
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More information about the file des
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Built-in commands are necessary to
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• Brace expansion • Tilde expan
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A number of methods can be used; on
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Chapter 2. Writing and debugging sc
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the documentation of your favorite
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Bash Guide for Beginners Comments a
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Bash Guide for Beginners + set +x I
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Chapter 3. The Bash environment In
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Bash Guide for Beginners local pid
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MOZILLA_FIVE_HOME=/usr/lib/mozilla
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Bash Guide for Beginners franky ~>
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the GNU Awk programming language is
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Bash Guide for Beginners 3.2.4. Res
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LINES MACHTYPE MAILCHECK OLDPWD OPT
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two is the second positional parame
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Bash Guide for Beginners 3.3.1. Why
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A correctly-formed brace expansion
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Bash Guide for Beginners If the sub
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of the whitespace characters "space
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Bash Guide for Beginners /dev/hda5
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Bash Guide for Beginners 3.7. Summa
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Two regular expressions may be conc
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LABEL=/ / ext3 defaults 1 1 If this
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cathy ~> ls -ld [[:digit:]]* drwxrw
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Bash Guide for Beginners p Print te
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sandy ~> sed -n '/a text/,/This/p'
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Bash Guide for Beginners # This is
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Chapter 6. The GNU awk programming
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