mirror of https://github.com/tLDP/LDP
89 lines
2.5 KiB
Bash
89 lines
2.5 KiB
Bash
#! /bin/bash
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# array-assign.bash
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# Array operations are Bash-specific,
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#+ hence the ".bash" in the script name.
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# Copyright (c) Michael S. Zick, 2003, All rights reserved.
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# License: Unrestricted reuse in any form, for any purpose.
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# Version: $ID$
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#
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# Clarification and additional comments by William Park.
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# Based on an example provided by Stephane Chazelas
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#+ which appeared in an earlier version of the
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#+ Advanced Bash Scripting Guide.
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# Output format of the 'times' command:
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# User CPU <space> System CPU
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# User CPU of dead children <space> System CPU of dead children
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# Bash has two versions of assigning all elements of an array
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#+ to a new array variable.
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# Both drop 'null reference' elements
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#+ in Bash versions 2.04 and later.
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# An additional array assignment that maintains the relationship of
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#+ [subscript]=value for arrays may be added to newer versions.
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# Constructs a large array using an internal command,
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#+ but anything creating an array of several thousand elements
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#+ will do just fine.
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declare -a bigOne=( /dev/* ) # All the files in /dev . . .
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echo
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echo 'Conditions: Unquoted, default IFS, All-Elements-Of'
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echo "Number of elements in array is ${#bigOne[@]}"
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# set -vx
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echo
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echo '- - testing: =( ${array[@]} ) - -'
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times
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declare -a bigTwo=( ${bigOne[@]} )
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# Note parens: ^ ^
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times
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echo
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echo '- - testing: =${array[@]} - -'
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times
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declare -a bigThree=${bigOne[@]}
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# No parentheses this time.
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times
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# Comparing the numbers shows that the second form, pointed out
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#+ by Stephane Chazelas, is faster.
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#
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# As William Park explains:
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#+ The bigTwo array assigned element by element (because of parentheses),
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#+ whereas bigThree assigned as a single string.
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# So, in essence, you have:
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# bigTwo=( [0]="..." [1]="..." [2]="..." ... )
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# bigThree=( [0]="... ... ..." )
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#
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# Verify this by: echo ${bigTwo[0]}
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# echo ${bigThree[0]}
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# I will continue to use the first form in my example descriptions
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#+ because I think it is a better illustration of what is happening.
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# The reusable portions of my examples will actual contain
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#+ the second form where appropriate because of the speedup.
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# MSZ: Sorry about that earlier oversight folks.
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# Note:
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# ----
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# The "declare -a" statements in lines 32 and 44
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#+ are not strictly necessary, since it is implicit
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#+ in the Array=( ... ) assignment form.
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# However, eliminating these declarations slows down
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#+ the execution of the following sections of the script.
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# Try it, and see.
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exit 0
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