mirror of https://github.com/tLDP/LDP
158 lines
4.3 KiB
Bash
158 lines
4.3 KiB
Bash
#!/bin/bash
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# empty-array.sh
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# Thanks to Stephane Chazelas for the original example,
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#+ and to Michael Zick and Omair Eshkenazi, for extending it.
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# And to Nathan Coulter for clarifications and corrections.
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# An empty array is not the same as an array with empty elements.
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array0=( first second third )
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array1=( '' ) # "array1" consists of one empty element.
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array2=( ) # No elements . . . "array2" is empty.
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array3=( ) # What about this array?
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echo
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ListArray()
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{
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echo
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echo "Elements in array0: ${array0[@]}"
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echo "Elements in array1: ${array1[@]}"
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echo "Elements in array2: ${array2[@]}"
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echo "Elements in array3: ${array3[@]}"
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echo
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echo "Length of first element in array0 = ${#array0}"
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echo "Length of first element in array1 = ${#array1}"
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echo "Length of first element in array2 = ${#array2}"
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echo "Length of first element in array3 = ${#array3}"
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echo
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echo "Number of elements in array0 = ${#array0[*]}" # 3
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echo "Number of elements in array1 = ${#array1[*]}" # 1 (Surprise!)
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echo "Number of elements in array2 = ${#array2[*]}" # 0
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echo "Number of elements in array3 = ${#array3[*]}" # 0
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}
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# ===================================================================
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ListArray
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# Try extending those arrays.
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# Adding an element to an array.
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array0=( "${array0[@]}" "new1" )
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array1=( "${array1[@]}" "new1" )
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array2=( "${array2[@]}" "new1" )
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array3=( "${array3[@]}" "new1" )
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ListArray
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# or
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array0[${#array0[*]}]="new2"
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array1[${#array1[*]}]="new2"
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array2[${#array2[*]}]="new2"
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array3[${#array3[*]}]="new2"
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ListArray
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# When extended as above, arrays are 'stacks' ...
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# Above is the 'push' ...
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# The stack 'height' is:
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height=${#array2[@]}
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echo
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echo "Stack height for array2 = $height"
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# The 'pop' is:
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unset array2[${#array2[@]}-1] # Arrays are zero-based,
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height=${#array2[@]} #+ which means first element has index 0.
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echo
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echo "POP"
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echo "New stack height for array2 = $height"
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ListArray
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# List only 2nd and 3rd elements of array0.
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from=1 # Zero-based numbering.
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to=2
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array3=( ${array0[@]:1:2} )
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echo
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echo "Elements in array3: ${array3[@]}"
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# Works like a string (array of characters).
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# Try some other "string" forms.
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# Replacement:
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array4=( ${array0[@]/second/2nd} )
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echo
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echo "Elements in array4: ${array4[@]}"
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# Replace all matching wildcarded string.
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array5=( ${array0[@]//new?/old} )
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echo
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echo "Elements in array5: ${array5[@]}"
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# Just when you are getting the feel for this . . .
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array6=( ${array0[@]#*new} )
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echo # This one might surprise you.
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echo "Elements in array6: ${array6[@]}"
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array7=( ${array0[@]#new1} )
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echo # After array6 this should not be a surprise.
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echo "Elements in array7: ${array7[@]}"
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# Which looks a lot like . . .
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array8=( ${array0[@]/new1/} )
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echo
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echo "Elements in array8: ${array8[@]}"
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# So what can one say about this?
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# The string operations are performed on
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#+ each of the elements in var[@] in succession.
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# Therefore : Bash supports string vector operations.
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# If the result is a zero length string,
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#+ that element disappears in the resulting assignment.
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# However, if the expansion is in quotes, the null elements remain.
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# Michael Zick: Question, are those strings hard or soft quotes?
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# Nathan Coulter: There is no such thing as "soft quotes."
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#! What's really happening is that
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#!+ the pattern matching happens after
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#!+ all the other expansions of [word]
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#!+ in cases like ${parameter#word}.
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zap='new*'
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array9=( ${array0[@]/$zap/} )
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echo
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echo "Number of elements in array9: ${#array9[@]}"
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array9=( "${array0[@]/$zap/}" )
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echo "Elements in array9: ${array9[@]}"
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# This time the null elements remain.
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echo "Number of elements in array9: ${#array9[@]}"
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# Just when you thought you were still in Kansas . . .
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array10=( ${array0[@]#$zap} )
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echo
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echo "Elements in array10: ${array10[@]}"
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# But, the asterisk in zap won't be interpreted if quoted.
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array10=( ${array0[@]#"$zap"} )
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echo
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echo "Elements in array10: ${array10[@]}"
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# Well, maybe we _are_ still in Kansas . . .
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# (Revisions to above code block by Nathan Coulter.)
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# Compare array7 with array10.
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# Compare array8 with array9.
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# Reiterating: No such thing as soft quotes!
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# Nathan Coulter explains:
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# Pattern matching of 'word' in ${parameter#word} is done after
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#+ parameter expansion and *before* quote removal.
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# In the normal case, pattern matching is done *after* quote removal.
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exit
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