Arrays (Bash)
Beginnetje
#!/bin/bash a=(1 2 "Drie" 4 "Vijf") echo $a # Returns "1" - First item of the array echo $a[0] # Returns "1[0]" - The [0] part is treated as a literal echo ${a[0]} # Returns "1" echo ${a[@]} # Returns "1 2 3 4" - whole array
Nog een beginnetje:
a[0]="Nul" a[1]="Eén" echo $a # Toont alleen eerste element echo $a[0] # Output: Null[0] - Waarschijnlijk niet wat de bedoeling was echo ${a[1]} # OK
Multi-dimensional arrays
Bash ondersteunt geen multidimensionale arrays, maar je kunt een heel eind komen middels associative arrays
Associative arrays
Length of an array
E.g.:
Return all indices
${!j[@]}
returns all indices. This may not be what I expected. E.g.:
unset j mapfile -t j < <( wp post list --post_type=product --field=ID --posts_per_page=64 ) echo "Number of items: ${!j[@]}"
Output (some output removed):
Number of items: 0 1 2 3 ... 64 65
Loop through array
Dit werkt. Merk op dat $i
de waarde van de cel bevat, niet de index:
array=( one two three ) for i in "${array[@]}" do echo "$i" done
Loop through the index of an array
Rather than going through the entries of an array, let's loop through its index. Note that this is base 0:
array=( one two three ) for i in "${!array[@]}" do echo "Index: $i - Value: ${array[i]}" done
Output:
Index: 0 - Value: one Index: 1 - Value: two Index: 2 - Value: three
Unset
Use unset to destroy an array. Note that the examples below, exhibit some dubious use of associative arrays:
#1/bin/bash # Create & fill an array ######################################## # declare -A tr echo "Newly declared array: ${tr[@]}" tr[1,1]="hoi"; tr[1,2]="doei" echo "Filled array: ${tr[@]}" # Re-declare doesn't reset ######################################## # declare -A tr echo "Array after redeclaring - It doesn't reset the array:" echo ${tr[@]} # Unset array ######################################## # unset tr echo "Array after unset - Now it's empty: ${tr[@]}" # Unset a non-existing array ######################################## # echo "Unsetting a non-existing array doesn't give an error:" unset ik_besta_niet
Uitvoer:
Newly declared array: Filled array: doei hoi Array after redeclaring - It doesn't reset the array: doei hoi Array after unset - Now it's empty: Unsetting a non-existing array doesn't give an error:
Reading output of a command into an array
I want to use WP-CLI command wp post list
to return IDs of products. I want to store these IDs in an array. Subsequently, I want to use parallelisation to do some nifty stuff with these IDs, without having to wait until tomorrow morning for the results.
This doesn't work:
j=$(wp post list --post_type=product --field=ID --posts_per_page=64)
Eventhough echo ${j[@]}
gives a promising result, I think it's just one string and not an array.
This works [1]:
unset j mapfile -t j < <( wp post list --post_type=product --field=ID --posts_per_page=64 ) echo "Whole array: ${j[@]}" echo "Item 5: ${j[5]}"
See also
Sources
- https://opensource.com/article/18/5/you-dont-know-bash-intro-bash-arrays
- https://linuxhint.com/associative_array_bash/
- https://stackoverflow.com/questions/11233825/multi-dimensional-arrays-in-bash
- https://stackoverflow.com/questions/3112687/how-to-iterate-over-associative-arrays-in-bash
- https://stackoverflow.com/questions/11426529/reading-output-of-a-command-into-an-array-in-bash