Category: Tech

Quick reference of things I discovered about how to use associative arrays in bash. Note: bash version 4 only.

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Update: see also Bash Arrays.

Basics

$ declare -A MYMAP     # Create an associative array
$ MYMAP[foo]=bar       # Put a value into an associative array
$ echo ${MYMAP[foo]}   # Get a value out of an associative array
bar
$ echo MYMAP[foo]      # WRONG
MYMAP[foo]
$ echo $MYMAP[foo]     # WRONG
[foo]

Creating

$ declare -A MYMAP     # Explicitly declare
$ MYMAP[foo]=bar       # Or this line implicitly makes it an associative array (in global scope, bash 4.2+ only)
$ MYMAP[baz]=quux      # Can add multiple values one by one
$ MYMAP[corge]=grault

$ declare -A MYMAP=( [foo]=bar [baz]=quux [corge]=grault ) # Initialise all at once
$ echo ${MYMAP[foo]}
bar
$ echo ${MYMAP[baz]}
quux

$ declare -A MYMAP                              # Or declare separately
$ MYMAP=( [foo]=bar [baz]=quux [corge]=grault ) # Then initialise
$ echo ${MYMAP[foo]}
bar
$ echo ${MYMAP[baz]}
quux

Variables as keys

$ K=baz
$ MYMAP[$K]=quux       # Use a variable as key to put a value into an associative array
$ echo ${MYMAP[$K]}    # Use a variable as key to extract a value from an associative array
quux
$ echo ${MYMAP[baz]}   # Obviously the value is accessible via the literal key
quux

Quoting keys

$ declare -A MYMAP
$ MYMAP[foo A]="bar B"         # Quoting keys makes no difference
$ MYMAP["corge X"]="grault Y"  # Quoting keys makes no difference
$ echo ${MYMAP["corge X"]}     # You can access by quoting
grault Y
$ echo ${MYMAP[corge X]}       # Or not bother
grault Y
$ echo ${MYMAP[foo A]}
bar B

$ MYMAP['waldo 1']="fred 2"    # Single quotes also make no difference
$ echo ${MYMAP['waldo 1']}
fred 2
$ echo ${MYMAP[waldo 1]}
fred 2

$ K=plugh
$ MYMAP['$K']=xyzzy   # Except single quotes prevent variable expansion, as usual
$ echo ${MYMAP[plugh]}

$ echo ${MYMAP['$K']}
xyzzy

Missing keys

$ MYMAP[foo]=bar
$ echo ${MYMAP[missing]}   # Accessing a missing value gives ""

$ # Testing whether a value is missing from an associative array
$ if [ ${MYMAP[foo]+_} ]; then echo "Found"; else echo "Not found"; fi
Found
$ if [ ${MYMAP[missing]+_} ]; then echo "Found"; else echo "Not found"; fi
Not found

Looping

$ declare -A MYMAP=( [foo a]=bar [baz b]=quux )
$ echo "${!MYMAP[@]}"  # Print all keys - quoted, but quotes removed by echo
foo a baz b

$ # Loop through all keys in an associative array
$ for K in "${!MYMAP[@]}"; do echo $K; done
foo a
baz b

$ for K in ${!MYMAP[@]}; do echo $K; done     # WRONG
foo
a
baz
b

$ # Looping through keys and values in an associative array
$ for K in "${!MYMAP[@]}"; do echo $K --- ${MYMAP[$K]}; done
foo a --- bar
baz b --- quux

$ # Loop through all values in an associative array
$ for V in "${MYMAP[@]}"; do echo $V; done
bar
quux

Clearing

$ declare -A MYMAP
$ MYMAP[foo]=bar
$ echo ${MYMAP[foo]}
bar
$ declare -A MYMAP    # Re-declaring DOES NOT clear an associative array
$ echo ${MYMAP[foo]}
bar
$ unset MYMAP         # You need to unset and re-declare to get a cleared associative array
$ declare -A MYMAP
$ echo ${MYMAP[foo]}

Deleting keys

$ MYMAP[foo]=bar
$ echo ${MYMAP[foo]}
bar
$ unset ${MYMAP[foo]} # WRONG
$ echo ${MYMAP[foo]}
bar
$ unset MYMAP[foo]    # To delete from an associative array, use "unset" with similar syntax to assigning
$                     # BUT see next section if key contains spaces
$ echo ${MYMAP[foo]}

$ MYMAP[baz]=quux
$ echo ${MYMAP[baz]}
quux
$ K=baz
$ unset MYMAP[$K]       # Can unset using a variable for the key too
$                       # BUT see next section if variable may contain spaces - always better to quote
$ echo ${MYMAP[baz]}

Deleting keys containing spaces

$ declare -A MYMAP
$ MYMAP[foo Z]=bar
$ echo ${MYMAP[foo Z]}
bar
$ unset MYMAP[foo Z]     # WRONG
bash: unset: `MYMAP[foo': not a valid identifier
bash: unset: `Z]': not a valid identifier
$ unset MYMAP["foo Z"]    # You must quote keys containing spaces when you unset in an associative array
$ echo ${MYMAP[foo Z]}

$ MYMAP[foo Z]=bar
$ unset MYMAP['foo Z']    # Single quotes work too
$ echo ${MYMAP[foo Z]}

$ MYMAP[baz A]=quux
$ echo ${MYMAP[baz A]}
quux
$ K="baz A"
$ unset MYMAP[$K]         # WRONG
bash: unset: `MYMAP[baz': not a valid identifier
bash: unset: `A]': not a valid identifier
$ unset MYMAP["$K"]       # You must quote variables whose values may contain spaces when you unset in an associative array
$ echo ${MYMAP[baz A]}

$ MYMAP[baz A]=quux
$ unset MYMAP['$K']       # Curiously, single quotes work here too, although I'd suggest avoiding them
$ echo ${MYMAP[baz A]}

Length

$ declare -A MYMAP=( [foo a]=bar [baz b]=quux )
$ echo ${#MYMAP[@]}  # Number of keys in an associative array
2
$ echo $#MYMAP[@]  # WRONG
0MYMAP[@]
$ echo ${#MYMAP}   # WRONG
0

Numeric indexing

$ declare -A MYMAP=( [foo a]=bar [baz b]=quux )
$ KEYS=("${!MYMAP[@]}")      # Make a normal array containing all the keys in the associative array
$ echo ${KEYS[0]}            # Find a key via an index
foo a
$ echo ${MYMAP[${KEYS[0]}]}  # Find a value via an index
bar

$ # Loop through using an index
$ for (( I=0; $I < ${#MYMAP[@]}; I+=1 )); do KEY=${KEYS[$I]};  echo $KEY --- ${MYMAP[$KEY]}; done
foo a --- bar
baz b --- quux

Scope

$ unset MYMAP
$ function createmap() { MYMAP[foo]=bar; }  # Implicit creation puts it in the global scope
$ echo ${MYMAP[foo]}

$ createmap
$ echo ${MYMAP[foo]}
bar

$ unset MYMAP
$ function createmaplocal() { declare -A MYMAP; MYMAP[foo]=bar; }  # Explicit creation puts it in the local scope
$ echo ${MYMAP[foo]}

$ createmaplocal
$ echo ${MYMAP[foo]}

Checking Bash version

$ bash --version   # Must be at least version 4 to have associative arrays
GNU bash, version 4.2.24(1)-release (x86_64-pc-linux-gnu)
...

Links

• Bash manual: Arrays
• Linux Journal: Associative Arrays in Bash
• Bash Hackers Wiki: Arrays
• Superuser: Test if element is in array in Bash
• Stackoverflow: How to iterate over associative array in bash