Bash arrays

September 18, 2013 [bash, Programming, Programming Languages, Tech]

Bash arrays are a lot like Bash Associative Arrays, but with numbers as keys.

Here's a quick reference.

Basics

$ declare -a MYARR  # Create an array
$ MYARR[3]=foo      # Put a value into an array
$ echo ${MYARR[3]}  # Get a value out of an array
foo
$ echo MYARR[3]     # WRONG
MYARR[0]
$ echo $MYARR[3]]   # WRONG
[3]

Creating, adding

$ declare -a MYARR    # Explicitly declare
$ MYARR[3]=foo        # Or this line implicitly makes it an array
$ MYARR[4]=bar        # Can add values one by one

$ declare -a MYARR=(a b c)   # Initialise all at once

$ echo ${MYARR[0]}
a
$ echo ${MYARR[1]}
b
$ echo ${MYARR[2]}
c

$ declare -a MYARR   # Or declare separately
$ MYARR=(a b c)      # Then initialise
$ echo ${MYARR[0]}
a
$ echo ${MYARR[1]}
b
$ echo ${MYARR[2]}
c

$ declare -a MYARR=(a b c)
$ MYARR=("${MYARR[@]}" d)  # Add an element
$ echo ${MYARR[@]}
a b c d
$ declare -a MYARR2=(e f g)
$ MYARR=("${MYARR[@]}" "${MYARR2[@]}")  # Concatenate arrays
$ echo ${MYARR[@]}
a b c d e f g

Keys/Indices

$ declare -a MYARR
$ MYARR[3]=foo
$ echo ${MYARR[0]}  # Unassigned values are empty

$ echo ${MYARR[4]}  # Unassigned values are empty

$ MYARR[seven]=bar     # A text index is treated as 0
$ echo ${MYARR[0]}
bar
$ echo ${MYARR[seven]} # A text index is treated as 0
bar

$ K=3

$ MYARR[$K]=baz      # Variables containing numbers work like numbers
$ echo ${MYARR[$K]}
baz
$ echo ${MYARR[3]}   # Obviously the value is accessible via the actual index
baz
$ K=foo
$ MYARR[$K]=bash     # Variables containing text are treated as 0
$ echo ${MYARR[0]}
bash

Length

$ declare -a MYARR=(a b c)
$ echo ${#MYARR[@]}  # Length of an array
3
$ echo $#MYARR[@]  # WRONG
0MYARR[@]
$ echo ${#MYARR}   # WRONG
1

$ MYARR[7]=x

$ echo ${#MYARR[@]}  # Only existing indices count in the length
4

$ declare -a MYARR=(a bb ccc)
$ echo ${#MYARR[0]}   # Length of an individual element
1
$ echo ${#MYARR[1]}
2
$ echo ${#MYARR[2]}
3

Looping

$ declare -a MYARR=("a 1" b c)
$ # Loop through array values
$ for V in "${MYARR[@]}"; do echo $V; done
a 1
b
c
$ for V in ${MYARR[@]}; do echo $V; done  #WRONG
a
1
b
c

$ echo "${!MYARR[@]}"  # Print all indices - quoted, but quotes removed by echo

0 1 2
$ echo "${MYARR[@]}"   # Print all values - quoted, but quotes removed by echo
a 1 b c

Clearing

$ declare -a MYARR
$ MYARR[3]=x

$ echo ${MYARR[3]}
x
$ unset MYARR
$ declare -a MYARR
$ echo ${MYARR[3]}

Deleting

$ MYARR[2]=foo
$ echo ${MYARR[2]}
foo
$ unset ${MYARR[2]} # WRONG
$ echo ${MYARR[2]}
foo
$ unset MYARR[2]    # To delete from an array, use "unset" with similar syntax to assigning
$ echo ${MYARR[2]}

$ MYARR[3]=quux
$ echo ${MYARR[3]}
quux
$ K=3
$ unset MYARR[$K]   # Can unset using a variable for the key too
$ echo ${MYARR[3]}


$ declare -a MYARR=(a b c d e f)
$ MYARR=("${MYARR[@]:0:3}" "${MYARR[@]:4}")  # Remove element 3, leaving no gap
$ echo ${MYARR[@]}

Cool stuff

$ declare -a MYARR=(a b c d e f g)
$ echo ${MYARR[@]:2:3}              # Extract a sub-array
c d e

$ declare -a MYARR=(a b c d e f g)
$ echo ${MYARR[@]/d/FOO}            # Replace elements that match
a b c FOO e f g

Scope

$ unset MYARR
$ function createmap() { MYARR[5]=bar; }  # Implicit creation puts it in the global scope
$ echo ${MYARR[5]}

$ createmap
$ echo ${MYARR[5]}
bar

$ unset MYARR

$ function createmaplocal() { declare -a MYARR; MYARR[3]=bar; }  # Explicit creation puts it in the local scope
$ echo ${MYARR[3]}


$ createmaplocal
$ echo ${MYARR[3]}