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Arrays in Shell Scripting
Newer versions of Bash support one-dimensional arrays. Array elements may be initialized with the variable[xx] notation. Alternatively, a script may introduce the entire array by an explicit declare -a variable statement. To dereference (retrieve the contents of) an array element, use curly bracket notation, that is, ${element[xx]}.
Example 26-1. Simple array usage
#!/bin/bash
area[11]=23
area[13]=37
area[51]=UFOs
# Array members need not be consecutive or contiguous.
# Some members of the array can be left uninitialized.
# Gaps in the array are okay.
# In fact, arrays with sparse data ("sparse arrays")
#+ are useful in spreadsheet-processing software.
echo -n "area[11] = "
echo ${area[11]} # {curly brackets} needed.
echo -n "area[13] = "
echo ${area[13]}
echo "Contents of area[51] are ${area[51]}."
# Contents of uninitialized array variable print blank (null variable).
echo -n "area[43] = "
echo ${area[43]}
echo "(area[43] unassigned)"
echo
# Sum of two array variables assigned to third
area[5]=`expr ${area[11]} + ${area[13]}`
echo "area[5] = area[11] + area[13]"
echo -n "area[5] = "
echo ${area[5]}
area[6]=`expr ${area[11]} + ${area[51]}`
echo "area[6] = area[11] + area[51]"
echo -n "area[6] = "
echo ${area[6]}
# This fails because adding an integer to a string is not permitted.
echo; echo; echo
# -----------------------------------------------------------------
# Another array, "area2".
# Another way of assigning array variables...
# array_name=( XXX YYY ZZZ ... )
area2=( zero one two three four )
echo -n "area2[0] = "
echo ${area2[0]}
# Aha, zero-based indexing (first element of array is [0], not [1]).
echo -n "area2[1] = "
echo ${area2[1]} # [1] is second element of array.
# -----------------------------------------------------------------
echo; echo; echo
# -----------------------------------------------
# Yet another array, "area3".
# Yet another way of assigning array variables...
# array_name=([xx]=XXX [yy]=YYY ...)
area3=([17]=seventeen [24]=twenty-four)
echo -n "area3[17] = "
echo ${area3[17]}
echo -n "area3[24] = "
echo ${area3[24]}
# -----------------------------------------------
exit 0 |
As we have seen, a convenient way of initializing an entire array is the array=( element1 element2 ... elementN ) notation.
Example 26-2. Formatting a poem
#!/bin/bash
# poem.sh: Pretty-prints one of the document author's favorite poems.
# Lines of the poem (single stanza).
Line[1]="I do not know which to prefer,"
Line[2]="The beauty of inflections"
Line[3]="Or the beauty of innuendoes,"
Line[4]="The blackbird whistling"
Line[5]="Or just after."
# Attribution.
Attrib[1]=" Wallace Stevens"
Attrib[2]="\"Thirteen Ways of Looking at a Blackbird\""
# This poem is in the Public Domain (copyright expired).
echo
tput bold # Bold print.
for index in 1 2 3 4 5 # Five lines.
do
printf " %s\n" "${Line[index]}"
done
for index in 1 2 # Two attribution lines.
do
printf " %s\n" "${Attrib[index]}"
done
tput sgr0 # Reset terminal.
# See 'tput' docs.
echo
exit 0
# Exercise:
# --------
# Modify this script to pretty-print a poem from a text data file. |
Array variables have a syntax all their own, and even standard Bash commands and operators have special options adapted for array use.
