If you are using the "standard" * NIX shell, you may not be familiar with the useful bash feature of arrays. While arrays in bash are not as cool as they are in P languages (Perl, Python, and PHP) and other programming languages, they are often useful.
Bash arrays only have numeric indices, but they are optional, which means you don't have to explicitly define the values of all the indices. An entire array can be defined by enclosing the entries in parentheses:
arr=(Hello World)
Individual records can be defined using the familiar syntax (from BASIC (forgive me Dijkstra - translator's note) to Fortran):
arr[0]=Hello
arr[1]=World
However, the opposite looks comparatively more ugly. If you need to refer to a specific entry, then:
echo ${arr[0]} ${arr[1]}
From the man page:
" Curly braces are needed to prevent conflicts when expanding full paths to files. "
In addition, the following weird constructs are available:
${arr[*]} #
${!arr[*]}#
${#arr[*]}#
${#arr[0]}# ( )
$ {! arr [*]} is a relatively new addition to bash and is not part of the original implementation. The following construction shows an example of a simple use of an array. Notice the "[index] = value", this allows you to assign a specific value to a specific record number.
#!/bin/bash
array=(one two three four [5]=five)
echo "Array size: ${#array[*]}" #
echo "Array items:" #
for item in ${array[*]}
do
printf " %s\n" $item
done
echo "Array indexes:" #
for index in ${!array[*]}
do
printf " %d\n" $index
done
echo "Array items and indexes:" #
for index in ${!array[*]}
do
printf "%4d: %s\n" $index ${array[$index]}
done
Running the script will generate the following output:
Array size: 5
Array items:
one
two
three
four
five
Array indexes:
0
1
2
3
5
Array items and indexes:
0: one
1: two
2: three
3: four
5: five
Note that the "@" character can be used instead of "*" in constructs like {arr [*]}, the result will be the same except for the expansion of the quoted entry. "$ *" and "$ @" will output quoted entries, "$ {arr [*]}" will return each entry as one word, "$ {arr [@]}" will return each entry as separate words.
The following example will show how quotes and unquoted constructs return strings (especially important when these strings contain spaces):
#!/bin/bash
array=("first item" "second item" "third" "item")
echo "Number of items in original array: ${#array[*]}"
for ix in ${!array[*]}
do
printf " %s\n" "${array[$ix]}"
done
echo
arr=(${array[*]})
echo "After unquoted expansion: ${#arr[*]}"
for ix in ${!arr[*]}
do
printf " %s\n" "${arr[$ix]}"
done
echo
arr=("${array[*]}")
echo "After * quoted expansion: ${#arr[*]}"
for ix in ${!arr[*]}
do
printf " %s\n" "${arr[$ix]}"
done
echo
arr=("${array[@]}")
echo "After @ quoted expansion: ${#arr[*]}"
for ix in ${!arr[*]}
do
printf " %s\n" "${arr[$ix]}"
done
Output at startup:
Number of items in original array: 4
first item
second item
third
item
After unquoted expansion: 6
first
item
second
item
third
item
After * quoted expansion: 1
first item second item third item
After @ quoted expansion: 4
first item
second item
third
item