eolas/Programming Languages/Shell Scripting/Lists and arrays.md

tags
Programming_Languages shell arrays

List variables

When we use the term list in bash, we are not actually referring to a specific type of data structure. Instead a list variable is really just a normal variable wrapped in quote marks that has strings separated by spaces. Despite the fact that this is not an actual iterative data structure, we are still able to loop through variables of this type.

A_STR_LIST="cat dog hamster"
AN_INT_LIST="1 2 3"

To iterate through a list variable, we can use a for loop:

for ele in $A_STR_LIST; do
    echo $ele
done

Brace expansion for listed variables

With a sequence of variables that follow a pattern, for example the natural numbers (1, 2, 3, 4, ...) we can represent them in a condensed format using something called brace expansion. For instance to represent the natural numbers from 1 through 10:

{1..10}

Here the two dots stand for the intervening values.

We can iterate through brace expanded variables just the same:

for num in {1..4}; do
    echo $num
done

Arrays

We define an array as follows:

words=(here are some words)

We can also explicitly define an array using declare :

declare -a words=("element1" "element2" "element3")

Index notation

We access specific array elements by their index using the same braces style we use with variables:

echo ${words[2]}

# element3

Iterating through arrays

for i in "${words[@]}"
do
   echo "$i"
   # or do whatever with individual element of the array
done
# element1 element2 element3

Note that @ here is a special symbol standing for all the members of the words array.

Looping through file system

The following script loops through all files in a directory that begin with l and which are of the bash file type (.sh) :

for x in ./l*.sh; do
    echo -n "$x "
done
echo