bash shell数组详解

原始文章：https://codefather.tech/blog/bash-array/
Bash Indexed Array of Strings
We will start by creating an indexed array of strings where the strings are directory names in a Linux system:

dirs=(“/etc” “/var” “/opt” “/tmp”)
First of all let’s see what gets printed when we echo the value of the array variable dirs:

$ echo $dirs
/etc
When you print a Bash array variable the result is the first element of the array.

Another way to print the first element of the array is by accessing the array based on its index.

Bash indexed arrays are zero based, this means that to access the first element we have to use the index zero.

$ echo ${dirs[0]}
/etc
Wonder why we are using curly brackets?

We can understand why by removing them to see what the ouput is:

$ echo $dirs[0]
/etc[0]
Bash prints the first element of the array followed by [0] because it only recognises $dirs as a variable. To include [0] as part of the variable name we have to use curly brackets.

In the same way, to print the second element of the array we will access the index 1 of the array:

$ echo ${dirs[1]}
/var
What if we want to access the last element of the array?

Before doing that we have to find out how to get the length of a Bash array…

How Do You Determine the Length of a Bash Array?
To find the length of an array in Bash we have to use the syntax ${#array_name[@]}.

Let’s apply it to our example:
$ echo ${#dirs[@]}
4
The syntax might seem hard to remember when you see it for the first time…

…but don’t worry, just practice it a few times and you will remember it.

Access the Last Element of a Bash Array
Now that we know how to get the number of elements in a Bash array, we can use this information to retrieve the value of the last element.

First we have to calculate the index of the last element that is equal to the number of elements in the array minus one (remember that Bash arrays are zero-based as usually happens in most programming languages).

${#dirs[@]}-1
This value will be the index to pass when we want to print the last element of the array:
$ echo KaTeX parse error: Expected '}', got 'EOF' at end of input: {dirs[{#dirs[@]}-1]}
/tmp
Definitely not one of the easiest ways to retrieve the last element of an array, if you are familiar with other programming languages

Since Bash 4.2 arrays also accept negative indexes that allow to access elements starting from the end of the array.

To verify your version of Bash use the following command:

$ bash --version