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Bash arrays
===========
:copyright 2013 Luke Shumaker
Way too many people don't understand Bash arrays. Many of them argue
that if you need arrays, you shouldn't be using Bash. If we reject
the notion that one should never use Bash for scripting, then thinking
you don't need Bash arrays is what I like to call "wrong".
The simple expanation of why everybody who programs in Bash needs to
understand arrays is this: command line arguments are exposed as an
array. Does your script take any arguments on the command line?
Great, you need to work with an array!
General array syntax
--------------------
The most important things to understanding arrays is to quote them,
and understanding the difference between `@` and `*`.
<table>
<caption>
<h1>Getting the entire array</h1>
<p>There is <em>no</em> valid reason to not wrap these in double
quotes.</p>
</caption>
<tbody>
<tr>
<td><code>"${array[@]}"</code></td>
<td>Returns every element of the array as a separate token.</td>
</tr><tr>
<td><code>"${array[*]}"</code></td>
<td>Returns every element of the array in a single
whitepace-separated string.</td>
</tr>
</tbody>
</table>
It's really that simple—that covers most usages of arrays, and most of
the mistakes made with them.
To help you understand the difference between `@` and `*`, here is a
sample.
<pre><code>#!/bin/bash
array=(foo bar baz)
for item in "${array[@]}"; do
echo " - <${item}>"
done<hr> - <foo>
- <bar>
- <baz></code></pre>
<pre><code>#!/bin/bash
array=(foo bar baz)
for item in "${array[@]}"; do
echo " - <${item}>"
done<hr> - <foo bar baz></code></pre>
To get individual entries, the syntax is
<code>${array[<var>n</var>]}</code>, where <var>n</var> starts at 0.
<table>
<caption>
<h1>Getting a single entry from the array</h1>
</caption>
<tbody>
<tr>
<td><code>"${array[<var>n</var>]}"</code></td>
<td>Returns the <var>n</var>th entry of the array, where the
first entry is at <var>n</var>=0.</td>
</tr>
</tbody>
</table>
To get a subset of the array, there are a few options (like normal,
switch between `@` and `*` to switch between
getting it as separate items, and as a whitespace-separated string):
<table>
<caption>
<h1>Getting subsets of an array</h1>
<p>Substitute <code>*</code> for <code>@</code> to get the subset
as a whitespace-separated string instead of separate tokens, as
described above.</p>
<p>Again, there is no valid reason to not wrap each of these in
double quotes.</p>
</caption>
<tbody>
<tr>
<td><code>"${array[@]:<var>start</var>}"</code></td>
<td>Returns from <var>n</var>=<var>start</var> to the end of the array.</td>
</tr><tr>
<td><code>"${array[@]:<var>start</var>:<var>count</var>}"</code></td>
<td>Returns <var>count</var> entries, starting at <var>n</var>=<var>start</var>.</td>
</tr><tr>
<td><code>"${array[@]::<var>count</var>}"</code></td>
<td>Returns <var>count</var> entries from the beginning of the array.</td>
</tr>
</tbody>
</table>
Notice that `"${array[@]}"` is equivalent to `"${array[@]:0}"`.
<table>
<caption>
<h1>Getting the length of an array</h1>
<p>The is the only situation where there is no difference
between <code>@</code> and <code>*</code>.</p>
</caption>
<tbody>
<tr>
<td>
<code>${#array[@]}</code>
<br>or<br>
<code>${#array[*]}</code>
</td>
<td>
Returns the length of the array
</td>
</tr>
</tbody>
</table>
Accessing the arguments array
-----------------------------
Accessing the arguments is mostly that simple, but that array doesn't
actually have a variable name. It's special. Instead, it is exposed
through a series of special variables (normal variables can only start
with letters and underscore), that *mostly* match up with the normal
array syntax.
<table>
<caption>
<h1>Accessing the arguments array</h1>
<aside>Note that for values of <var>n</var> with more than 1
digit, you need to wrap it in <code>{}</code>.
Otherwise, <code>"$10"</code> would be parsed
as <code>"${1}0"</code>.</aside>
</caption>
<tbody>
<tr><th colspan=2>Individual entries</th></tr>
<tr><td><code>${array[0]}</code></td><td><code>$0</code></td></tr>
<tr><td><code>${array[1]}</code></td><td><code>$1</code></td></tr>
<tr><td colspan=2 style="text-align:center">...</td></tr>
<tr><td><code>${array[9]}</code></td><td><code>$9</code></td></tr>
<tr><td><code>${array[10]}</code></td><td><code>${10}</code></td></tr>
<tr><td colspan=2 style="text-align:center">...</td></tr>
<tr><td><code>${array[<var>n</var>]}</code></td><td><code>${<var>n</var>}</code></td></tr>
<tr><th colspan=2>Subset arrays (array)</th></tr>
<tr><td><code>"${array[@]}"</code></td><td><code>"${@:0}"</code></td></tr>
<tr><td><code>"${array[@]:1}"</code></td><td><code>"$@"</code></td></tr>
<tr><td><code>"${array[@]:<var>pos</var>}"</code></td><td><code>"${@:<var>pos</var>}"</code></td></tr>
<tr><td><code>"${array[@]:<var>pos</var>:<var>len</var>}"</code></td><td><code>"${@:<var>pos</var>:<var>len</var>}"</code></td></tr>
<tr><td><code>"${array[@]::<var>len</var>}"</code></td><td><code>"${@::<var>len</var>}"</code></td></tr>
<tr><th colspan=2>Subset arrays (string)</th></tr>
<tr><td><code>"${array[*]}"</code></td><td><code>"${*:0}"</code></td></tr>
<tr><td><code>"${array[*]:1}"</code></td><td><code>"$*"</code></td></tr>
<tr><td><code>"${array[*]:<var>pos</var>}"</code></td><td><code>"${*:<var>pos</var>}"</code></td></tr>
<tr><td><code>"${array[*]:<var>pos</var>:<var>len</var>}"</code></td><td><code>"${*:<var>pos</var>:<var>len</var>}"</code></td></tr>
<tr><td><code>"${array[*]::<var>len</var>}"</code></td><td><code>"${*::<var>len</var>}"</code></td></tr>
<tr><th colspan=2>Array length</th></tr>
<tr><td><code>${#array[@]}</code></td><td><code>$#</code> + 1</td></tr>
</tbody>
</table>
Did notice what was inconsistent? The variables `$*`, `$@`, and `$#`
behave like the <var>n</var>=0 entry doesn't exist.
<table>
<caption>
<h1>Inconsistencies</h1>
</caption>
<tbody>
<tr>
<th colspan=3><code>@</code> or <code>*</code></th>
</tr><tr>
<td><code>"${array[@]}"</code></td>
<td>→</td>
<td><code>"${array[@]:0}"</code></td>
</tr><tr>
<td><code>"${@}"</code></td>
<td>→</td>
<td><code>"${@:1}"</code></td>
</tr><tr>
<th colspan=3><code>#</code></th>
</tr><tr>
<td><code>"${#array[@]}"</code></td>
<td>→</td>
<td>length</td>
</tr><tr>
<td><code>"${#}"</code></td>
<td>→</td>
<td>length-1</td>
</tr>
</tbody>
</table>
These make sense because argument 0 is the name of the script—we
almost never want that when parsing arguments. You'd spend more code
getting the values that it currently gives you.
|
