echo output with % and %

Question

When solving some exercises in Shell Script, I find this script:

x="Este texto para teste."
echo ${x% *}

I confess that, when performing the table test, I could not solve it. When running this script the result was:

Este texto para

However if you modify echo to echo ${x%% *}, the result will be:

Este

What is the explanation for these results, both with % as to %%? Is there any practical use in these expressions or their emphasis is only on teaching exercises?

Answer 1

Those are just two of the many ways to manipulate strings in Bash. In this case, both serve to remove a snippet from the end of a string.

The difference is that % removes as little as possible, while %% removes as much as possible. For example, if I have:

texto='abcdefbxyz'

And make:

echo ${texto%b*z}

It will remove the smallest snippet of the end of the string that corresponds to b*z (the letter b, zero or more characters and the letter z). The shortest stretch that satisfies this condition is bxyz, therefore result of this echo is abcdef.

Now if I do:

echo ${texto%%b*z}

It will remove the longest stretch of the end of the string that corresponds to b*z (the letter b, zero or more characters and the letter z). The longest stretch that satisfies this condition is bcdefbxyz, therefore result of this echo is a.

---

In your case, the string is "Este texto para teste.". In doing echo ${x% *}, it removes the smallest snippet of the end of the string that corresponds to a space followed by zero or more characters. The snippet that satisfies this condition is teste., so the result is Este texto para.

And what to do echo ${x%% *}, it removes the longest stretch of the end of the string that corresponds to a space followed by zero or more characters. The snippet that satisfies this condition is texto para teste., so the result is Este.

It’s a very useful resource (for any situation that involves removing a final chunk of a string), far from being something "just for exercises".

In the documentation has some examples.