[linux] What linux shell command returns a part of a string?

In "pure" bash you have many tools for (sub)string manipulation, mainly, but not exclusively in parameter expansion :

${parameter//substring/replacement}
${parameter##remove_matching_prefix}
${parameter%%remove_matching_suffix}

Indexed substring expansion (special behaviours with negative offsets, and, in newer Bashes, negative lengths):

${parameter:offset}
${parameter:offset:length}
${parameter:offset:length}

And of course, the much useful expansions that operate on whether the parameter is null:

${parameter:+use this if param is NOT null}
${parameter:-use this if param is null}
${parameter:=use this and assign to param if param is null}
${parameter:?show this error if param is null}

They have more tweakable behaviours than those listed, and as I said, there are other ways to manipulate strings (a common one being $(command substitution) combined with sed or any other external filter). But, they are so easily found by typing man bash that I don't feel it merits to further extend this post.

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