How to remember which expansion ${var%} ${var#} work from which end?

Question

I am having trouble remembering which one of the parameter expansions ${var%subst} or ${var#subst} remove from the front and which one from the back of the string. Example:

$ var=/a/b/c
$ echo dirname=${var#/*} filename=${var%%*/}
dirname=a/b/c filename=/a/b/c                      # Wrong!
$ echo dirname=${var%/*} filename=${var##*/}
dirname=/a/b filename=c

I always mix them and either end up writing some test commands or checking the manual. It's easy to remember that %% removes more then %, because %% is a longer string then %, 2 characters vs 1 character, same for ## vs #. But I always mix % with #.

Is there a memory rule to know which % or # remove from which end of the string?

consider a cheat sheet of examples; could be a `*.txt` file you can quickly double-click from the desktop, or perhaps define an alias to `cat` the cheat sheet at the command prompt; if you end up with a lot of cheat sheets consider a shell script that runs an input arg through a `case` statement to determine which cheat sheet to display — markp-fuso, Mar 01 '21 at 17:46

score 13 · Answer 1 · answered Mar 01 '21 at 11:46

13

Percent symbols % always come last in numbers (e.g. 86%), so they remove from the end.

The hash symbols # start comments, so they remove from the start.

answered Mar 01 '21 at 11:46

Quasímodo

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"The #1 rule, useful 100% of the time" - That's the phrase I use. Hence `#` removes from the start because it's at the start, `%` removes from the end for the opposite reason. Then the only complication is greedy/non-greedy mode. Obviously people who want *more* (such as `##` rather than `#`) are greedy. – paxdiablo Mar 04 '21 at 07:55

RavinderSingh13 · Answer 2 · 2021-03-01T12:28:27.063

8

Remember only 1 and other will be Opposite of it.

# Shebang starts from a hash which means it will remove from starting till pattern, if you remember this and you know what other does IMHO :)

edited Mar 01 '21 at 12:28

answered Mar 01 '21 at 12:00

RavinderSingh13

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pynexj · Answer 3 · 2021-03-01T12:46:12.047

6

For keyboards of US English layout, # is on the left and % on the right.

edited Mar 01 '21 at 12:46

answered Mar 01 '21 at 11:45

pynexj

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On **American** keyboards at least. On mine, % is on the left and # is on the right. – user1934428 Mar 01 '21 at 12:14

score 1 · Accepted Answer · answered Mar 01 '21 at 12:19

1

I use a figurative representation of the symbols:

%: Looks like a pair of scissors to cut the right part of a text strip while you hold the scissors in your right hand and the strip in your left hand.
#: Looks like an eraser you use to erase the left part of a text strip while you hold the right part with your right hand and the eraser in your left hand.
//: This bash specific text-replace, looks like the cuts in a strip to re-assemble or edit in-between parts.

answered Mar 01 '21 at 12:19

Léa Gris

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Thank You! `%: Looks like a pair of scissors to cut the right part of a text strip` This, worked for me, it just "clicked", because scissors are so visual, I imagined that once, and now I always think of scissors when writing scripts. Then I remember that `#` is the opposite of `%`, and I have it. – KamilCuk Oct 06 '21 at 00:18

score 0 · Answer 5 · answered Mar 01 '21 at 13:30

And I thought I was alone with this problem.

My mnemonic is that since percent sign (%) looks like a slash surrounded by a couple of circles (°/o), put the slashes alongside ie. %/ - also, that also looks like it says o lol (°/o/).

With the asterisk in place (°/o/*) it also looks like a ~~guy~~person holding an orb and a BZZRT in ~~his~~hir raised hands (can not be unseen).

How to remember which expansion ${var%} ${var#} work from which end?

5 Answers5

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