c/linux · by unglueclass23@programming.dev · 42d

A Surprisingly Common Mistake Involving Wildcards & The Find Command

https://blog.robertelder.org/find-command-wildcard-globbing/

I came across this older article from 2020 and I found it informative. It's about how the shell does globbing and the potential issues it can cause if not understood correctly.

TLDR:

find . -not -name *.py -delete and find . -not -name '*.py' -delete will behave differently in certain scenarios.

In the first example, the shell will replace the wildcard pattern with a list of matching file names IF there are any matches in the current directory. If there isn't, then it won't do anything and will pass *.py to find.

In the second example, the shell won't do any globbing at all and will just pass *.py

52 points · 9 comments · view on lemmy.world

9 Comments

vk6flab@lemmy.radio · 22 pts · 42d (4 replies)

Basic habit to get into:

If you're contemplating doing something destructive, do a dry-run first.

In this case, remove the -delete flag, run the command and see what you get.

A good approach is to build a command step by step and test your assumptions each iteration.

Things might take a few moments longer, but one day it's going to save your bacon.

As for unexpected globbing, learn the difference between quoted and unquoted, and single versus double quotes.

Source: Linux user for 25+ years

unglueclass23@programming.dev · 4 pts · 42d (2 replies)

What's the difference between single vs double quotes?

IanTwenty@piefed.social · 11 pts · 42d

Presuming BASH:

https://www.gnu.org/software/bash/manual/bash.html#Quoting

Single quotes are much safer/predictable because:

Enclosing characters in single quotes (‘'’) preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.

Not to say they are always the right choice though.

lagoon8622@sh.itjust.works · 6 pts · 42d

The shell doesn't expand or interpolate single-quoted strings, they are literals

supersquirrel@sopuli.xyz · 1 pts · 42d

If you're contemplating doing something destructive, do a dry-run first.

@vk6flab@lemmy.radio is a WITCH I tell you! This is black magic!!

p_consti@lemmy.world · 3 pts · 42d (2 replies)

Using zsh will gelp with that mistake, because by default it will turn the 'no matches' scenario into an error. But seriously, always quote your arguments if there is any chance for ambiguity (also for variables)

unglueclass23@programming.dev · 2 pts · 42d

I use bash and it did throw that error, however the find . | egrep *.py example at the end did not.

fruitcantfly@programming.dev · 1 pts · 42d

It took a while to get used to zsh bailing on non-matching wildcards, but it honestly makes a lot more sense

toynbee@piefed.social · 2 pts · 42d

Don't forget backslashes as an option.