a glob w/ no wildcards returns itself

jeberle has asked for the wisdom of the Perl Monks concerning the following question:

When I supply a pattern w/o any glob characters, I get the pattern back, even if no file matches it. Why does Perl's glob do this?

$ echo foo.c
foo.c

$ echo foo.?
foo.?

$ ls foo.c
ls: foo.c: No such file or directory

$ ls foo.?
ls: foo.?: No such file or directory

$ perl -le 'print glob "foo.c"'
foo.c  # shell expansion w/o existence check

$ perl -le 'print glob "foo.?"'
# shell expansion w/ existence check
[download]

Glob's behavior w/o wildcards matches a simple shell expansion, while glob's behavior w/ wildcards matches a file existence test.

Just to make this sting a little, Python's behavior is consistent:

$ python -c 'from glob import glob; print(glob("foo.c"))'
[]

$ python -c 'from glob import glob; print(glob("foo.?"))'
[]
[download]

Comment on a glob w/ no wildcards returns itself Select or Download Code

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Re: a glob w/ no wildcards returns itself by Eliya (Vicar) on Aug 23, 2011 at 19:08 UTC
While you can certainly argue for (or against) any of the different globbing behaviors - ranging from "the shell"'s (interpret wildcards literally, if no match in the file system) to Python's (always test for existence) - ultimately, it's a matter of preference. Personally, I find Perl's mixed behavior rather "do-what-I-mean". I.e., when I specify a `?` or `` wildcard, I'm typically interested in what matching files there are in the file system, while if I use a "limited set" wildcard like `{a,b,c}`, I can compactly compute permutations with it, e.g. `$ perl -le 'print for glob "{foo,bar}{baz,quux}"' foobaz fooquux barbaz barquux` [download] which Python doesn't do because of its consistent behavior. IOW, consistent isn't always the most practically useful... :) And, as for the shell, I always found its behavior to treat a "" literally (in case nothing matches) a little counter-intuitive... (if I really mean a literal "*", I can always escape it in Perl). Anyhow, YMMV.	[reply] [d/l] [select]
Re^2: a glob w/ no wildcards returns itself by jeberle (Initiate) on Aug 23, 2011 at 19:32 UTC
Thank you Eliya. The use case I have is to do actual file matching, where the user specifies the pattern. For this, the behavior is counter-productive. Therefore: `$ perl -le 'print grep { -f } glob "foo.c"'` [download] will have to rule the day. In bash you can do: `$ shopt -s nullglob $ echo foo.?` [download] but then `ls` generates full dir listings on failed globs.	[reply] [d/l] [select]
Re: a glob w/ no wildcards returns itself by ikegami (Patriarch) on Aug 23, 2011 at 20:01 UTC
It makes no sense to check if the file exists when wild cards aren't present. `perl foo.pl --help shouldn't become perl foo.pl perl foo.pl infile outfile shouldn't become perl foo.pl infile` [download] It makes no sense to not honour wild cards. If you ask for all files matching «`foo.?`», it makes no sense to return «`foo.?`» unless «`foo.?`» exists. If you want the glob to return «`foo.?`», you'd pass «`foo.\?`». PS — Perl matches csh's behaviour. You tested using the wrong shell.	[reply] [d/l] [select]