What is the meaning of asterisk, backslash, colon and two in (*\:2)?

What is the meaning of following ls command?

ls -l *\:2,*T

ls = list
-l = long (list)
*T probably means T at the end

But what about that *\:2 and the comma?

jugmac00

Posted 2017-08-18T06:40:40.597

Reputation: 259

Answers

*\:2,*T is the glob pattern of files to list. To understand it, we need to remember a few things:

: has to be escaped in the shell, becoming \:
File names can easily contain commas

So *\:2,*T would e.g. match a file called TranscationNumber:2,EventType:XYT

EDIT

From the comments, the necessity (or not) of escaping the : needs a few words: It is not strictly necessary to escape the : sign, but bash itself suggests it when autocompleting.

Eugen Rieck

Posted 2017-08-18T06:40:40.597

Reputation: 15 128

Thank you, both Eugen and HBruijn. I suspected some complicated globbing pattern, but actually it should just match the colon and the comma, which I did not think of when working with files. Now, the command totally makes sense: list all email files in the Maildir directories, which are marked as "Deleted" (T). Btw. The number 2 means Maildir in version 2 (cf https://wiki2.dovecot.org/MailboxFormat/Maildir).

– jugmac00 – 2017-08-18T11:20:35.040

11Since when does : have to be escaped? – Barmar – 2017-08-18T17:48:20.623

7: doesn't need to be escaped. It is only special in specific circumstances (when used as a command), not in glob patterns. The \: is equivalent to :, escaping makes no difference. – terdon – 2017-08-18T18:11:30.147

@terdon I was wondering in which case it would be interesting to have \: as a command, and found out that bash still seems to parse \: as the NOP command. See this ideone. I guess it was different in older versions of bash or other shells?

– Aaron – 2017-08-19T00:59:04.973

@Barmar Edited my answer, thanks for pointing this out! – Eugen Rieck – 2017-08-19T01:33:02.270

1@Aaron, it's not exactly no-op: the shell will still process variable substitutions, particularly the assignment variety (i.e. to set default values): unset var; : ${var:=x}; echo $var – glenn jackman – 2017-08-19T13:23:40.440

@Aaron I wouldn't expect \: to protect if from being recognized as a command since that is another common construct used, for example, to ignore aliases. On the other hand, tab completion on a file whose name has a : results in \:. I don't know why that would happen but I would guess that it is historical and that older shells would choke on it. I tested on bash, dash, tcsh and ksh and all of them had no issue dealing with :. Presumably, some older shells did.

– terdon – 2017-08-22T15:15:13.373

@terdon thanks, I didn't know about that use of backslash, I would have used command or builtin to achieve the same effect. There's definitely something weird with the auto-completion : I've tested with bash on both a Cygwin and an RHEL and got some weird behaviors, including the one you mention, the inability to complete just after the : or even worse, completion proposals based only on the part following the : (i.e. completion after test:te proposing every file starting with te). Looks like escaping them can be a good idea. – Aaron – 2017-08-22T16:42:11.420

3@terdon & @Aaron The colon : is part of the default values for the COMP_WORDBREAKS environment variable. In Bash autocompletion those characters are used as word separators by the read line library and a file-name containing a colon should of course be a single word. Hence the reason that bash autocompletion requires the colon to be escaped, although otherwise it is "mostly" not a special character... – HBruijn – 2017-08-23T07:49:04.980

List all files that match the wildcard pattern *:2,*T

There the wildcard * matches anything (any number of all possible characters)
:2, are characters that need to be present in the file/directory names.
The colon : is a special character that needs to be escaped, hence the form of \:2,.
The file/directory names need to end with a T.

File names that would match would be

:2,T
a:2,T
a:2,bT
abbY-$fafaf:2,<hskjhsgdfhjk>T

HBruijn

Posted 2017-08-18T06:40:40.597

Reputation: 1 024

1There's nothing special about :, it doesn't need to be escaped. – Kevin – 2017-08-18T19:05:45.730

@Kevin bash auto-completion automatically escapes the colon in file/directory name patterns, because is included in the list of COMP_WORDBREAK environment values that get set by default. - So it is indeed not an actual special character, but when you use autocompletion on the commandline and want/need to match filenames with a colon , it does need to be escaped. – HBruijn – 2017-08-23T07:39:59.357

As others have noted, this will list in long format, files containing :2, and ending in T

This looks like a search in a Maildir folder for files that were deleted (trashed). However, for robustness it should have had another * at the end, though. New flags with a later alphabetical position could be added, and Dovecot for instance adds another field with the file size at the end.

https://cr.yp.to/proto/maildir.html

Ángel

Posted 2017-08-18T06:40:40.597

Reputation: 960