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lazarus
Asked: 2014-09-04 01:18:31 +0800 CST

use of * in file searching

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while reading about linux file searching I got the following...

To search for files using the * wildcard, replace the unknown string with *, e.g. if you remember only that the extension was .out, type ls *.out

and when I tried the following command on system(ubuntu 14.04 LTS) ..I got 

anupam@JAZZ:~$ ls  *.bash* 

ls: cannot access  *.bash* : No such file or directory

anupam@JAZZ:~$ ls  .bash*

.bash_history   .bash_logout  .bash_profile  .bashrc

anupam@JAZZ:~$

in the first case why it is showing No such directory ( *.bash* ) and in next case its showing the files ( .bash* )

I am new to this flavor, and according to my automata theory classes (*) means any combination of strings including epsilon(empty),,So why the results are different in both the cases....?

14.04

1 Answers

  1. Best Answer

    When you do ls * the * is being expanded before it is passed to ls. That is to say if we have three files (a, b and c) in a directory ls * is actually running ls a b c.

    When Bash can't expand, it passes through the raw string¹. That's why you see the wildcards in the error, along with a not found message. ls tried to show the listing for a file literally called *.bash*.

    So why didn't that expand? Well by default globbing (what this wildcard expansion is called) won't return hidden files. You can change this with shopt -s dotglob (that won't persist unless you stick it in your .bashrc — it might be disabled by default for a good reason so be careful with it), here's a quick demo:

    $ ls  *.bash*
ls: cannot access *.bash*: No such file or directory
$ shopt -s dotglob
$ ls  *.bash*
.bash_aliases  .bash_history  .bash_logout  .bashrc  .bashrc.save

    The exception to this is —as you've already shown— when you've already explicitly stated the files will be hidden with a pattern like .bash*. It simply overrides the default dotglob setting:

    $ shopt -u dotglob  # unset dotglob
$ ls .bash*
.bash_aliases  .bash_history  .bash_logout  .bashrc  .bashrc.save

    Anyway besides that quirk, I hope this helps you understand what's going on under the surface.

    There are other shopt flags that alter how globbing works: extglob, failglob, globstar, nocaseglob and nullglob. They and a raft of other shopt flags are documented as part of the Bash manual.

    Similarly, the page on Pattern Matching should make for some good reading.

    ¹ Unless failglob or nullglob are set.

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