Pathname expansion (globbing)

General

Unlike on other platforms you may have seen, on UNIX(r), the shell is responsible for interpreting and expanding globs ("filename wildcards"). A called program will never see the glob itself; it will only see the expanded filenames as its arguments (here, all filenames matching *.log):

grep "changes:" *.log

The base syntax for the pathname expansion is the pattern matching syntax. The pattern you describe is matched against all existing filenames and the matching ones are substituted. Since this substitution happens after word splitting, all resulting filenames are literal and treated as separate words, no matter how many spaces or other IFS-characters they contain.

Normal behaviour

with the set command (-f, noglob) you can entirely disable pathname expansion
when matching a pathname, the slash-character (/) always needs to be matched explicitly
the dot at the beginning of a filename must be matched explicitly (also one following a / in the glob)
a glob that doesn't match a filename is unchanged and remains what it is

Customization

when the shell option nullglob is set, non-matching globs are removed, rather than preserved
when the shell option failglob is set, non-matching globs produce an error message and the current command is not executed
when the shell option nocaseglob is set, the match is performed case-insensitive
when the shell option dotglob is set, wildcard-characters can match a dot at the beginning of a filename
when the shell option dirspell is set, Bash performs spelling corrections when matching directory names
when the shell option globstar is set, the glob ** will recursively match all files and directories. This glob isn't "configurable", i.e. you can't do something like **.c to recursively get all *.c filenames.
when the shell option globasciiranges is set, the bracket-range globs (e.g. [A-Z]) use C locale order rather than the configured locale's order (i.e. ABC...abc... instead of e.g. AaBbCc...) - since 4.3-alpha
the variable GLOBIGNORE can be set to a colon-separated list of patterns to be removed from the list before it is returned

nullglob

Normally, when no glob specified matches an existing filename, no pathname expansion is performed, and the globs are not removed:

$ echo "Textfiles here:" *.txt
Textfiles here: *.txt

In this example, no files matched the pattern, so the glob was left intact (a literal asterisk, followed by dot-txt).

This can be very annoying, for example when you drive a for-loop using the pathname expansion:

for filename in *.txt; do
  echo "=== BEGIN: $filename ==="
  cat "$filename"
  echo "=== END: $filename ==="
done

When no file name matches the glob, the loop will not only output stupid text ("BEGIN: *.txt"), but also will make the cat-command fail with an error, since no file named *.txt exists.

Now, when the shell option nullglob is set, Bash will remove the entire glob from the command line. In case of the for-loop here, not even one iteration will be done. It just won't run.

So in our first example:

$ shopt -s nullglob
$ echo "Textfiles here:" *.txt
Textfiles here:

and the glob is gone.

Glob characters

* - means 'match any number of characters'. '/' is not matched (and depending on your settings, things like '.' may or may not be matched, see above)
? - means 'match any single character'
[abc] - match any of the characters listed. This syntax also supports ranges, like [0-9]

For example, to match something beginning with either 'S' or 'K' followed by two numbers, followed by at least 3 more characters:

[SK][0-9][0-9]???*

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