|
Size: 4872
Comment: good enough for a first draft
|
Size: 4878
Comment: argh. one ambiguity already.
|
| Deletions are marked like this. | Additions are marked like this. |
| Line 3: | Line 3: |
| Traditional shell globs use a very simple syntax, which is less expressive than a regular expression. Most characters in a glob are treated literally, but a {{{*}}} matches 0 or more characters, a {{{?}}} matches precisely one character, and {{{[...]}}} matches any characters in a specified set (see the previous reference for details). All globs are implicitly anchored at both start and end. For example: | Traditional shell globs use a very simple syntax, which is less expressive than a regular expression. Most characters in a glob are treated literally, but a {{{*}}} matches 0 or more characters, a {{{?}}} matches precisely one character, and {{{[...]}}} matches any single character in a specified set (see the previous reference for details). All globs are implicitly anchored at both start and end. For example: |
"Glob" is the common name for a set of Bash features that match or expand specific types of patterns. Some synonyms for globbing (depending on the context in which it appears) are [http://tiswww.case.edu/php/chet/bash/bashref.html#SEC36 pattern matching], pattern expansion, filename expansion, and so on. A glob may look like *.txt and, when used to match filenames, is sometimes called a "wildcard".
Traditional shell globs use a very simple syntax, which is less expressive than a regular expression. Most characters in a glob are treated literally, but a * matches 0 or more characters, a ? matches precisely one character, and [...] matches any single character in a specified set (see the previous reference for details). All globs are implicitly anchored at both start and end. For example:
* |
Matches any string, of any length |
foo* |
Matches any string beginning with foo |
*x* |
Matches any string containing an x (beginning, middle or end) |
*.tar.gz |
Matches any string ending with .tar.gz |
*.[ch] |
Matches any string ending with .c or .h |
foo? |
Matches foot or foo$ but not fools |
Bash expands globs which appear unquoted in commands, by matching filenames relative to the current directory. The expansion of the glob results in 1 or more words (0 or more, if certain options are set), and those words (filenames) are used in the command. For example:
{{{tar xvf *.tar # Expands to: tar xvf file1.tar file2.tar file42.tar ... # (which is generally not what one wants)}}}
Even if a file contains internal whitespace, the expansion of a glob that matches that file will still preserve each filename as a single word. For example,
{{{# This is safe even if a filename contains whitespace: for f in *.tar; do
- tar tvf "$f"
done
# But this one is not: for f in $(ls | grep '\.tar$'); do
- tar tvf "$f"
done}}}
In the second example above, the output of ls is filtered, and then the result of the whole pipeline is divided into words, to serve as iterative values for the loop. This word-splitting will occur at internal whitespace within each filename, which makes it useless in the general case. The first example has no such problem, because the filenames produced by the glob do not undergo any further word-splitting. For more such examples, see BashPitfalls.
Globs are also used to match patterns in a few places in bash. The most traditional is in the case command:
{{{case "$input" in
[Yy]|) confirm=1;; [Nn]*) confirm=0;;
- ) echo "I don't understand. Please try again.";;
esac}}}
Patterns (which are separated by | characters) are matched against the first word after the case itself. The first pattern which matches, "wins", causing the corresponding commands to be executed.
Bash also allows globs to appear on the right-hand side of a comparison inside a [[ command:
{{{if $output = *[Ee]rror*; then ... }}}
Finally, globs are used during [:BashFAQ#faq73:parameter expansion] to indicate patterns which may be stripped out, or replaced, during a substitution. Simple examples (there are many more on the previously referenced page):
{{{filename=${path##*/} # strip leading pattern that matches */ (be greedy) dirname=${path%/*} # strip trailing pattern matching