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87 lines
4.6 KiB
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87 lines
4.6 KiB
Plaintext
====== Pathname expansion (globbing) ======
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===== General =====
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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''):
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<code>
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grep "changes:" *.log
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</code>
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The base syntax for the pathname expansion is the [[syntax:pattern | pattern matching]] syntax. The pattern you describe is matched against all existing filenames and the matching ones are substituted. Since this substitution happens **after [[syntax:expansion:wordsplit | word splitting]]**, all resulting filenames are literal and treated as separate words, no matter how many spaces or other ''IFS''-characters they contain.
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===== Normal behaviour =====
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* with [[commands:builtin:set | the set command]] (''-f'', ''noglob'') you can entirely disable pathname expansion
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* when matching a pathname, the slash-character (''/'') always needs to be matched explicitly
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* the dot at the beginning of a filename must be matched explicitly (also one following a ''/'' in the glob)
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* a glob that doesn't match a filename is unchanged and remains what it is
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===== Customization =====
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* when the shell option ''[[internals:shell_options#nullglob | nullglob]]'' is set, non-matching globs are removed, rather than preserved
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* when the shell option ''[[internals:shell_options#failglob | failglob]]'' is set, non-matching globs produce an error message and the current command is not executed
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* when the shell option ''[[internals:shell_options#nocaseglob | nocaseglob]]'' is set, the match is performed case-insensitive
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* when the shell option ''[[internals:shell_options#dotglob | dotglob]]'' is set, wildcard-characters can match a dot at the beginning of a filename
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* when the shell option ''[[internals:shell_options#dirspell | dirspell]]'' is set, Bash performs spelling corrections when matching directory names
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* when the shell option ''[[internals:shell_options#globstar | globstar]]'' is set, the glob ''<nowiki>**</nowiki>'' will recursively match all files and directories. This glob isn't "configurable", i.e. you **can't** do something like ''<nowiki>**</nowiki>.c'' to recursively get all ''*.c'' filenames.
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* when the shell option ''[[internals:shell_options#globasciiranges | 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
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* the variable [[syntax:shellvars#GLOBIGNORE|GLOBIGNORE]] can be set to a colon-separated list of patterns to be removed from the list before it is returned
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==== nullglob ====
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Normally, when no glob specified matches an existing filename, no pathname expansion is performed, and the globs are __**not**__ removed:
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<code>
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$ echo "Textfiles here:" *.txt
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Textfiles here: *.txt
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</code>
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In this example, no files matched the pattern, so the glob was left intact (a literal asterisk, followed by dot-txt).
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This can be very annoying, for example when you drive a [[syntax:ccmd:classic_for | for-loop]] using the pathname expansion:
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<code>
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for filename in *.txt; do
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echo "=== BEGIN: $filename ==="
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cat "$filename"
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echo "=== END: $filename ==="
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done
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</code>
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When no file name matches the glob, the loop will not only output stupid text ("''BEGIN: *.txt''"), but also will make the ''cat''-command fail with an error, since no file named ''*.txt'' exists.
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Now, when the shell option ''[[internals:shell_options#nullglob | nullglob]]'' is set, Bash will remove the entire glob from the command line. In case of the for-loop here, not even one iteration will be done. It just won't run.
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So in our first example:
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<code>
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$ shopt -s nullglob
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$ echo "Textfiles here:" *.txt
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Textfiles here:
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</code>
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and the glob is gone.
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====Glob characters=====
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* * - means 'match any number of characters'. '/' is not matched (and depending on your settings, things like '.' may or may not be matched, see above)
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* ? - means 'match any single character'
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* [abc] - match any of the characters listed. This syntax also supports ranges, like [0-9]
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For example, to match something beginning with either 'S' or 'K' followed by two numbers, followed by at least 3 more characters:
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<code>[SK][0-9][0-9]???*</code>
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===== See also =====
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* [[syntax:expansion:intro | Introduction to expansion and substitution]]
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* [[syntax:pattern | pattern matching syntax]]
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* [[commands:builtin:set | the set builtin command]]
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* [[commands:builtin:shopt | the shopt builtin command]]
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* [[internals:shell_options | list of shell options]]
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