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GNU/Linux Command-Line Tools Summary


20.4.3. Useful categories of characters (as defined by the POSIX standard)

This information has been taken from the grep info page with a tiny amount of editing, see [10] in the Prev for further information.

  • [:upper:] uppercase letters

  • [:lower:] lowercase letters

  • [:alpha:] alphabetic (letters) meaning upper+lower (both uppercase and lowercase letters)

  • [:digit:] numbers in decimal, 0 to 9

  • [:alnum:] alphanumeric meaning alpha+digits (any uppercase or lowercase letters or any decimal digits)

  • [:space:] whitespace meaning spaces, tabs, newlines and similar

  • [:graph:] graphically printable characters excluding space

  • [:print:] printable characters including space

  • [:punct:] punctuation characters meaning graphical characters minus alpha and digits

  • [:cntrl:] control characters meaning non-printable characters

  • [:xdigit:] characters that are hexadecimal digits.

       These are used with

The above commands will work with most tools which work with text (for example: tr ).

For example (advanced example), this command scans the output of the dir command, and prints lines containing a capital letter followed by a digit:


ls -l | grep '[[:upper:]][[:digit:]]'

The command greps for [upper_case_letter][any_digit], meaning any uppercase letter followed by any digit. If you remove the [ ] (square brackets) in the middle it would look for an uppercase letter or a digit, because it would become [upper_case_letter any_digit]

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