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Top Forums Shell Programming and Scripting ksh, difference between double bracket and single bracket Post 302584139 by BeefStu on Thursday 22nd of December 2011 09:24:57 AM
Old 12-22-2011
ksh, difference between double bracket and single bracket

Can somebody tell me the difference between double brackets and single
brackets, when doing a test.

I have always been acustomed to using single brackets and have not
encountered any issues to date. Why would somebody use double brackets.

Ie

Code:
 
if [[ "$a" = "$b ]]
 
vs
 
if [ "$a" = "$b" ]

Thanks to all who answer
 

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SHELLEXP(3)						     Library Functions Manual						       SHELLEXP(3)

NAME
shellexp - match string against a cruft filter pattern SYNOPSIS
extern int shellexp(const char *string, const char *pattern); DESCRIPTION
The shellexp() function is similar to fnmatch(3), but works with cruft patterns instead of standard glob(7) patterns. The function returns a true value if string matches the cruft pattern pattern, and a false value (0) otherwise. Returns -1 in case of pattern syntax error. Cruft patterns are similar to glob(7) patterns, but are not fully compatible. The following special characters are supported: ? (a question mark) matches exacly one character of string other than a slash. * matches zero or more characters of string other than a slash. /** or /**/ matches zero or more path components in string. Please note that you can only use ** when directly following a slash, and further- more, only when either directly preceding a slash or at the very end of pattern. A ** followed by anything other than a slash makes pattern invalid. A ** following anything else than a slash reduces it to having the same effect as *. [character-class] Matches any character between the brackets exactly once. Named character classes are NOT supported. If the first character of the class is ! or ^, then the meaning is inverted (matches any character NOT listed between the brackets). If you want to specify a literal closing bracket in the class, then specify it as the first (or second, if you want to negate) character after the opening bracket. Also, simple ASCII-order ranges are supported using a dash character (see examples section). Any other character matches itself. EXAMPLES
/a/b*/*c matches /a/b/xyz.c, as well as /a/bcd/.c, but not /a/b/c/d.c. /a/**/*.c matches all of the following: /a/a.c, /a/b/a.c, /a/b/c/a.c and /a/b/c/d/a.c. /a/[0-9][^0-9]* matches /a/1abc, but not /a/12bc. BUGS
Uses constant-length 1000 byte buffers to hold filenames. Also uses recursive function calls, which are not very efficient. Does not vali- date the pattern before matching, so any pattern errors (unbalanced brackets or misplaced **) are only reported when and if the matching algorithm reaches them. SEE ALSO
fnmatch(3), glob(3), cruft(8) and dash-search(1). AUTHOR
This manual page was written by Marcin Owsiany <porridge@debian.org>, for the Debian GNU/Linux system (but may be used by others). October 17, 2007 SHELLEXP(3)
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