if/else comparison with wildcard Post: 302447105

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Top Forums Shell Programming and Scripting if/else comparison with wildcard Post 302447105 by agama on Saturday 21st of August 2010 02:08:45 AM

08-21-2010

Registered User

Quote:

Originally Posted by cola

What's the problem if single bracket is used?

The open single bracket is a symlink to the test command. Shells (Ksh, bash) must treat it as an external command, and thus all of the command line is treated as an external command. Therefore, wildcards, unquoted, are expanded in the same manner as would be on any other command: as matching files in the filesystem. Quoted wildcard characters are treated as literals and passed to test which treats them as literals. So executing the following code takes the false branch:

Code:

if [ foo = "*o" ]
then
    echo "seemingly broken because this doesn't echo"
else
    echo "this will echo because * is interpreted literally by test"
fi

While this if statement takes the expected true branch:

Code:

if [[ foo == *"o" ]]

Beyond those differences, the double square bracketed expressions are interpreted by the shell, and no fork/exec overhead is needed, so they are much more efficient than using a single bracket.

Once in a long time does it make sense to use a single bracket expression (except when needing to write a pure bourn shell script), and even then I'd question its use.

agama

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wildmat

WILDMAT(3)						     Library Functions Manual							WILDMAT(3)

NAME

       wildmat - perform shell-style wildcard matching

SYNOPSIS

       int
       wildmat(text, pattern)
	   char       *text;
	   char       *pattern;

DESCRIPTION

       Wildmat	is  part  of libinn (3).  Wildmat compares the text against the pattern and returns non-zero if the pattern matches the text.  The
       pattern is interpreted according to rules similar to shell filename wildcards, and not as a full regular expression such as  those  handled
       by the grep(1) family of programs or the regex(3) or regexp(3) set of routines.

       The pattern is interpreted as follows:

       x     Turns  off the special meaning of x and matches it directly; this is used mostly before a question mark or asterisk, and is not spe-
	      cial inside square brackets.

       ?      Matches any single character.

       *      Matches any sequence of zero or more characters.

       [x...y]
	      Matches any single character specified by the set x...y.	A minus sign may be used to indicate a	range  of  characters.	 That  is,
	      [0-5abc] is a shorthand for [012345abc].	More than one range may appear inside a character set; [0-9a-zA-Z._] matches almost all of
	      the legal characters for a host name.  The close bracket, ], may be used if it is the first character in the set.  The  minus  sign,
	      -, may be used if it is either the first or last character in the set.

       [^x...y]
	      This  matches any character not in the set x...y, which is interpreted as described above.  For example, [^]-] matches any character
	      other than a close bracket or minus sign.

HISTORY

       Written by Rich $alz <rsalz@uunet.uu.net> in 1986, and posted to Usenet several times since then,  most	notably  in  comp.sources.misc	in
       March, 1991.

       Lars Mathiesen <thorinn@diku.dk> enhanced the multi-asterisk failure mode in early 1991.

       Rich and Lars increased the efficiency of star patterns and reposted it to comp.sources.misc in April, 1991.

       Robert Elz <kre@munnari.oz.au> added minus sign and close bracket handling in June, 1991.

       This is revision 1.10, dated 1992/04/03.

SEE ALSO

       grep(1), regex(3), regexp(3).

																	WILDMAT(3)

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