09-23-2011
The significance is as follow:
First look at this : [ "$a"="$b" ]
The shell takes "$a", appends an "=" to it, and then it appends "$b" to it.
This is a single longer string containing an equal sign. There is no comparison evaluation done in this case. The only evaluation done is to check that the long string is not empty, which it will never be since there is always at least an equal sign in it even if both $a and $b are empty.
Second, take this:
[ "$a" = "$b" ]
The shell sees three strings: $a, and equal sign, and $b. The equal sign is interpreted as an operator and evaluated as you expect.
Hope this helps.
---------- Post updated at 07:49 AM ---------- Previous update was at 07:44 AM ----------
Extrapolate the same for the spaces between the [ ... ] and the arguments.
When the [ and ] is not separate (surrounded by spaces) then the shell do not recognize them as special characters.
Essentially you have
if "[$a" = "$b]"
Because the brackets are added to the strings before evaluation is done.
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LEARN ABOUT OPENSOLARIS
wsdup
wstring(3C) Standard C Library Functions wstring(3C)
NAME
wstring, wscasecmp, wsncasecmp, wsdup, wscol - Process Code string operations
SYNOPSIS
#include <widec.h>
int wscasecmp(const wchar_t *s1, const wchar_t *s2);
int wsncasecmp(const wchar_t *s1, const wchar_t *s2, int n);
wchar_t *wsdup(const wchar_t *s);
int wscol(const wchar_t *s);
DESCRIPTION
These functions operate on Process Code strings terminated by wchar_t null characters. During appending or copying, these routines do not
check for an overflow condition of the receiving string. In the following, s, s1, and s2 point to Process Code strings terminated by a
wchar_t null.
wscasecmp(), wsncasecmp()
The wscasecmp() function compares its arguments, ignoring case, and returns an integer greater than, equal to, or less than 0, depending
upon whether s1 is lexicographically greater than, equal to, or less than s2. It makes the same comparison but compares at most n Process
Code characters. The four Extended Unix Code (EUC) codesets are ordered from lowest to highest as 0, 2, 3, 1 when characters from different
codesets are compared.
wsdup()
The wsdup() function returns a pointer to a new Process Code string, which is a duplicate of the string pointed to by s. The space for the
new string is obtained using malloc(3C). If the new string cannot be created, a null pointer is returned.
wscol()
The wscol() function returns the screen display width (in columns) of the Process Code string s.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
+-----------------------------------------------------------+
|ATTRIBUTE TYPE ATTRIBUTE VALUE |
|MT-Level MT-Safe |
+-----------------------------------------------------------+
SEE ALSO
malloc(3C), string(3C), wcstring(3C), attributes(5)
SunOS 5.11 29 Dec 1996 wstring(3C)