'test', also known as '[', is rarely used outside of UNIX shell scripts. These two commands are equivalent:
[ 5 -eq 5 ] test 5 -eq 5
test returns an error level of 0 if the test is true, and something other than 0 if the result is not true; it is almost always used as part of an "if" or "while" statement. See examples of use in the web scripting examples at the link above.
The character "!" negates the sense of a test, and comes before the expression to be tested. E.g.,
[ ! -r heidigger ]returns true is hiedigger is not readable.
The man page for test is below.
test - condition evaluation command
test expr
[ expr ]
test evaluates the expression expr and, if its value is true, sets a zero
(true) exit status; otherwise, a non-zero (false) exit status is set;
test also sets a non-zero exit status if there are no arguments. When
permissions are tested, the effective user ID of the process is used.
All operators, flags, and brackets (brackets used as shown in the second
SYNOPSIS line) must be separate arguments to the test command; normally
these items are separated by spaces.
The following primitives are used to construct expr:
-r file true if file exists and is readable.
-l file true if file exists and is a symbolic link.
-w file true if file exists and is writable.
-x file true if file exists and is executable.
-f file true if file exists and is a regular file.
-d file true if file exists and is a directory.
-h file true if file exists and is a symbolic link.
-c file true if file exists and is a character special file.
-b file true if file exists and is a block special file.
-p file true if file exists and is a named pipe (fifo).
-u file true if file exists and its set-user-ID bit is set.
-g file true if file exists and its set-group-ID bit is set.
-k file true if file exists and its sticky bit is set.
-s file true if file exists and has a size greater than zero.
-t [ fildes ]
true if the open file whose file descriptor number is fildes
(1 by default) is associated with a terminal device.
-z s1 true if the length of string s1 is zero.
-n s1 true if the length of the string s1 is non-zero.
s1 = s2 true if strings s1 and s2 are identical.
s1 != s2 true if strings s1 and s2 are not identical.
s1 true if s1 is not the null string.
n1 -eq n2 true if the integers n1 and n2 are algebraically equal. Any
of the comparisons -ne, -gt, -ge, -lt, and -le may be used in
place of -eq.
-L file true if file exists and is a symbolic link.
These primaries may be combined with the following operators:
! unary negation operator.
-a binary and operator.
-o binary or operator (-a has higher precedence than -o).
( expr ) parentheses for grouping. Notice also that parentheses are
meaningful to the shell and, therefore, must be quoted.
find(1), sh(1).
If you test a file you own (the -r, -w, or -x tests), but the permission
tested does not have the owner bit set, a non-zero (false) exit status
will be returned even though the file may have the group or other bit set
for that permission. The correct exit status will be set if you are
super-user.
The = and != operators have a higher precedence than the -r through -n
operators, and = and != always expect arguments; therefore, = and !=
cannot be used with the -r through -n operators.
If more than one argument follows the -r through -n operators, only the
first argument is examined; the others are ignored, unless a -a or a -o
is the second argument.
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