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[ expression ]
The test command evaluates the expression parameter. In some shells (such as FreeBSD sh(1)), it is actually a shell builtin, even though external version still exists. In the second form of the command, the [ ] (brackets) must be surrounded by blank spaces, this is because /bin/[ is a program and POSIX compatible shells require a space between the program name and its arguments. You must test explicitly for file names in the C shell. File-name substitution (globbing) causes the shell script to exit. Functions and operators are treated as separate parameters by the test command. The expression parameter refers to a statement that is checked for a true or false condition.
The following functions are used to construct this parameter:
-e FileName - FileName exists.
Note: All remaining functions return true if the object (file or string) exists, and the condition specified is true.
-b Filename - Returns a True exit value if the specified FileName exists and is a block special file. -c FileName - FileName is a character special file. -d FileName - FileName is a directory. -f FileName - FileName is a regular file. -g FileName - FileName's Set Group ID bit is set. -h FileName - FileName is a symbolic link. -k FileName - FileName's sticky bit is set. -L FileName - FileName is a symbolic link. -p FileName - FileName is a named pipe (FIFO). -r FileName - FileName is readable by the current process. -s FileName - FileName has a size greater than 0. -t FileDescriptor - FileDescriptor is open and associated with a terminal. -u FileName - FileName's Set User ID bit is set.
-w FileName - FileName's write flag is on. However, the FileName will not be writable on a read-only file system even if test indicates true.
-x FileName - FileName's execute flag is on. If the specified file exists and is a directory, the True exit value indicates that the current process has permission to change (chdir) into the directory.
file1 -nt file2 - file1 is newer than file2. file1 -ot file2 - file1 is older than file2. file1 -ef file2 - file1 is another name for file2. (symbolic link or hard link)
String functions 
Note that in Perl, these sections are reversed: eq is a string operator and == is a numerical operator, and so on for the others.
-n String1 - the length of the String1 variable is nonzero. -z String1 - the length of the String1 variable is 0 (zero). String1 = String2 - String1 and String2 variables are identical. String1 != String2 - String1 and String2 variables are not identical. String1 - String1 variable is not a null string.
Number functions 
Integer1 -eq Integer2 - Integer1 and Integer2 variables are algebraically equal. Any of the following comparisons can be used in place of -eq. -ne (not equal) -gt (greater than) -ge (greater or equal) -lt (less than) -le (less or equal)
These functions can be combined with the following operators:
! - Unary negation operator -a - Binary AND operator -o - Binary OR operator (the -a operator has higher precedence than the -o operator) \(Expression\) - Parentheses for grouping must be escaped with a backslash (\).
The -a and -o operators, along with parentheses for grouping, are XSI extensions and are therefore not portable. In portable shell scripts, the same effect may be achieved by connecting multiple invocations of test together with the
|| operators and parentheses.
Exit Status 
This command returns the following exit values:
0 - The Expression parameter is true. 1 - The Expression parameter is false or missing. >1 - An error occurred.
1. To test whether a file is nonexistent or empty, type:
if test ! -s "$1" then echo $1 does not exist or is empty. fi
If the file specified by the first positional parameter to the shell procedure, $1, does not exist or is of size 0, the test command displays the message. If $1 exists and has a size greater than 0, the test command displays nothing.
Note: There must be a space between the -s function and the file name.
The quotation marks around $1 ensure that the test works properly even if the value of $1 is a null string. If the quotation marks are omitted and $1 is the empty string, the test command displays the error message
test: argument expected.
2. To do a complex comparison, type:
if [ $# -lt 2 -o ! -e "$1" ] then exit fi
If the shell procedure is given fewer than two positional parameters or the file specified by $1 does not exist, then the shell procedure exits. The special shell variable $# represents the number of positional parameters entered on the command line that starts this shell procedure.
See also