View source for bash Conditional Expressions

Conditional expressions are used by the [[ compound command and the test and [ builtin commands.

Expressions may be unary or binary. Unary expressions are often used to examine the status of a file. There are string operators and numeric comparison operators as well. If the file argument to one of the primaries is of the form /dev/fd/N, then file descriptor N is checked. If the file argument to one of the primaries is one of /dev/stdin, /dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2, respectively, is checked.

When used with ‘[[’, the ‘<’ and ‘>’ operators sort lexicographically using the current locale. The test command uses ASCII ordering.

Unless otherwise specified, primaries that operate on files follow symbolic links and operate on the target of the link, rather than the link itself.

 -a file
True if file exists.

 -b file
True if file exists and is a block special file.

 -c file
True if file exists and is a character special file.

 -d file
True if file exists and is a directory.

 -e file
True if file exists.

 -f file
True if file exists and is a regular file.

 -g file
True if file exists and its set-group-id bit is set.

 -h file
True if file exists and is a symbolic link.

 -k file
True if file exists and its "sticky" bit is set.

 -p file
True if file exists and is a named pipe (FIFO).

 -r file
True if file exists and is readable.

 -s file
True if file exists and has a size greater than zero.

 -t fd
True if file descriptor fd is open and refers to a terminal.

 -u file
True if file exists and its set-user-id bit is set.

 -w file
True if file exists and is writable.

 -x file
True if file exists and is executable.

 -G file
True if file exists and is owned by the effective group id.

 -L file
True if file exists and is a symbolic link.

 -N file
True if file exists and has been modified since it was last read.

 -O file
True if file exists and is owned by the effective user id.

 -S file
True if file exists and is a socket.

 file1 -ef file2
True if file1 and file2 refer to the same device and inode numbers.

 file1 -nt file2
True if file1 is newer (according to modification date) than file2, or if file1 exists and file2 does not.

 file1 -ot file2
True if file1 is older than file2, or if file2 exists and file1 does not.

 -o optname
True if the shell option optname is enabled. The list of options appears in the description of the -o option to the set builtin (see The Set Builtin).

 -v varname
True if the shell variable varname is set (has been assigned a value).

 -z string
True if the length of string is zero.

 -n string
string
True if the length of string is non-zero.

 string1 == string2
 string1 = string2
True if the strings are equal. ‘=’ should be used with the test command for POSIX conformance.

 string1 != string2
True if the strings are not equal.

 string1 < string2
True if string1 sorts before string2 lexicographically.

 string1 > string2
True if string1 sorts after string2 lexicographically.

 arg1 OP arg2
OP is one of ‘-eq’, ‘-ne’, ‘-lt’, ‘-le’, ‘-gt’, or ‘-ge’. These arithmetic binary operators return true if arg1 is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to arg2, respectively. Arg1 and arg2 may be positive or negative integers.

[http://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html taken from]