bash [ -flag ] CONDITIONAL EXPRESSIONS

CONDITIONAL EXPRESSIONS
       Conditional  expressions  are  used by the [[ compound command and the test and [ builtin commands to test file attributes and perform string and arith-
       metic comparisons.  Expressions are formed from the following unary or binary primaries.  If any 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.

       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 is set-group-id.
       -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.
       -O file
              True if file exists and is owned by the effective user id.
       -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.
       -S file
              True if file exists and is a socket.
       -N file
              True if file exists and has been modified since it was last read.
       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.
       file1 -ef file2
              True if file1 and file2 refer to the same device and inode numbers.
       -o optname
              True if shell option optname is enabled.  See the list of options under the description of the -o option to the set builtin below.
       -z string
              True if the length of string is zero.
       string
       -n string
              True if the length of string is non-zero.

       string1 == string2
              True if the strings are equal.  = may be used in place of == for strict POSIX compliance.

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

       string1 < string2
              True if string1 sorts before string2 lexicographically in the current locale.

       string1 > string2
              True if string1 sorts after string2 lexicographically in the current locale.

       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.