https://www.gnu.org/software/bash/manual/bashref.html#Bash-Conditional-Expressions
6.4 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 fileTrue if file exists.
-b fileTrue if file exists and is a block special file.
-c fileTrue if file exists and is a character special file.
-d fileTrue if file exists and is a directory.
-e fileTrue if file exists.
-f fileTrue if file exists and is a regular file.
-g fileTrue if file exists and its set-group-id bit is set.
-h fileTrue if file exists and is a symbolic link.
-k fileTrue if file exists and its "sticky" bit is set.
-p fileTrue if file exists and is a named pipe (FIFO).
-r fileTrue if file exists and is readable.
-s fileTrue if file exists and has a size greater than zero.
- 그리고 더 많다~
