Conditionals

Conditional expressions are used by the [[ compound command and by the test and [ built-in commands.

Expressions may be unary or binary. Unary expressions are often used to examine the status of a file. You only need one object, for instance a file, to do the operation on.

There are string operators and numeric comparison operators as well; these are binary operators, requiring two objects to do the operation on. If the FILE argument to one of the primaries is in 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, then file descriptor 0, 1 or 2 respectively is checked.

Conditionals are discussed in detail in Chapter 7.

More information about the file descriptors in Section 8.2.3.