Built-in test

The [ built-in takes a series of arguments to perform one or more tests and return an exit status of zero (success) or non-zero (failure). Multiple expressions are separated by one or more -a (AND) or -o (OR) operators and can be grouped using parentheses (parentheses must be escaped or quoted to prevent interpolation).

Tests are separated into unary, binary, and string operators depending on number of operands.

Unary operators requiring a single operand include:

Operator(s)	Description
! expression	True if expression is false
-[bcdefghkprsuwxGLOS] file	Various checks against a single file/directory
-[nz] string	Test if a string is empty (-z) or non-empty (-n)
-t file_descriptor	True if file descriptor is open and to a terminal

Binary operators requiring two operands include:

Operator(s)	Description
file1 { -nt, -ot, -ef } file2	Various checks involving two files/directories
string1 { =, !=, <, > } string2	Various checks involving two strings
number1 { -eq, -ne, -gt, -ge, -lt, -le } number2	Various checks involving two numbers

When given an argument with no operator or when an operator is given zero operands, it is treated as a string and the exit status is zero (success) if the string is non-empty and non-zero (failure) otherwise.

The behavior of being given only string is the same as if given -n string. Combined with the unary NOT operator, ! string is the same as -z string.

Given the alphabet soup of unary operators, it is suggested that the readability of code is improved by using string and ! string instead of -n string and -z string, respectively.