Test (Unix)

The test command evaluates an expression and returns an exit status of 0 (true) or 1 (false). It is often used in shell scripts within if, while, and until constructs. The basic syntax is test expression or [ expression ]. The latter form requires a space after the opening bracket and before the closing bracket. The command supports a wide range of unary and binary operators for file attributes, string comparisons, and integer comparisons.¹

The test command appeared in early Unix versions, with the bracket alias [ introduced later as a link to the same executable. It was standardized by POSIX and is now part of the POSIX.1-2017 specification. The command is implemented as a shell built-in in most modern shells (e.g., bash, zsh, dash) for efficiency, but a standalone binary is still provided for compatibility.²

The command supports file operators such as -e (exists), -f (regular file), -d (directory), and -r (readable). String operators include -z (empty string), -n (non-empty), and comparison operators = and !=. Integer comparisons use -eq, -ne, -lt, -le, -gt, and -ge. Logical operators are ! (negation), -a (and), and -o (or), though the latter two are considered obsolete and should be replaced with && and || in modern scripts.¹

The [ command is a synonym for test, but it requires a final argument of ]. In some shells, [ is a built-in that behaves slightly differently from the external command, particularly regarding argument handling and error messages. For example, in bash, the built-in [ may treat certain expressions differently than the external test binary.³

One lesser-known fact is that the test command can be used to compare file modification times with -nt (newer than) and -ot (older than). Another is that the -t operator checks if a file descriptor is associated with a terminal, which is useful for detecting interactive sessions. Additionally, the command has a rarely used -a and -o for logical and/or, but these are not recommended due to ambiguity. The test command is also available in Windows via Unix-like environments such as Cygwin and MSYS2.

While the external test command is defined by POSIX, many shells implement it as a built-in, which can lead to subtle differences. For example, in bash, the built-in test supports additional operators like -v (variable is set) and -R (variable is a name reference). Scripts that rely on these extensions are not portable to other shells. The [[ keyword in bash and ksh is an enhanced version that avoids word-splitting and pathname expansion issues, but it is not POSIX-compliant.⁴

Exit status

A numeric code returned by a command to indicate success (0) or failure (non-zero).

Shell built-in

A command that is implemented within the shell itself, rather than as an external executable.

POSIX

A family of standards for maintaining compatibility between operating systems, defining a common API and command-line utilities.