System.Environment

Miscellaneous information about the system environment.

Documentation

getArgs :: IO [String]

Computation getArgs returns a list of the program's command line arguments (not including the program name).

getProgName :: IO String

Computation getProgName returns the name of the program as it was invoked.

However, this is hard-to-impossible to implement on some non-Unix OSes, so instead, for maximum portability, we just return the leafname of the program as invoked. Even then there are some differences between platforms: on Windows, for example, a program invoked as foo is probably really FOO.EXE, and that is what getProgName will return.

executablePath :: Maybe (IO (Maybe FilePath))

Get an action to query the absolute pathname of the current executable.

If the operating system provides a reliable way to determine the current executable, return the query action, otherwise return Nothing. The action is defined on FreeBSD, Linux, MacOS, NetBSD, and Windows.

Even where the query action is defined, there may be situations where no result is available, e.g. if the executable file was deleted while the program is running. Therefore the result of the query action is a Maybe FilePath.

Note that for scripts and interactive sessions, the result is the path to the interpreter (e.g. ghci.)

Note also that while most operating systems return Nothing if the executable file was deleted/unlinked, some (including NetBSD) return the original path.

Since: base-4.17.0.0

getExecutablePath :: IO FilePath

Returns the absolute pathname of the current executable, or argv[0] if the operating system does not provide a reliable way query the current executable.

Note that for scripts and interactive sessions, this is the path to the interpreter (e.g. ghci.)

Since base 4.11.0.0, getExecutablePath resolves symlinks on Windows. If an executable is launched through a symlink, getExecutablePath returns the absolute path of the original executable.

If the executable has been deleted, behaviour is ill-defined and varies by operating system. See executablePath for a more reliable way to query the current executable.

Since: base-4.6.0.0

getEnv :: String -> IO String

Computation getEnv var returns the value of the environment variable var. For the inverse, the setEnv function can be used.

This computation may fail with:

• isDoesNotExistError if the environment variable does not exist.

lookupEnv :: String -> IO (Maybe String)

Return the value of the environment variable var, or Nothing if there is no such value.

For POSIX users, this is equivalent to getEnv.

Since: base-4.6.0.0

setEnv :: String -> String -> IO ()

setEnv name value sets the specified environment variable to value.

Early versions of this function operated under the mistaken belief that setting an environment variable to the empty string on Windows removes that environment variable from the environment. For the sake of compatibility, it adopted that behavior on POSIX. In particular

setEnv name ""

has the same effect as

unsetEnv name

If you'd like to be able to set environment variables to blank strings, use setEnv.

Throws IOException if name is the empty string or contains an equals sign.

Since: base-4.7.0.0

unsetEnv :: String -> IO ()

unsetEnv name removes the specified environment variable from the environment of the current process.

Throws IOException if name is the empty string or contains an equals sign.

Since: base-4.7.0.0

withArgs :: [String] -> IO a -> IO a

withArgs args act - while executing action act, have getArgs return args.

withProgName :: String -> IO a -> IO a

withProgName name act - while executing action act, have getProgName return name.

getEnvironment :: IO [(String, String)]

getEnvironment retrieves the entire environment as a list of (key,value) pairs.

If an environment entry does not contain an '=' character, the key is the whole entry and the value is the empty string.