System.Environment

base-4.8.2.0: Basic libraries

Copyright | (c) The University of Glasgow 2001
License | BSD-style (see the file libraries/base/LICENSE)
Maintainer | libraries@haskell.org
Stability | provisional
Portability | portable
Safe Haskell | Safe
Language | Haskell2010

System.Environment

Description

Miscellaneous information about the system environment.

Synopsis

- getArgs :: IO [String]
- getProgName :: IO String
- getExecutablePath :: IO FilePath
- getEnv :: String -> IO String
- lookupEnv :: String -> IO (Maybe String)
- setEnv :: String -> String -> IO ()
- unsetEnv :: String -> IO ()
- withArgs :: [String] -> IO a -> IO a
- withProgName :: String -> IO a -> IO a
- getEnvironment :: IO [(String, String)]

# 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.

getExecutablePath :: IO FilePath

Returns the absolute pathname of the current executable.

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

Since: 4.6.0.0

getEnv :: String -> IO String

Computation `getEnv` `var` returns the value of the environment variable `var`. For the inverse, POSIX users can use `putEnv`.

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: 4.6.0.0

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

`setEnv name value` sets the specified environment variable to `value`.

On Windows setting an environment variable to the empty string removes that environment variable from the environment. For the sake of compatibility we adopt that behavior. In particular

    setEnv name ""

has the same effect as

    unsetEnv name

If you don't care about Windows support and want to set an environment variable to the empty string use `System.Posix.Env.setEnv` from the `unix` package instead.

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

Since: 4.7.0.0

unsetEnv :: String -> IO ()

`unSet 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: 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.