# System Calls

## `Alarm(s): RngIntElt`

A procedure which when used on UNIX systems, sends the signal SIGALRM to the Magma process after $s$ seconds. This allows the user to specify that a Magma-process should self-destruct after a certain period.

## `ChangeDirectory(s): MonStgElt`

Change to the directory specified by the string $s$. Tilde expansion is allowed.

## `GetCurrentDirectory()`

Returns the current directory as a string.

## `Getpid()`

Returns Magma’s process ID (value of the Unix C system call `getpid()`).

## `Getuid()`

Returns the user ID (value of `the Unix C system call getuid()`).

## `Fork() -> RngIntElt`

```magma
ExitSummary: BoolElt                    Default: false
```

Execute the Unix/Linux system call `fork()` and return the process ID of the child (if now the parent), or return 0 (if now the child). If the parameter `ExitSummary` is set to `true`, then the child process prints the usual exit summary (`"Total time: ..."`) when it exits; otherwise this is suppressed (the default).

## `System(C): MonStgElt -> RngIntElt`

Execute the system command specified by the string $C$. This is done by calling the C function `system()`. This also returns the system command’s return value as an integer. On most Unix systems, the lower 8 bits of this value give the process status while the next 8 bits give the value given by the command to the C function `exit()` (see the Unix manual entries for `system(3)` or `wait(2)`, for example). Thus one should normally divide the result by 256 to get the exit value of the program on success. See also the `Pipe` intrinsic function.

## `Example: Get Date (ex-52e3aa)`

The `System` intrinsic can be used to access the current date and time.

```magma
> System("date");
Tue Feb 28 13:02:03 EST 2017

```

## `%! shell-command`

Execute the given command in the Unix shell then return to Magma. Note that this type of shell escape (contrary to the one using a `System` call) takes place entirely outside Magma and does not show up in Magma’s history.