cmd – interface to host operating system commands

bind –a '#C' /


Cmd provides a way to run commands in the underlying operating system's command interpreter of drawterm or when Inferno is running hosted. It serves a three–level directory that is conventionally bound behind the root directory. The top of the hierarchy is a directory cmd, that contains a clone file and zero or more numbered directories. Each directory represents a distinct connection to the host's command interpreter. The directory contains five files: ctl, data, stderr, status and wait, used as described below. Opening the clone file reserves a connection: it is equivalent to opening the ctl file of an unused connection directory, creating a new one if necessary.

The file ctl controls a connection. When read, it returns the decimal number n of its connection directory. Thus, opening and reading clone allocates a connection directory and reveals the number of the allocated directory, allowing the other files to be named (eg, /cmd/n/data).

Ctl accepts the following textual commands, allowing quoting as interpreted by parsecmd(2):
dir wdir
Run the host command in directory wdir, which is a directory on the host system. Issue this request before starting the command. By default, commands are run in the Inferno root directory on the host system.
exec command args ...
Spawn a host process to run the command with arguments as given. The write returns with an error, setting the error string, if anything prevents starting the command. If write returns successfully, the command has started, and its standard input and output may be accessed through data, and its error output accessed through stderr (see below). If arguments containing white space are quoted (following the conventions of rc(1) or parsecmd(2)), they are requoted by cmd using the host command interpreter's conventions so that command sees exactly the same arguments as were written to ctl.
killKill the host command immediately.
Set the device to kill the host command when the ctl file is closed (normally all files must be closed, see below).
nice [n]
Run the host command at less than normal scheduling priority. Issue this request before starting the command. The optional value n, in the range 1 to 3, indicates the degree of `niceness' (default: 1).

The data file provides a connection to the input and output of a previously–started host command. It must be opened separately for reading and for writing. When opened for reading, it returns data that the command writes to its standard output; when closed, further writes by the command will receive the host equivalent of `write to closed pipe'. When opened for writing, data written to the file can be read by the command on its standard input; when closed, further reads by the command will see the host equivalent of `end of file'. (Unfortunately there is no way to know when the command needs input.)

The stderr file provides a similar read–only connection to the error output from the command. If the stderr file is not opened, the error output will be discarded.

Once started, a host command runs until it terminates or until it is killed, by using the kill or killonclose requests above, or by closing all ctl, data and wait files for a connection.

The read–only status file provides a single line giving the status of the connection (not the command), of the form:
cmd/n opens state wdir arg0

where the fields are separated by white space. The meaning of each field is:
n     The cmd directory number.
opensThe decimal number of open file descriptors for ctl, data and wait.
state   The status of the interface in directory n:
Open      Allocated for use but not yet running a command.
Execute   Running a command.
Done      Command terminated: status available in the status file (or via wait).
Closed    Command completed. Available for reallocation via clone.
wdir   The command's initial working directory on the host.
arg0   The host command name (without arguments).

The read–only wait file must be opened before starting a command via ctl. When read, it blocks until the command terminates. The read then returns with a single status line, to be parsed using tokenize (see getfields(2)). There are five fields: host process ID (or 0 if unknown); time the command spent in user code in milliseconds (or 0); time spent in system code in milliseconds (or 0); real time in milliseconds (or 0); and a string giving the exit status of the command. The exit status is host–dependent, except that an empty string means success, and a non–empty string contains a diagnostic.

Command execution
In all cases, the command runs in the host operating system's own file name space. All file names will be interpreted in that space, not Plan 9's. For example, on Unix / refers to the host's file system root, not Plan 9's; the effects of mounts and binds will not be visible.


A write to ctl returns with an error and sets the error string if a command cannot be started or killed successfully.