NAME

cmd − interface to host operating system commands

SYNOPSIS

­bind -a ’#C’ /

­/cmd/clone
/cmd/n/ctl
/cmd/n/data
/cmd/n/status

DESCRIPTION

The ­cmd device provides an interface to commands on the host operating system, when running emu(1). By convention, this kernel device (#C) is bound to the root directory (with the ­-a option). 

Opening the ­clone file provides a handle for starting a command, managing the command’s I/O, and checking the command’s status.  It creates or allocates a currently unused directory, ­n under ­/cmd (where ­n is a decimal integer).  It contains the files ctl, data, and status.  The file descriptor returned from the open of ­clone provides access to the ­ctl file of the directory selected by cmd.

Reading ­ctl via the file descriptor returned from opening ­clone provides the directory number, n, which can then be used to construct filenames for the related files. Writes to ­ctl must have the format:

exec oscmd args ... 

File offset is ignored for both reading and writing. 

Reading ­data obtains the data that the command writes to its standard output and standard error.  Writing to ­data sends the data into the command’s standard input. 

The read-only ­status file provides a status line (to a maximum of 128 bytes) of the format:

cmd/n refcnt state cmd arg ... 

where each field separator is a single blank. The meaning of each field is:

­n The ­cmd directory number. 

­refcnt The number (decimal) of open file descriptors for ctl. 

­state The status of the interface in directory n:

­Open Allocated for use but not yet running a command. 

­Execute Running a command

­Close Command completed. Available for reallocation via clone. 

­cmd arg ... 
The active command. Output only for state Execute. 

Command execution

On Unix systems, the command is run via ­/bin/sh as follows:

/bin/sh -c ’cmdargs...’

The command will be run using the Unix user and group ID of the user that started emu(1), unless it was started by the super-user, in which case ­cmd attempts to set the Unix user ID and group ID to those of a Unix user corresponding to the current Inferno user’s name, and failing that, as user and group nobody. 

On Plan 9, the command is run via ­/bin/rc as follows:

/bin/rc -c ’cmdargs...’

The command will be run using the Plan 9 identity of the user that started emu(1).

On Windows systems, the command must be a binary executable (not built into the command interpreter) in the current path.  It is always run with the same Windows user identity as started emu(1). Furthermore, the ­ctl file transforms all slashes not quoted or escaped by backslashes to backslashes in the command string before executing it. 

SOURCE

­/emu/devcmd.c

SEE ALSO

emu(1), os(1)

BUGS

Standard output and standard error ideally should not be mingled. 
Exit status should be available.
The maximum length of a command string is 128 bytes.

Plan 9  —  April 27, 2000