subprocess

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Run and interact with a child process in R! subprocess brings a new R API to create, control the life cycle and shutdown a child process in Linux, Windows and Mac OS. Check this out.

Remote shell example

Here's an example of running a ssh client child process. It connects to a ssh server using public key (thus, no password). Then we list files in the remote account and finally gracefully shutdown the child process.

Load the subprocess package and start a new child process:

library(subprocess)

ssh_path <- '/usr/bin/ssh'
handle <- spawn_process(ssh_path, c('-T', 'test@example'))

Here is the description of the child process:

print(handle)
#> Process Handle
#> command   : /usr/bin/ssh -T test@example
#> system id : 17659
#> state     : running

And here is what the child process has sent so far to its output streams:

process_read(handle, PIPE_STDOUT, timeout = TIMEOUT_INFINITE)
#> [1] "Welcome to Ubuntu 16.10 (GNU/Linux 4.8.0-27-generic x86_64)"
#> [2] ""                                                           
#> [3] " * Documentation:  https://help.ubuntu.com"                 
#> [4] " * Management:     https://landscape.canonical.com"         
#> [5] " * Support:        https://ubuntu.com/advantage"            
#> [6] ""                                                           
#> [7] "0 packages can be updated."                                 
#> [8] "0 updates are security updates."                            
#> [9] ""
process_read(handle, PIPE_STDERR)
#> character(0)

Nothing in the standard error output. Good! Now we ask the remote shell to list files.

process_write(handle, 'ls\n')
#> [1] 3
process_read(handle, PIPE_STDOUT, timeout = TIMEOUT_INFINITE)
#> [1] "Desktop"          "Download"         "examples.desktop"
#> [4] "Music"            "Public"           "Video"
process_read(handle, PIPE_STDERR)
#> character(0)

The first number in the output is the value returned by process_write() which is the number of characters written to standard input of the child process. The final character(0) is the output read from the standard error stream.

We are now ready to close the connection by exiting the remote shell:

process_write(handle, 'exit\n')
#> [1] 5
process_read(handle, PIPE_STDOUT, timeout = TIMEOUT_INFINITE)
#> character(0)
process_read(handle, PIPE_STDERR)
#> character(0)

The last thing is making sure that the child process is no longer alive:

process_wait(handle, TIMEOUT_INFINITE)
#> [1] 0
process_status(handle)
#> [1] "exited"