(use-modules (ice-9 popen))

— Scheme Procedure: open-pipe command mode
— Scheme Procedure: open-pipe* mode prog [args...]

Execute a command in a subprocess, with a pipe to it or from it, or with pipes in both directions.

open-pipe runs the shell command using `/bin/sh -c'. open-pipe* executes prog directly, with the optional args arguments (all strings).

mode should be one of the following values. OPEN_READ is an input pipe, ie. to read from the subprocess. OPEN_WRITE is an output pipe, ie. to write to it.

— Variable: OPEN_READ
— Variable: OPEN_WRITE
— Variable: OPEN_BOTH

For an input pipe, the child's standard output is the pipe and standard input is inherited from current-input-port. For an output pipe, the child's standard input is the pipe and standard output is inherited from current-output-port. In all cases cases the child's standard error is inherited from current-error-port (see Default Ports).

If those current-X-ports are not files of some kind, and hence don't have file descriptors for the child, then /dev/null is used instead.

Care should be taken with OPEN_BOTH, a deadlock will occur if both parent and child are writing, and waiting until the write completes before doing any reading. Each direction has PIPE_BUF bytes of buffering (see Ports and File Descriptors), which will be enough for small writes, but not for say putting a big file through a filter.

— Scheme Procedure: open-input-pipe command

Equivalent to open-pipe with mode OPEN_READ.

          (let* ((port (open-input-pipe "date --utc"))
                 (str  (read-line port)))
            (close-pipe port)
            str)
          => "Mon Mar 11 20:10:44 UTC 2002"
     

— Scheme Procedure: open-output-pipe command

Equivalent to open-pipe with mode OPEN_WRITE.

          (let ((port (open-output-pipe "lpr")))
            (display "Something for the line printer.\n" port)
            (if (not (eqv? 0 (status:exit-val (close-pipe port))))
                (error "Cannot print")))
     

— Scheme Procedure: open-input-output-pipe command

Equivalent to open-pipe with mode OPEN_BOTH.

— Scheme Procedure: close-pipe port

Close a pipe created by open-pipe, wait for the process to terminate, and return the wait status code. The status is as per waitpid and can be decoded with status:exit-val etc (see Processes)