with_create_args specifies the arguments used to create a child process.
create ~prog ~args ?working_dir ?env () uses fork and exec to create a child
process that runs the executable prog with args as arguments. It creates pipes to
communicate with the child process's stdin, stdout, and stderr.
Unlike exec, args should not include prog as the first argument.
If working_dir is supplied, then the child process will chdir() there before
calling exec().
env specifies the environment of the child process.
create returns Error if it is unable to create the child process. This can happen
in any number of situations (unable to fork, unable to create the pipes, unable to cd
to working_dir, unable to exec, etc.).
collect_output_and_wait t closes stdin t and then begins collecting the output
produced on t's stdout and stderr, continuing to collect output until t
terminates and the pipes for stdout and stderr are closed.
run creates a process and waits for it to complete. If the process exits with
an acceptable status, then run returns its stdout. Acceptable statuses are zero,
and any nonzero values specified in accept_nonzero_exit. If the process exits
unacceptably, then run returns an error indicating what went wrong that includes
stdout and stderr.
Some care is taken so that an error displays nicely as a sexp---in particular, if the child's output can already be parsed as a sexp, then it will display as a sexp (rather than a sexp embedded in a string). Also, if the output isn't a sexp, it will be split on newlines into a list of strings, so that it displays on multiple lines rather than a single giant line with embedded " "'s.
run_lines is like run but returns the lines of stdout as a string list, using
String.split_lines.
run_expect_no_output is like run but expects the command to produce no output, and
returns an error if the command does produce output.
collect_stdout_and_wait and collect_stdout_lines_and_wait are like run and
run_lines but work from an existing process instead of creating a new one.