Execution of shell commands.
color_default controls whether color will actually do coloring.
show msg outputs the message msg on stdout.
warn msg outputs the warning message msg on stderr.
error msg outputs the error message msg on stderr.
fatal_error i msg output the error message msg on stderr
and stops the program with exit code i.
verbose_default defines the default value for the ?verbose argument
of this module.
has_cmd cmd is true iff the shell has the command cmd.
Execute a shell command.
Execute a shell command and returns its output.
Execute a command in a given directory.