If flag is true, turn debugging on. Otherwise, turn debugging
off. When debugging is on, commands to be executed are printed, and
the shell is given set -x command to be more verbose.
append(
cmd, kind)
Append a new action at the end. The cmd variable must be a valid
bourne shell command. The kind variable consists of two letters.
The first letter can be either of '-' (which means the command
reads its standard input), 'f' (which means the commands reads
a given file on the command line) or '.' (which means the commands
reads no input, and hence must be first.)
Similarly, the second letter can be either of '-' (which means
the command writes to standard output), 'f' (which means the
command writes a file on the command line) or '.' (which means
the command does not write anything, and hence must be last.)
prepend(
cmd, kind)
Add a new action at the beginning. See append() for explanations
of the arguments.
open(
file, mode)
Return a file-like object, open to file, but read from or
written to by the pipeline. Note that only one of 'r',
'w' may be given.