Simple Shell is lab1 from UCLA CS 111 Operating Systems (Winter 2019) with Professor Eggert.

Example Usage and Explanation

simpsh \
  --rdonly a \
  --pipe \
  --pipe \
  --creat --trunc --wronly c \
  --creat --append --wronly d \
  --command 3 5 6 tr A-Z a-z \
  --command 0 2 6 sort \
  --command 1 4 6 cat b - \
  --close 2 \
  --close 4 \
  --wait

This example invocation creates seven file descriptors:

1. A read only descriptor for the file a, created by the --rdonly option.

2. The read end of the first pipe, created by the first --pipe option.

3. The write end of the first pipe, also created by the first --pipe option.

4. The read end of the second pipe, created by the second --pipe option.

5. The write end of the second pipe, also created by the second --pipe option.

6. A write only descriptor for the file c, created by the first --wronly option as modified by the preceding --creat and --trunc.

7. A write only, append only descriptor for the file d, created by the --wronly option as modified by the preceding --creat and --append options.

It then creates three subprocesses:

1. A subprocess with standard input, output, and error being the file descriptors numbered 3, 5, and 6 above. This subprocess runs the command tr with the two arguments A-Z and a-z.

2. A subprocess with standard input, standard output, and standard error being the file descriptors numbered 0, 2, and 6 above, respectively. This subprocess runs the command sort with no arguments

3. A subprocess with standard input, output, and error being the file descriptors numbered 1, 4, and 6 above. This subprocess runs the command cat with the two arguments b and -.

It then closes the write ends of the pipes, and waits for all three subprocesses to finish. As each finishes, it outputs "exit N" if it exited with status N or "signal S" if it terminated with signal number S, followed by the command and arguments. The output might look like this:

exit 0 sort
exit 0 cat b -
exit 0 tr A-Z a-z

although not necessarily in that order, depending on which order the subprocesses finished.

Simple Shell Options

Here is a detailed list of the command-line options that simpsh supports. Each option should be executed in sequence, left to right.

First are the file flags. These flags affect the next file that is opened. They are ignored if no later file is opened. Each file flag corresponds to an oflag value of open; the corresponding oflag value is listed after the option. Also see Opening and Closing Files and Open-time Flags.

--append

O_APPEND

--cloexec

O_CLOEXEC

--creat

O_CREAT

--directory

O_DIRECTORY

--dsync

O_DSYNC

--excl

O_EXCL

--nofollow

O_NOFOLLOW

--nonblock

O_NONBLOCK

--rsync

O_RSYNC

--sync

O_SYNC

--trunc

O_TRUNC

Second are the file-opening options. These flags open files. Each file-opening option also corresponds to an oflag value, listed after the option. Each opened file is given a file number; file numbers start at 0 and increment after each file-opening option. Normally they increment by 1, but the --pipe option causes them to increment by 2.

--rdonly f

O_RDONLY. Open the file f for reading only.

--rdwr f

O_RDWR. Open the file f for reading and writing.

--wronly f

O_WRONLY. Open the file f for writing only.

--pipe

Open a pipe. Unlike the other file options, this option does not take an argument. Also, it consumes two file numbers, not just one. Third is the subcommand options:

--command i o e cmd args

Execute a command with standard input i, standard output o and standard error e; these values should correspond to earlier file or pipe options. The executable for the command is cmd and it has zero or more arguments args. None of the cmd and args operands begin with the two characters "--".

--wait

Wait for all commands to finish. As each finishes, output its exit status or signal number as described above, and a copy of the command (with spaces separating arguments) to standard output.

Finally, there are some miscellaneous options:

--close N

Close the Nth file that was opened by a file-opening option. For a pipe, this closes just one end of the pipe. Once file N is closed, it is an error to access it, just as it is an error to access any file number that has never been opened. File numbers are not reused by later file-opening options.

--verbose

Just before executing an option, output a line to standard output containing the option. If the option has operands, list them separated by spaces. Ensure that the line is actually output, and is not merely sitting in a buffer somewhere.

--profile

Just after executing an option, output a line to standard output containing the resources used. Use getrusage and output a line containing as much useful information as you can glean from it.

--abort

Crash the shell. The shell itself should immediately dump core, via a segmentation violation.

--catch N

Catch signal N, where N is a decimal integer, with a handler that outputs the diagnostic N caught to stderr, and exits with status N. This exits the entire shell. N uses the same numbering as your system; for example, on GNU/Linux, a segmentation violation is signal 11.

--ignore N

Ignore signal N.

--default N

Use the default behavior for signal N.

--pause

Pause, waiting for a signal to arrive.