psh - "the pipe shell"

Invocation

[$ make]
$ ./psh

Built-in commands

The pipe shell, or psh, is a simplified shell for use on UNIX-like systems. The following are built-in commands:

exit - Halt the shell

psh $ exit
psh $ CTRL-D

setvar - Set key to value

psh $ setvar $SOMEVAR "somestring"
psh $ setvar $SOMEVAR $SOMEOTHERVAR

cd - Change directory

psh $ cd path/to/directory
psh $ cd $SOME_DIR_VAR

Note that the pipe shell displays the current leaf directory to the user like so:

[currentdir] psh $

This has been omitted from the documentation to reduce clutter.

User commands

Users may use an absolute path to the command, a path relative to ./ or ../, or a command that can be found by searching through directories in the $PATH variable. The default $PATH is:

/bin
/usr/bin
/usr/local/bin
/usr/local/gnu/bin
/usr/local/sbin
/usr/sbin
/sbin
/usr/games

The user, of course, can set his or her own path using setvar. Another built-in shell variable is $ShowParseOutput, which allows diagnostic information to be printed about parsing when set to "1". Initially, this is set to "0".

User Commands may provide arguments in the form of strings, quoted strings, and shell variables.

Multiple user commands can be chained together using pipes like so:

psh $ cmd1 | cm2 | cmd3

In this case, the output of cmd1 would be piped as the input of cmd2, whose output would be piped to cmd3.

Stdin and stdout can be redirected to other files like so:

psh $ cmd < filein > fileout

Note that one or both can be used, but if both are used, redirecting stdin must precede redirecting stdout. Filenames can be strings, quoted strings, or variables.

Finally, commands can be sent to the background by typing an ampersand at the end of the command like so:

psh $ cmd &
[1354] Sent to background

To summarize:

usage - psh $ cmd [arg]* [| cmd [arg]* ]* [< filein] [> fileout] [&]

Pitfalls

Commands cannot start with shell variables. Shell variables must have whitespace on either side. Each of the built-in commands takes an exact number of arguments - no fewer, no more. Strings packed into shell variables are not unpacked as multiple arguments to commands.

Examples

Saving arguments in a variable

psh $ setvar $LSFLAGS "-laG"
psh $ ls $LSFLAGS

Storing a directory in a variable

psh $ setvar $HOME "/mounts/u-zon-d2/ugrad/somestudent"
psh $ cd $HOME

Running a command found on the $PATH

psh $ python -c "print 'hello world!'"

Running a user command relative to ./

psh $ ./psh

Piping data through user commands

psh $ ps -e | sort | grep -in "psh"

Redirecting stdin

psh $ wc < somefile.txt

Letting a long job complete in the background

psh $ ls -R / > bigfile.txt &

cdparks / pipeshell