Submission

Please, submit your solution by e-mail to: martin.kuettler (at) os.inf.tu-dresden.de. Don't attach anything to the e-mail (like pdf, docs etc.) other than the requested source code and a Makefile within a tar-archive.

Exercise

In this exercise you are going to write a simple shell that is able to execute command lines, interpret pipes and use input/and output redirection. You are going to use Flex and Bison in order to specify a parser for your command lines and act upon these.

First, implement a shell with the following required features:

• Execute command lines with the following syntax:
  command <args> [< input_redirect] (| command <args>)* [> output_redirect] [&]
  As a starting point you may (but need not) consider the files provided in the resources section below.
• Support an arbitrary amount of arguments to programs as well as an arbitrary length for the pipe chain.
• Detect erroneous input and give useful feedback to the user.

Second, add convenience features to your shell:

1. The readline library provides line editing features, such as deleting a word. AlibReadline comes along with libHistory which provides means to store strings in a list and navigate through this list. Use both libraries in order to implement easy-to-use line editing as well as access to previously entered commands using the arrow keys.
2. Add support for the following shell builtins to your shell
   • cd to change to another working directory
   • pwd to print the current working directory
   • kill <signo> <pid> to send a signal to a process
   • alias/unalias to manage aliases (which are replaced by by their meaning during execution of commands).

Resources

• Flex and Bison are successors to Lex and Yacc, so all relevant information for Lex and Yacc is also useful for these tools. There is a pretty good tutorial available.
• Manual for the Flex scanner generator
• Manual for the Bison parser generator
• Sample Flex input file
• Sample Bison input file
• The GNU readline library
• When implementing builtins you might have a look at the BASH reference manual.
• For executing processes, have a look at the man pages for fork(), execve(), and wait().
• For input and output redirection, take a look at open(), close(), and dup2().
• Pipes are created by pipe(), signal handling can be set up using sigaction().