psh - Naive shell with recursive descent parser

Exercise on C and system programming.

Prerequisites

• GNU readline

Build

    $ make clean; make all
    $ ./psh

Goal

1. Enable to redirect
   • e.g.) <command> > <filename>
2. Enable to pipe
   • e.g.) <command 1> | <command 2>
   • Pipe should enable to be nested
     • e.g.) <command 1> | <command 2> | <command 3> | ... <command n>
     • Depth of nests can be limited by realistic restriction
3. Enable to interpret ~ as a part of paths
4. Redirect and pipe can be recognized even if they have no space arround them
5. Enable to deal with both absolute and relative path
6. Enable to read environment variables

Options

1. Enable to modify environment variables (5 points)
2. Enable to interpret back quoted commands and return its result to outer command (20 points)
3. Enable to interpret if statements and loops (15 points)

Shell input syntax in BNF

    <piped_commands> ::= <command> { '|' <piped_commands> }
    <command> ::= <command_element> { <command> }
    <redirect_in> ::=  { <num> } '<' { '&' } <word>
    <redirect_out> ::= { <num> } '>' { ('>' | '&') } <word>
    <redirection> ::= <redirect_in>
                    | <redirect_out>
    <redirection_list> ::= <redirection> { <redirection_list> } 
    <command_element> ::= <word>
                        | <env_assignment>
                        | <redirection_list>

    <digit> ::= '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' 
    <alpha> ::= 'a' | 'b' | 'c' | 'd' | 'e' | 'f' | 'g' | 'h' | 'i' | 'j' 
              | 'k' | 'l' | 'm' | 'n' | 'o' | 'p' | 'q' | 'r' | 's' | 't' 
              | 'u' | 'v' | 'w' | 'x' | 'y' | 'z' 
              | 'A' | 'B' | 'C' | 'D' | 'E' | 'F' | 'G' | 'H' | 'I' | 'J' 
              | 'K' | 'L' | 'M' | 'N' | 'O' | 'P' | 'Q' | 'R' | 'S' | 'T' 
              | 'U' | 'V' | 'W' | 'X' | 'Y' | 'Z' 
    <alphanum> ::= (<alpha> | <num> | '_' ) { <alphanum> }
    <special> ::= '!' | '"' | '#' | '%' | ''' | '(' | ')' | '*' | '+' | ',' 
                | '-' | '.' | '/' | ':' | ';" | '?' | '@' | '[' | ']' | '&' 
                | '\' | '^' | '_' | '`' | '{' | '|' | '}' 
    <word> ::= (<env> | <letter> | <home>) { <word> }
    <home> ::= '~' { <alphanum> }
    <env> ::= '$' { '{' } <word> { '}' }
    <num> ::= <digit> { <num> }
    <env_assignment> ::= <alphanum> '=' <word>
    <letter> ::= (<alphanum> | <special>) { <letter> }

Copyright

Copyright (c) 2010 Taku Fukushima. All rights reserved.

License

psh is MIT Lisense