spec.md

Description:

Implement the Unix shell described in the accompanying man page.

Summary of details enumerated in the manual page (in order of appearance):
    rc file (~/.ushrc)
    hostname%
    special chars:
    & | ; < > >> |& >& >>&
    backslash
    strings
    commands:
    cd echo logout nice pwd setenv unsetenv where 
Major tasks
    Command line parsing
        ush.tar.gz 
    Command execution
        fork(2)
        exec family: execvp(2) might be the best 
    I/O redirection
        open(2), close(2) (not fopen, fclose)
        dup(2), dup2(3c)
        stdin, stdout, stderr 
    Environment variable
        putenv(3c), getenv(3c) 
    Signal handling
        signal(5)
        signal(3c)
        getpid(2), getpgrp(2), getppid(2), getpgid(2)
        setpgid(2), setpgrp(2) 

Job control is not implemented including these commands

the fg command,
the bg command,
the kill command,
the jobs command, or
the & operator. 

Behaviour:

- disabled standard input buffer
- Signal Handling
    + ignore sigint(ctrl+c) in parent shell
    + ignore sigquit(ctrl+\) in parent shell
    + ignore sigterm in parent shell
    + ignore sigtstp(ctrl+z) in parent shell
    + if cannot set sigaction perror "cannot handle signal"
- .ushrc
    + search in home directory
    + no error if not found
    + open in read-only mode 
    + execute commands line by line
    + command behaviour is exactly like when normally run in shell
    + works by counting the number of lines in ushrc, redirecting the file to shells standard input and reading and executing line number of times.
    + restores the standard input to command line after execution
- prompt "%hostname" - hostname is the environment variable "USER"
- print prompt at standard output
- print everything else at standard input