Shell

Building

make

Running

./shell

Implemented features

Processing of all sorts of incorrect commands:

ls;|>>
((pwd)(
...

Built-in commands

cd   # changes the current directory,
pwd  # prints the full path to the current directory
exit # terminates the interpreter.

Quotes allow to insert a space in an argument or a program name, for example:

cat "new file" #print a file with name "new file" to the screen.

Character escaping

Character escaping cancels the special action of the following character, for example

\"

is treated as a regular quote, and not as a delimiter for a quoted string, two backslashes

\\ 

allow to enter a regular single backslash.

Comments

If the input string contains a # symbol (not inside quotes) then the rest of the line is ignored.

ls #print files in current directory

Substitution of system variables:

$HOME  # user's home directory
$SHELL # path to the current shell
$USER  # username of the current user
$EUID  # user ID with the rights of which the process is running

Pipes

program1 | program2 | ... | programN

Redirecting input-output

<, >, >>

Running in background

&

Sequential execution of commands

as if they were passed to the interpreter one by one in a line

program1 ; program2 ; ... ; programN

Logical operators

Run program1, if successful run program2

program1 && program2

Run program1, if failed run program2

program1 || program2

Execution of commands in parentheses in a child process:

(ls | wc) | cat
(mv f1.c f2.c; cp f2.c f3.c) &
((((ls) | cat –n ) | cat –n) | sort –r)

Implementation

The interpreter is written in the C programming language, using the POSIX standard.

Each command to be executed in a separate process is described by the following structure:

struct Tree {
    int argc;             // number of arguments
    char **argv;          // list of command name and arguments
    char *infile;         // redirected standard input file
    char *outfile;        // redirected standard output file
    int backgrnd;         // 1 if the command is to be executed in the background
    next_type type;       // NXT, AND, OR, if ; or && or || are used respectively
    int append;           // 1 if ">>" is used to append to file
    struct Tree *psubcmd; // commands to be run in child shell
    struct Tree *pipe;    // next command after "|"
    struct Tree *next;    // next command after ";", "&&", "||" (or after "&")
};

Examples

cat f; date; pwd > zz

date; (ls -l|cat -n)>f & pwd