Simple Shell

The files in this repository are functions and programs that deal with setting up a UNIX command line interpreter.

• Usage: command arguments(optional)

Examples

• ls /bin
• history
• pwd
• echo $PATH

The shell:

1. Displays a prompt and waits for the user to type a command. Command lines always end with a new line.
2. Displays a prompt again each time a command has been executed.
3. If an executable cannot be found, an error message is displayed, followed by a prompt.
4. Handles the "end-of-file" condition (CTRL+D)

Compilation instructions

1. Clone this repository to your local machine.
2. Run gcc *c -o shell
3. Run shell program (./shell)

buildstr.c

Takes 3 pointers to strings and concatenates them to from a complete string. Creates a buffer using malloc to concatenate the passed strings to. Returns a pointer to a string that should be freed by the caller.

built_in_shell.c

Checks for whether the passed commands are part of the built_in shell functions. If a match is found, that function is run using the passed arguments, if any.

cd.c

One of the shell built_in functions. Changes the directory we're currently in, and updates the PWD and OLDPWD environment variables.

check_args.c

Checks the arguments passed for the echo command and does the required substitution. If no substitution is required, the argument remains unchanged.

check_path.c

Checks for the location of a passed command in the directories listed in the path variable. If the command is found, the full path string is built using the buildstr.c function.

exec_function.c

Runs the passed command using the arguments given. First checks whether the command is a built_in shell function using the built_in_shell.c function. If not, it searches for the command using the check_path.c function. If a match is found, it creates a child process and runs the command in the child process. If no match is found even in the current directory, the function fails.

free_2d.c

Frees an array of char arrays that were created using malloc.

getenv.c

Retrieves the value of an environment variable. Checks whether the passed environment variable exists. If so, it returns its value. If no match is found, it returns NULL.

handle_sigint.c

Handles the SIGINT signal (Ctrl c or ^C) using the signal function. If SIGINT is recieved, it prints a newline and a prompt and waits for more user input.

parsecommands.c

Parses the string that has been read from the command line. Separates the string into its constituent words using space as a delimiter and stores the words in an array of words. This 2d array is created using malloc and thus should be freed once used. Returns this 2d array.

printenv.c

Prints the environment variables to stdout when the "env" command is received.

setenv.c

Creates a new environment variable and sets its value or updates the value of an existing environment variable when the setenv command is received.

shelberton.h

Header file for the shell program. Contains all the function prototyes as well as the include statements for all the required header files.

shell.c

Contains the main function for the shell program. Reads in the command and arguments from stdin and calls the necessary functions to parse the string and run the command. Also sets up the environment variables that will be used.

shell_exit.c

Called when the exit command is passed. It frees all the malloc'ed variables and then exits the shell with the given status, or 0 if none is given.

strcat.c

Our implementation of the strcat function. Takes 4, arguments; 1 destination string and 3 source strings. The destination string should be large enough to fit all the passed strings.

strchr.c

Our implementation of the strchr.c function. It finds the first occurence of a given character in a given string. If not found, it returns NULL.

strcmp.c

Our implementation of the strcmp function. It takes 2 pointers to strings as arguments and compares them. If the 2 strings match, it returns 0. If a mismatch is found, it returns the difference in the ASCII codes of the mismatched characters.

strdup.c

Our implementation of the strdup function. It takes a pointer to a string as an argument. It creates a buffer of required size using malloc and copies the given string to the buffer and returns a pointer to the created buffer. Calling function should free the return value.

strlen.c

Takes a pointer to a string and returns the length of the string.

strncmp.c

Our implementation of the strncmp function. Compares the first n bytes of the 2 strings it receives and returns 0 if they match. If a mismatch is found, it returns the difference in ASCII codes of the mismatched characters.

strtok.c

Our implementation of the strtok function. It parses a given string using the given delimeter character and returns a pointer to the resulting string. Uses malloc and thus the calling function should free the returned pointer.

toint.c

Takes in a nunmber contained in a string and returns it as an integer variable.

tostring.c

Takes an integer variable as an argument and converts it to a string. Returns a pointer to this string. Uses malloc to create the buffer to store the number, thus the calling function should free the returned pointer.

unsetenv.c

Removes an existing environment variable. Called when the "unset" command is received.

putchar.c

Our implementation of the putchar function that prints a given character to STDOUT.

history.c

Our implementation of the built-in history command that stores and displays commands previously entered by the user in the terminal.It stores the commands in a file named ".simple_shell_history" and displays that file's content when called. It contains different helper functions which are aptly named to indicate their functionality.

help_fun.c

This contains helper functions to some of our functions;display_help is used to execute the inbuilt help command which enables their execution once they are called.

getline.c

This is our own implementation of the getline function that reads user input from STDIN and whose return value is the read user input.It has inbuilt functionality to ignore comments denoted by a "#" sign at the start thus providing clear and required outcome.