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fabagaipo/c_shell.c

Created February 4, 2025 14:14
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c_shell.c
#include <sys/wait.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
/*
Function Declarations for builtin shell commands:
*/
int lsh_cd(char **args);
int lsh_help(char **args);
int lsh_exit(char **args);
/*
List of builtin commands, followed by their corresponding functions.
*/
char *builtin_str[] = {
"cd",
"help",
"exit"
};
int (*builtin_func[]) (char **) = {
&lsh_cd,
&lsh_help,
&lsh_exit
};
int lsh_num_builtins() {
return sizeof(builtin_str) / sizeof(char *);
}
/*
Builtin function implementations.
*/
/**
@brief Builtin command: change directory.
@param args List of args. args[0] is "cd". args[1] is the directory.
@return Always returns 1, to continue executing.
*/
int lsh_cd(char **args)
{
if (args[1] == NULL) {
fprintf(stderr, "lsh: expected argument to \"cd\"\n");
} else {
if (chdir(args[1]) != 0) {
perror("lsh");
}
}
return 1;
}
/**
@brief Builtin command: print help.
@param args List of args. Not examined.
@return Always returns 1, to continue executing.
*/
int lsh_help(char **args)
{
int i;
printf("Stephen Brennan's LSH\n");
printf("Type program names and arguments, and hit enter.\n");
printf("The following are built in:\n");
for (i = 0; i < lsh_num_builtins(); i++) {
printf(" %s\n", builtin_str[i]);
}
printf("Use the man command for information on other programs.\n");
return 1;
}
/**
@brief Builtin command: exit.
@param args List of args. Not examined.
@return Always returns 0, to terminate execution.
*/
int lsh_exit(char **args)
{
return 0;
}
/**
@brief Launch a program and wait for it to terminate.
@param args Null terminated list of arguments (including program).
@return Always returns 1, to continue execution.
*/
int lsh_launch(char **args)
{
pid_t pid;
int status;
pid = fork();
if (pid == 0) {
// Child process
if (execvp(args[0], args) == -1) {
perror("lsh");
}
exit(EXIT_FAILURE);
} else if (pid < 0) {
// Error forking
perror("lsh");
} else {
// Parent process
do {
waitpid(pid, &status, WUNTRACED);
} while (!WIFEXITED(status) && !WIFSIGNALED(status));
}
return 1;
}
/**
@brief Execute shell built-in or launch program.
@param args Null terminated list of arguments.
@return 1 if the shell should continue running, 0 if it should terminate
*/
int lsh_execute(char **args)
{
int i;
if (args[0] == NULL) {
// An empty command was entered.
return 1;
}
for (i = 0; i < lsh_num_builtins(); i++) {
if (strcmp(args[0], builtin_str[i]) == 0) {
return (*builtin_func[i])(args);
}
}
return lsh_launch(args);
}
/**
@brief Read a line of input from stdin.
@return The line from stdin.
*/
char *lsh_read_line(void)
{
#ifdef LSH_USE_STD_GETLINE
char *line = NULL;
ssize_t bufsize = 0; // have getline allocate a buffer for us
if (getline(&line, &bufsize, stdin) == -1) {
if (feof(stdin)) {
exit(EXIT_SUCCESS); // We received an EOF
} else {
perror("lsh: getline\n");
exit(EXIT_FAILURE);
}
}
return line;
#else
#define LSH_RL_BUFSIZE 1024
int bufsize = LSH_RL_BUFSIZE;
int position = 0;
char *buffer = malloc(sizeof(char) * bufsize);
int c;
if (!buffer) {
fprintf(stderr, "lsh: allocation error\n");
exit(EXIT_FAILURE);
}
while (1) {
// Read a character
c = getchar();
if (c == EOF) {
exit(EXIT_SUCCESS);
} else if (c == '\n') {
buffer[position] = '\0';
return buffer;
} else {
buffer[position] = c;
}
position++;
// If we have exceeded the buffer, reallocate.
if (position >= bufsize) {
bufsize += LSH_RL_BUFSIZE;
buffer = realloc(buffer, bufsize);
if (!buffer) {
fprintf(stderr, "lsh: allocation error\n");
exit(EXIT_FAILURE);
}
}
}
#endif
}
#define LSH_TOK_BUFSIZE 64
#define LSH_TOK_DELIM " \t\r\n\a"
/**
@brief Split a line into tokens (very naively).
@param line The line.
@return Null-terminated array of tokens.
*/
char **lsh_split_line(char *line)
{
int bufsize = LSH_TOK_BUFSIZE, position = 0;
char **tokens = malloc(bufsize * sizeof(char*));
char *token, **tokens_backup;
if (!tokens) {
fprintf(stderr, "lsh: allocation error\n");
exit(EXIT_FAILURE);
}
token = strtok(line, LSH_TOK_DELIM);
while (token != NULL) {
tokens[position] = token;
position++;
if (position >= bufsize) {
bufsize += LSH_TOK_BUFSIZE;
tokens_backup = tokens;
tokens = realloc(tokens, bufsize * sizeof(char*));
if (!tokens) {
free(tokens_backup);
fprintf(stderr, "lsh: allocation error\n");
exit(EXIT_FAILURE);
}
}
token = strtok(NULL, LSH_TOK_DELIM);
}
tokens[position] = NULL;
return tokens;
}
/**
@brief Loop getting input and executing it.
*/
void lsh_loop(void)
{
char *line;
char **args;
int status;
do {
printf("> ");
line = lsh_read_line();
args = lsh_split_line(line);
status = lsh_execute(args);
free(line);
free(args);
} while (status);
}
/**
@brief Main entry point.
@param argc Argument count.
@param argv Argument vector.
@return status code
*/
int main(int argc, char **argv)
{
// Load config files, if any.
// Run command loop.
lsh_loop();
// Perform any shutdown/cleanup.
return EXIT_SUCCESS;
}
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