Simple shell implementation

Shell is an important tool in Unix/Linux to parse user input commands. Next we will implement a simple shell program to practiceFork/exec/Wait/exitBy the way, we recommend a book titled understanding Unix/Linux programming-A Guide to theory and practice, which is well written and suitable for beginners in Unix/Linux programming.

The main program of our shell is as follows:

 1 int main(void) 2 { 3     char *cmdline; 4     char *prompt; 5     char **arglist; 6     int result; 7  8     prompt = PROMPT; 9     signal(SIGINT, SIG_IGN);10     signal(SIGQUIT,SIG_IGN);11 12     while ((cmdline = next_cmd(prompt, stdin)) != NULL) {13         if ((arglist = split_line(cmdline)) != NULL) {14             result = execute(arglist);15             freelist(arglist);16         }17         free(cmdline);18     }19 20     return 0;21 }

Where,Next_cmd ()The main function of a function is to read the next command from the input stream, and return null when the file Terminator is reached. The code for this function is as follows:

 1 char *next_cmd(char *prompt, FILE *file) 2 { 3     char *cmdline; 4     int  length = 0; 5     int  c; 6     int  location = 0; 7  8     printf("%s", prompt); 9 10     while ((c = getc(file)) != ‘\n‘) {11         if (location + 1 >= length) {12             cmdline = (char *)malloc(BUFSIZ);13             length = BUFSIZ;14         } 15         else if (location >= BUFSIZ) {16             cmdline = realloc(cmdline, length + BUFSIZ);17             length += BUFSIZ;18         }19         cmdline[location++] = c;20     }21     cmdline[location] = ‘\0‘;22 23     return cmdline;24 }

Split_line ()The main function of the function is to split the input string into a string array, which ends with null. The code for this function is as follows:

 1 char **split_line(char *cmd) 2 { 3     char **arglist; 4     int  row = 0; 5     int  len = 0; 6     char *cp = cmd; 7     char *start = cmd; 8     arglist = malloc(BUFSIZ); 9 10     while (*cp != ‘\0‘) {11         while (*cp != ‘ ‘ && *cp != ‘\t‘) {12             ++cp;13             ++len;14         }15         arglist[row] = malloc(len + 1);16         strncpy(arglist[row], start, len);17         ++row;18         len = 0;19         while (*cp == ‘ ‘ || *cp == ‘\t‘) {20             ++cp;21         }22         start = cp;23     }24     arglist[row] = NULL;25 26     return arglist;27 }

Execute ()The main function of a function is to use fork, execvp, and wait functions to run a command and return the end state of the command. The code for this function is as follows:

 1 int execute(char **argv)  2 { 3     pid_t pid; 4     int child_info = -1; 5  6     if ((pid = fork()) == -1) { 7         perror("fork error"); 8     } 9 10     if (pid == 0) {11         signal(SIGINT, SIG_DFL);12         signal(SIGQUIT, SIG_DFL);13         execvp(argv[0], argv);14         perror("exec error");15         exit(1);16     }else {17         if (wait(&child_info) == -1) {18             perror("wait error");19         }20     }21 22     return child_info;23 }

Freelist ()The function is to release the space of the string array allocated above. The code for this function is as follows:

 1 void freelist(char **list)  2 { 3     char **cp = list; 4     while (*cp) { 5         free(*cp); 6         ++cp; 7     } 8  9     free(list);10 }

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Simple shell implementation