c语言字符串拼接——snprintf

获取可执行文件绝对路径的时候遇到了一个问题，比如给了可执行文件的名字 “ls”，要求在/bin和/usr/bin里面找存在的可执行的名叫ls的可执行文件，返回绝对路径比如/bin/ls，如果高效拼接“/bin”、“/”、“ls”。


借鉴上面的回答，得到代码：

char *pathSet[] = {"/bin", "/usr/bin", NULL};
#define BUFF_SIZE 256
int searchPath(char *execute, char *path) {
    for (int i = 0; pathSet[i] != NULL; i++) {
        snprintf(path, BUFF_SIZE, "%s/%s", pathSet[i], execute);
        if (access(path, X_OK) == 0) return 0;
    }
    return -1;
}

答案（未实现并行指令）

#include <ctype.h>  // isspace
#include <regex.h>  // regcomp, regexec, regfree
#include <stdio.h>  // fopen, fclose, fileno, getline, feof
#include <stdlib.h> // exit
#include <sys/types.h>
#include <sys/wait.h> // waitpid
#include <string.h>  // strlen, strsep, strcat, strdup, strcmp
#include <unistd.h>  // STDERR_FILENO, fork, exec, access, exit, chdir
#define INTERACTIVE_MODE 1
#define BATCH_MODE 2
#define BUFF_SIZE 256
char *pathSet[BUFF_SIZE] = {"/bin", "/usr/bin", NULL};
char *line = NULL;
FILE *in;
void printError() {
    char error_message[30] = "An error has occurred\n";
    write(STDERR_FILENO, error_message, strlen(error_message));
}
void parseInputAndExecute(char *arg);
void executeCommands(char *args[], int argsNum, FILE *out);
int searchPath(char *execute, char *path);
void redirect(FILE *out);
char *trim(char *s);
int main(int argc, char *argv[]) {
    size_t len = 0;
    ssize_t readlen = 0;
    char *command = NULL;
    //int commandsNum = 0;
    if (argc == 1) {
        while (1) {
            printf("wish> ");
            if ((readlen = getline(&line, &len, stdin)) != -1) {
                if (line[readlen - 1] == '\n') line[readlen - 1] = '\0';
                while ((command = strsep(&line, "&")) != NULL) {
                    //printf("cmd:%s--line:%s--|", command, line);
                    if (command[0] == '\0') continue;
                    parseInputAndExecute(command);
                }
            } else {
                free(line);
                printError();
                exit(1);
            }
        }
    } else if (argc == 2){
        in = fopen(argv[1], "r");
        if (in == NULL) {
            free(line);
            printError();
            exit(1);
        }
        while ((readlen = getline(&line, &len, in)) != -1) {
            if (line[readlen - 1] == '\n') line[readlen - 1] = '\0';
            while ((command = strsep(&line, "&")) != NULL) {
                //printf("cmd:%s--line:%s--|", command, line);
                if (command[0] == '\0') continue;
                parseInputAndExecute(command);
            }
        }
        fclose(in);
        free(line);
        exit(0);
    } else {
        printError();
        exit(1);
    }
    return 0;
}
void parseInputAndExecute(char *arg) {
    FILE *out = stdout;
    int argsNum = 0;
    char *args[BUFF_SIZE];
    char *token;
    char *command = strsep(&arg, ">");
    if (command == NULL || *command == '\0') {
        printError();
        return;
    }
    if (arg != NULL) {
        // contain white space in the middle or ">"
        char *file = trim(arg);
        token = strsep(&file, " \t");
        //printf("file:%s---token:%s", file, token);
        if (file != NULL) {
            printError();
            return;
        }
        if ((out = fopen(token, "w")) == NULL) {
            printError();
            return;
        }
    }
    //printf("command:%s--arg:%s\n", line, arg);
    char *const delim = " \t";
    command = trim(command);
    while (command && *command != '\0') {
        token = strsep(&command, delim);
        if (token != NULL && *token != '\0') args[argsNum++] = token;
        if (argsNum >= BUFF_SIZE) break;
    }
    // for (int i = 0; i < argsNum; i++) {
    //     printf("token:%s--argsNum:%d\n", args[i], argsNum);
    // }
    if (argsNum < BUFF_SIZE) args[argsNum] = NULL;
    else args[BUFF_SIZE - 1] = NULL;
    if (argsNum > 0) {
        executeCommands(args, argsNum, out);
    }
}
void executeCommands(char *args[], int argsNum, FILE *out) {
    // check built-in command: exit, cd, path
    if (strcmp(args[0], "exit") == 0) {
        if (argsNum > 1) printError();
        else {
            free(line);
            exit(EXIT_SUCCESS); //exit(0);
        }
    } else if (strcmp(args[0], "cd") == 0) {
        if (argsNum != 2) printError();
        else {
            if (chdir(args[1]) == -1) {
                printError();
            }
        }
    } else if (strcmp(args[0], "path") == 0) {
        int i = 0;
        // while (args[i]) {
        //     printf("args:%s\n", args[i]);
        //     i++;
        // }
        // i = 0;
        while (pathSet[i] != NULL) i++;
        int j = 0;
        while (j < argsNum - 1) {
            pathSet[i] = strdup(args[j + 1]);
            i++;
            j++;
        }
        pathSet[i] = NULL;
        if (argsNum == 1) pathSet[0] = NULL;
        // i = 0;
        // while (pathSet[i]) {
        //     printf("pathset:%s\n", pathSet[i]);
        //     i++;
        // }
    } else {
        // not built-in command
        char path[BUFF_SIZE];
        int rc = searchPath(args[0], path);
        if (rc == -1) {
            printError(); // not fond in path
            return;
        }
        //printf("path: %s,args: %s", path, args[0]);
        pid_t pid = fork();
        if (pid < 0) {
            printError();
            exit(1);
        } else if (pid == 0) {
            redirect(out);
            execv(path, args);
            printError();
        } else {
            waitpid(pid, NULL, 0); // parent process waits child
        }
    }
}
int searchPath(char *execute, char *path) {
    for (int i = 0; pathSet[i] != NULL; i++) {
        snprintf(path, BUFF_SIZE, "%s/%s", pathSet[i], execute);
        int rc = access(path, X_OK);
        //printf("path:%s---rc:%d\n", path, rc);
        if (rc == 0) {
            return 0;
        }
    }
    return -1;
}
void redirect(FILE *out) {
    int outFileno = fileno(out);
    if (outFileno == -1) {
        printError();
        exit(1);
    }
    if (outFileno != STDOUT_FILENO) {
        // redirect output
        if (dup2(outFileno, STDOUT_FILENO) == -1) {
            printError();
            return;
        }
        if (dup2(outFileno, STDERR_FILENO) == -1) {
            printError();
            return;
        }
        fclose(out);
    }
}
char *trim(char *s) {
    // trim leading spaces
    while (isspace(*s)) s++;
    if (*s == '\0') return s;   //empty string
    // trim trailing spaces
    char *end = s + strlen(s) - 1;
    while (end > s && isspace(*end)) end--; 
    *(end + 1) = '\0';
    return s;
}