環境：centos 7.3 x86_64

如果搜一下man就會發現，裡面有兩個chmod，一個是chmod(1)，一個是chmod(2)。根據牛頓-萊布尼茲公式，立即推，第一個是使用者命令，第二個是系統呼叫。系統呼叫裡，函式的原型是這樣的：

int chmod(const char *pathname, mode_t mode);

所以要實現的chmod命令，無非就是把使用者的輸入解釋成對應的mode_t型別，然後交給chmod系統呼叫進行處理即可。

目標，支援符號和八進位制數兩種表示法。符號表示法僅支援[ugoa...][+-=][rwxst...]。八進位制表示法支援1-4位八制數，沒有設定的位預設為0，同man 1 chmod

Usage: chmod mode file...

程式流程大概是這樣的：

1. 解析mode
2. 對於每個檔案呼叫chmod進行設定或修改

程式碼大概是這樣的，用了一些位操作去減少程式碼的長度：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>

void oops(const char* str);
mode_t parse_mode(const char *mode);
void xc_chmod(const char *path, mode_t mode);
mode_t modify_mode(mode_t mode);

#define USAGE "Usage: chmod mode file...\n"
 


enum{
    CW_ADD,
    CW_REMOVE,
    CW_SET,
    CW_COVER
}chmod_way;
enum{
    MOD_READ = 0444,
    MOD_WRITE = 0222,
    MOD_EXEC = 0111,
};
enum{
    ER_ALL = 0777,
    ER_USR = 0700,
    ER_GRP = 070,
    ER_OTH = 07
}effect_range = 0;

int mode_read = 0;
int mode_write = 0; 
int mode_exec = 0;
int mode_sgid = 0
 
;
int mode_sticky = 0;

int main(int ac, char *av[])
{
    mode_t mode;
    if(ac < 3){
        fprintf(stderr, USAGE);
        exit(1);
    }
    mode = parse_mode(*++av);
    ac -= 2;
    while(ac--)
        xc_chmod(*++av, mode);
    return 0;
}
void xc_chmod(const char *path, mode_t mode)
{
    struct stat info;
    if(chmod_way != CW_COVER){
        if(stat(path, &info) == -1)
            oops("chmod");
        mode = info.st_mode;
        mode = modify_mode(mode);   
    }
    if(chmod(path, mode) == -1)
            oops("chmod");
}
mode_t modify_mode(mode_t mode)
{
    mode_t amode = mode;
    int add_or_remove;
    if(chmod_way == CW_SET)
    {
        if(mode_read){
            // printf("MODE_READ: %o & effect_range: %o = %o\n", MOD_READ, effect_range, MOD_READ& effect_range);
            amode |= MOD_READ & effect_range;
        }
        if(mode_write)
            amode |= MOD_WRITE & effect_range;
        if(mode_exec)
            amode |= MOD_EXEC & effect_range;

        if(mode_sgid){
            if(effect_range & ER_USR)
                amode |= S_ISUID;
            if(effect_range & ER_GRP)
                amode |= S_ISGID;
        }
        if(mode_sticky  && (effect_range & ER_OTH))
            amode |= S_ISVTX;
    }else{
        add_or_remove = chmod_way == CW_ADD;

        if(mode_read)
            if(add_or_remove)
                amode |= MOD_READ & effect_range;
            else
                amode &= ~(MOD_READ & effect_range);
        if(mode_write)
            if(add_or_remove)
                amode |= MOD_WRITE & effect_range;
            else
                amode &= ~(MOD_WRITE & effect_range);
        if(mode_exec)
            if(add_or_remove)
                amode |= MOD_EXEC & effect_range;
            else
                amode &= ~(MOD_EXEC & effect_range);

        if(mode_sgid){
            if(effect_range & ER_USR)
                if(add_or_remove)
                    amode |= S_ISUID;
                else
                    amode &= ~S_ISUID;

            if(effect_range & ER_GRP)
                if(add_or_remove)
                    amode |= S_ISGID;
                else
                    amode &= ~S_ISGID;

        }
        if(mode_sticky && (effect_range & ER_OTH))
            if(add_or_remove)
                amode |= S_ISVTX;
            else
                amode &= ~S_ISVTX;
    }
    return amode;
}
mode_t parse_mode(const char *mode)
{
    char *p;
    mode_t amode;
    if(mode[0] >= '0' && mode[0] <= '9'){
        /* parse use oct digits */
        int len = 0;
        int base = 1;
        int tmp = 0;
        int i;
        int legal = 1;
        p = mode;
        while(*p){
            if(!(*p >= '0' && *p <= '7'))
                legal = 0;
            p++;
            len++;
        }
        if(len > 4)
            legal = 0;
        if(!legal){
            fprintf(stderr, USAGE);
            exit(1);
        }
        sscanf(mode, "%o", &tmp);
        amode = tmp;
        chmod_way = CW_COVER;
    }else{
        /* parse use symbolic */
        p = mode;
        while(*p != '+' && *p != '-' && *p != '='){
            switch(*p){
                case 'a':
                    effect_range |= ER_ALL;
                    break;
                case 'g':
                    effect_range |= ER_GRP;
                    break;
                case 'o':
                    effect_range |= ER_OTH;
                    break;
                case 'u':
                    effect_range |= ER_USR;
                    break;
                default:
                    fprintf(stderr, USAGE);
                    exit(1);
                    break;
            }
            p++;
        }
        switch(*p){
            case '+':
                chmod_way = CW_ADD;
                break;
            case '-':
                chmod_way = CW_REMOVE;
                break;
            case '=':
                chmod_way = CW_SET;
                break;
            default:
                fprintf(stderr, USAGE);
                exit(1);
                break;
        }
        p++;
        while(*p){
            switch(*p){
                case 'r':
                    mode_read = 1;
                    break;
                case 'w':
                    mode_write = 1;
                    break;
                case 'x':
                    mode_exec = 1;
                    break;
                case 's':
                    mode_sgid = 1;
                    break;
                case 't':
                    mode_sticky = 1;
                    break;
                default:
                    fprintf(stderr, USAGE);
                    exit(1);
                    break;
            }
            ++p;
        }
    }
    return amode;
}
void oops(const char* str)
{
    perror(str);
    exit(1);
}

效果：

測試指令碼：

gcc chmod.c

./a.out 7777 aaa
ls -l aaa > bbb
./a.out 644 aaa
ls -l aaa >> bbb
./a.out 0000 aaa
ls -l aaa >> bbb
./a.out u=rwxst aaa
ls -l aaa >> bbb
./a.out g=rwxst aaa
ls -l aaa >> bbb
./a.out o=rwxst aaa
ls -l aaa >> bbb
./a.out 0000 aaa
ls -l aaa >> bbb
./a.out a=rwxst aaa
ls -l aaa >> bbb
./a.out 0000 aaa
ls -l aaa >> bbb
./a.out a=rw aaa
ls -l aaa >> bbb
./a.out 0000 aaa
ls -l aaa >> bbb
./a.out ug=rw aaa
ls -l aaa >> bbb
./a.out o+rw aaa
ls -l aaa >> bbb
./a.out u+x aaa
ls -l aaa >> bbb
./a.out u+s aaa
ls -l aaa >> bbb
./a.out u-x aaa
ls -l aaa >> bbb
./a.out a-r aaa
ls -l aaa >> bbb
./a.out a+t aaa
ls -l aaa >> bbb

chmod 7777 aaa
ls -l aaa > ccc
chmod 644 aaa
ls -l aaa >> ccc
chmod 0000 aaa
ls -l aaa >> ccc
chmod u=rwxst aaa
ls -l aaa >> ccc
chmod g=rwxst aaa
ls -l aaa >> ccc
chmod o=rwxst aaa
ls -l aaa >> ccc
chmod 0000 aaa
ls -l aaa >> ccc
chmod a=rwxst aaa
ls -l aaa >> ccc
chmod 0000 aaa
ls -l aaa >> ccc
chmod a=rw aaa
ls -l aaa >> ccc
chmod 0000 aaa
ls -l aaa >> ccc
chmod ug=rw aaa
ls -l aaa >> ccc
chmod o+rw aaa
ls -l aaa >> ccc
chmod u+x aaa
ls -l aaa >> ccc
chmod u+s aaa
ls -l aaa >> ccc
chmod u-x aaa
ls -l aaa >> ccc
chmod a-r aaa
ls -l aaa >> ccc
chmod a+t aaa
ls -l aaa >> ccc

diff bbb ccc