Unix/Linux编程实践教程–chmod在Centos7.3的实现

时间：2021-05-08 16:42:59

环境：centos 7.3 x86_64

如果搜一下man就会发现，里面有两个chmod，一个是chmod(1)，一个是chmod(2)。根据牛顿-莱布尼兹公式，立即推，第一个是用户命令，第二个是系统调用。系统调用里，函数的原型是这样的：

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

所以要实现的chmod命令，无非就是把用户的输入解释成对应的mode_t类型，然后交给chmod系统调用进行处理即可。

目标，支持符号和八进制数两种表示法。符号表示法仅支持[ugoa...][+-=][rwxst...]。八进制表示法支持1-4位八制数，没有设置的位默认为0，同man 1 chmod一致。

Usage: chmod mode file...

程序流程大概是这样的：

  1. 解析mode
  2. 对于每个文件调用chmod进行设置或修改

代码大概是这样的，用了一些位操作去减少代码的长度：

#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：

Unix/Linux编程实践教程–chmod在Centos7.3的实现

测试脚本：

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
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