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Write a program in C using Unix system calls and functions that will change the permissions on a ...

Write a program in C using Unix system calls and functions that will change the permissions on a file. The executable shall be called “mychmod” and will be executed by:

mychmod -u rwx -g rwx -o rwx -U rwx -G rwx -O rwx file1 file2 ...

The lowercase options will add permissions while the uppercase options will remove permissions. Each of the switches is optional and should be interpreted as the ones in the Unix command chmod(1), you can review the manual (man chmod) pages for more information. Furthermore, the permissions can be specified as any permutation of the string rwx, and can be split over multiple switches such as:

mychmod -u r -u wx -g rx -G w -O wrx file1 file2 ...

If the command cannot be properly parsed, or if there is no file specified, you should print a help message to show usage of the command and exit with a status value of 1. Send the help message to “stderr”. More information in the manual (man stderr) Before you access a file, make sure that the file exists and that you have permissions to access and change the file permissions. Remember, you cannot change permissions if you are not the user owner of the file. In such a case, you should continue with other specified files but return an exit status of 2 at the end of command. Check out the man page for “stat” or “fstat” to get information about the specified file.

A successful execution of command is indicated by a return status of 0.

Here is what I have so far.... I can't remove the file permission, and If I did, I removed all permission instead the one I speficied...

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

void DisplayUsage(char *);

/*************************
* main() *
*************************/
int main(int argc, char **argv) {

int c, i;
int fileCount = 0;
//to verify the ownership of a file
// check: "man getuid"

struct stat fileStat;
mode_t addPermissions = 0;
mode_t removePermissions = 0;

c = getopt (argc, argv, "u:g:o:U:G:O:");
//check: "man getopt"
if (c == 'u') {
for (i=0; i < strlen(optarg); i++) {
if (optarg[i] == 'r') { //check: "man optarg"
addPermissions |= S_IRUSR; //man fstat
} else if (optarg[i] == 'w') {
addPermissions |= S_IWUSR;
} else if (optarg[i] == 'x') {
addPermissions |= S_IXUSR;
}
}
}
if (c == 'g') {
for (i=0; i < strlen(optarg); i++) {
if (optarg[i] == 'r') { //check: "man optarg"
addPermissions |= S_IRGRP; //man fstat
} else if (optarg[i] == 'w') {
addPermissions |= S_IWGRP;
} else if (optarg[i] == 'x') {
addPermissions |= S_IXGRP;
}
}
}
if (c == 'o') {
for (i=0; i < strlen(optarg); i++) {
if (optarg[i] == 'r') { //check: "man optarg"
addPermissions |= S_IROTH; //man fstat
} else if (optarg[i] == 'w') {
addPermissions |= S_IWOTH;
} else if (optarg[i] == 'x') {
addPermissions |= S_IXOTH;
}
}
}
if (c == 'U') {
for (i = 0; i < strlen(optarg); i++) {
if (optarg[i] == 'r') { //check: "man optarg"
removePermissions &= ~(S_IRUSR);
} else if (optarg[i] == 'w') {
removePermissions &= ~(S_IWUSR);
} else if (optarg[i] == 'x') {
removePermissions &= ~(S_IXUSR);
}
}
}
if (c == 'G') {
for (i = 0; i < strlen(optarg); i++) {
if (optarg[i] == 'r') { //check: "man optarg"
removePermissions &= ~(S_IRGRP);
} else if (optarg[i] == 'w') {
removePermissions &= ~(S_IWGRP);
} else if (optarg[i] == 'x') {
removePermissions &= ~(S_IXGRP);
}
}
}
if (c == 'O') {
for (i = 0; i < strlen(optarg); i++) {
if (optarg[i] == 'r') { //check: "man optarg"
removePermissions &= ~(S_IROTH);
} else if (optarg[i] == 'w') {
removePermissions &= ~(S_IWOTH);
} else if (optarg[i] == 'x') {
removePermissions &= ~(S_IXOTH);
}
}
}

// Change permissions
// here we can use optind
stat(argv[3], &fileStat);
fileStat.st_mode &= removePermissions;
fileStat.st_mode |= addPermissions;
chmod(argv[3], fileStat.st_mode);

//check: "man optind"

return 0;
}

/************************
* DisplayUsage() *
************************/
void DisplayUsage(char *programName) {
printf("Usage:\n ./Vo_Vananh_CMPS3600_Lab11 -u rwx -g rwx -o rwx -U rwx -G rwx -O rwx file1 file2 ...\n");
}

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Homework Answers

Answer #1

Solution:-

  • I have implemented the above question as per the requirement given in the question.
  • i have attached the screenshot of the running without argument and runningwith with invalid option.

Source code-

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

void DisplayUsage(char *);

/*****************************************************************************
 *    main()                                                                 *
 *****************************************************************************/

int main(int argc, char **argv) {

  int c;
  int i;
  int index;
  int fileCount = 0;

  char isFileError = 0;
  uid_t myUid = getuid();

  struct stat fileStat;
  mode_t addPermissions = 0;
  mode_t removePermissions = 0;

  opterr = 0; // prevents the default error messages of getopt() 

  while ((c = getopt (argc, argv, "u:g:o:U:G:O:")) != -1) { // Parse cli args
    switch (c) {
 
    case 'u':
      for (i=0; i<strlen(optarg); i++) {
        if (optarg[i] == 'r') 
          addPermissions |= S_IRUSR;
        else if (optarg[i] == 'w') 
          addPermissions |= S_IWUSR;
        else if (optarg[i] == 'x') 
          addPermissions |= S_IXUSR;
      }
      break;

    case 'g':
      for (i=0; i<strlen(optarg); i++) {
        if (optarg[i] == 'r') 
          addPermissions |= S_IRGRP;
        else if (optarg[i] == 'w') 
          addPermissions |= S_IWGRP;
        else if (optarg[i] == 'x') 
          addPermissions |= S_IXGRP;
      }
      break;

    case 'o':
      for (i=0; i<strlen(optarg); i++) {
        if (optarg[i] == 'r') 
          addPermissions |= S_IROTH;
        else if (optarg[i] == 'w') 
          addPermissions |= S_IWOTH;
        else if (optarg[i] == 'x') 
          addPermissions |= S_IXOTH;
      }
      break;

    case 'U':
      for (i=0; i<strlen(optarg); i++) {
        if (optarg[i] == 'r') 
          removePermissions |= S_IRUSR;
        else if (optarg[i] == 'w') 
          removePermissions |= S_IWUSR;
        else if (optarg[i] == 'x') 
          removePermissions |= S_IXUSR;
      }
      break;

    case 'G':
      for (i=0; i<strlen(optarg); i++) {
        if (optarg[i] == 'r') 
          removePermissions |= S_IRGRP;
        else if (optarg[i] == 'w') 
          removePermissions |= S_IWGRP;
        else if (optarg[i] == 'x') 
          removePermissions |= S_IXGRP;
      }
      break;

    case 'O':
      for (i=0; i<strlen(optarg); i++) {
        if (optarg[i] == 'r') 
          removePermissions |= S_IROTH;
        else if (optarg[i] == 'w') 
          removePermissions |= S_IWOTH;
        else if (optarg[i] == 'x') 
          removePermissions |= S_IXOTH;
      }
      break;

    case '?': // getopt detected invalid option character or missing target

      if (optopt == 'u' || optopt == 'g' || optopt == 'o' ||\
          optopt == 'U' || optopt == 'G' || optopt == 'O')
        fprintf (stderr, "\n\t***Option -%c requires an argument***\n", optopt);
      else if (isprint (optopt))
        fprintf (stderr, "\n\t***Unknown option `-%c'***\n", optopt);
      else
        fprintf (stderr, "\n\t***Unknown option character `\\x%x'***\n", optopt);

      DisplayUsage(argv[0]);

      printf("Exit status: 1\n");
      return 1; // Error!

    default:
      abort (); // Critical Error!
    }
  }

  if (argc == 1)
    DisplayUsage(argv[0]);

  else { // Change permission for each file

    for (index = optind; index < argc; index++) { 
      ++fileCount;
  
      if ( stat(argv[index], &fileStat) < 0) { // Check if file exists
        perror("Stat()");
        isFileError = 1; 
      }
  
      // Check if user owns file or if root
      else if (fileStat.st_uid != myUid && myUid != 0) { 
        fprintf(stderr, "UID ERROR: You do not own this file\n");
        isFileError = 1;
      }
      
      fileStat.st_mode |= addPermissions;
      fileStat.st_mode &= ~removePermissions;

      if ( chmod(argv[index], fileStat.st_mode) == -1) {
        perror(argv[index]);
        isFileError = 1;
      }
    }  
  }
  
  if (fileCount == 0) {
    printf("No file specified\nExit Status: 2\n");
    return 2;
  }

  if (isFileError) {
    printf("Exit status: 2\n");
    return 2;
  }
  else {
    printf("Exit status: 0\n");
    return 0;
  }
}

/*****************************************************************************
 *    DisplayUsage()                                                         *
 *****************************************************************************/

void DisplayUsage(char *programName) {
  fprintf(stderr, "\nSYNOPSIS\n    %s OPTION... PERMISSION... FILE...\n",\
          programName);
      
  fprintf(stderr, "\nDESCRIPTION\n    Uses unix system calls and functions");
  fprintf(stderr, " to change\n    the permissions on one or more files.\n");

  fprintf(stderr, "\nOPTIONS\n    \t-u\n\t    Add permissions for user.");
  fprintf(stderr, "\n\t-g\n\t    Add permissions for group.");
  fprintf(stderr, "\n\t-o\n\t    Add permissions for other.");
  fprintf(stderr, "\n\t-U\n\t    Remove permissions for user.");
  fprintf(stderr, "\n\t-G\n\t    Remove permissions for group.");
  fprintf(stderr, "\n\t-O\n\t    Remove permissions for group.\n");

  fprintf(stderr, "\nPERMISSIONS\n    \tr\n\t    READ");
  fprintf(stderr, "\n\tw\n\t    WRITE");
  fprintf(stderr, "\n\tx\n\t    EXECUTE\n");
}

----------------------------------------------------------------------------------------------------------------------------

Makefile:-

CC=gcc
CFLAGS=-c -Wall -g

SOURCES=$(wildcard src/*.c)
OBJECTS=$(notdir $(SOURCES:.c=.o))

EXECUTABLE=mychmod

all: $(EXECUTABLE)

$(EXECUTABLE): $(OBJECTS)  
        $(CC) $(OBJECTS) -o $(EXECUTABLE) 

%.o: src/%.c
        $(CC) $(CFLAGS) $< -o $@

.PHONY: clean

clean:
        rm $(OBJECTS) $(EXECUTABLE)

--------------------------------------------------------------------------------------------------

Output:-

initial compilation and running without arguments

sentient@Wilderness:-/dev/C/Project-2$ pwd home/sentient/dev/C/Project-2 sentient@Wilderness: /dev/C/Project-2$ ls Makefile M

Running with invalid option

sentient@Wilderness:~/dev/C/Project-2$ ./mychmod f ***Unknown option f ** SYNOPSIS ./nychnod OPTION.. PERMISSION... FILE... D

-------------------------------------------------------------------------------------------------------------

if you are satisfy with my answer then please,please like my answer.

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