A simple shell with functions: interpreting and executing commands, supporting input and output redirection, supporting pipelines, and running programs in the background
Supported command formats:
Single command: such as ls
Commands with 1 or more parameters: such as ls -l /tmp
Commands with an output redirection or input redirection or pipe: e.g. ls -l > a, wc -c < a, ls -l / | wc -c
The background operator & can be added at the end of each of the above commands, such as ls &
Type exit or logout to exit myshell
1 #include <stdio.h>
2 #include <stdlib.h> // exit
3 #include <string.h>
4 #include <fcntl.h> // O_RDWR
5 #include <dirent.h> // DIR, struct dirent
6
7 #define BUFFSIZE 64
8
9 #define normal 0 // normal command
10 #define out_redirect 1 // output redirection
11 #define in_redirect 2 // input redirection
12 #define have_pipe 3 // There is a pipe in the command
13
14 void print_prompt(); // print prompt
15 void get_input(char *); // read the command entered by the user
16 // A two-dimensional array is used as a function parameter, the length of the first dimension can not be specified, but the length of the second dimension must be specified
17 // You can also use a pointer to a one-dimensional array with 8 elements, char (*a)[8]
18 void explain_input(char *, int *, char (*a)[8]); // Parse the input command
19 void do_cmd(int, char a[][8]); // execute command
20 int find_command(char *); // find the executable program in the command
21
22 int main(int argc, char *argv[])
23 {
24 char buf[BUFFSIZE];
25 char fraudulent[10][8];
26 int argcount;
27
28 while(1)
29 {
30 memset(buf, 0, BUFFSIZE);
31 print_prompt(); // print prompt
32 get_input(buf); // read user input
33 if(strcmp(buf, "exit")==0 || strcmp(buf, "logout")==0)
34 break;
35 int i;
36 for(i=0; i<10; i++) // initialize the array holding the command and its parameters
37 arglist [i] [0] = '\ 0';
38 argcount = 0; // count, the number of words in the command
39 explain_input(buf, &argcount, arglist); // parse command
40 do_cmd(argcount, arglist); // execute command
41 }
42 return 0;
43 }
44
45 void print_prompt()
46 {
47 printf("myshell$$ ");
48 // fflush(stdout); // fflush flushes stdout
49 }
50
51 void get_input(char *buf)
52 {
53 fgets(buf, BUFFSIZE, stdin); // fgets
54 int len = strlen(buf); // newline character \n is counted, terminator character is not counted \0
55 if(len >= BUFFSIZE)
56 {
57 printf("ERROR: command is too long !\n");
58 exit(-1);
59 }
60 buf[len-1] = '\0'; // remove newline characters read
61 }
62
63 // Parse the command in buf, the result is stored in arglist, the number of commands and their parameters is argcount
64 // For example, "ls -l" command, then arglist[0], arglist[1] are ls, -l respectively
65 void explain_input(char *buf, int *argcount, char arglist[][8])
66 {
67 char *p = buf;
68
69 // Split the entire string entered by the user into words
70 // Store in each row of the 2D array
71 while(*p != '\0')
72 {
73 if(*p == ' ')
74 p++;
75 else
76 {
77 char *q = p;
78 int len = 0; // word length
79 while((*q!=' ') && (*q!='\0'))
80 {
81 q ++; len ++;
82 }
83 // Store the currently disassembled word in a row in a two-dimensional array
84 strncpy (arglist [* argcount], p, len + 1);
85 arglist [* argcount] [len] = '\ 0';
86 (*argcount)++;
87 p = q;
88 }
89 }
90 }
91
92 //
93 void do_cmd(int argcount, char arglist[10][8])
94 {
95 // Array of pointers, each element points to a row in a two-dimensional array
96 // arg stores all commands and their parameters, argnext stores the command after the pipe character
97 char *arg[argcount+1], *argnext[argcount+1];
98 int i, flag = 0, how = 0, background = 0;
99 char *file;
100 pid_t pid;
101
102 // Extract command
103 for(i=0; i<argcount; i++)
104 arg [i] = arglist [i];
105 arg[argcount] = NULL;
106
107 // Check if the command line has a background runner
108 for(i=0; i<argcount; i++)
109 {
110 if(strncmp(arg[i], "&", 1) == 0) // strncmp
111 { // The background runner must be at the end of the command, otherwise the command format is wrong
112 if(i == argcount-1)
113 {
114 background = 1;
115 arg[argcount-1] = NULL;
116 break;
117 }
118 else
119 {
120 printf("ERROR: wrong command about backgrount\n");
121 return;
122 }
123 }
124 }
125
126 for(i=0; arg[i]!=NULL; i++)
127 {
128 if(strcmp(arg[i], ">") == 0)
129 {
130 flag++;
131 how = out_redirect;
132 if(arg[i+1] == NULL) // output redirector at the end
133 flag++; // Make flag greater than 1 to inform the command format error
134 }
135 if(strcmp(arg[i], "<") == 0)
136 {
137 flag++;
138 how = in_redirect;
139 if(i == 0) // input redirector is at the top
140 flag++;
141 }
142 if(strcmp(arg[i], "|") == 0)
143 {
144 flag++;
145 how = have_pipe;
146 if(arg[i+1] == NULL) // pipe last
147 flag++;
148 if(i == 0) // pipe first
149 flag++;
150 }
151 }
152 // flag is greater than 1, indicating that it contains two or more of >,<,| at the same time, this program does not support
153 // or command format error
154 if(flag > 1)
155 {
156 printf("ERROR: wrong command about >,<,|\n");
157 return;
158 }
159
160 if(how == out_redirect) // command contains only one output redirector
161 {
162 for(i=0; arg[i]!=NULL; i++)
163 if(strcmp(arg[i], ">") == 0)
164 {
165 file = arg[i+1]; // Get the file name of the output redirection
166 arg[i] = NULL;
167 }
168 }
169
170 if(how == in_redirect) // command contains only one input redirector
171 {
172 for(i=0; arg[i] != NULL; i++)
173 if(strcmp(arg[i], "<") == 0)
174 {
175 file = arg[i+1];
176 arg[i] = NULL;
177 }
178 }
179
180 if(how == have_pipe) // command contains only one pipe symbol
181 {
182 for(i=0; arg[i]!=NULL; i++)
183 if(strcmp(arg[i], "|") == 0)
184 {
185 arg[i] = NULL;
186 i++;
187 int j = 0;
188 // Store the command following the pipe character in argnext
189 while(arg[i] != NULL)
190 {
191 argnext[j++] = arg[i++];
192 }
193 argnext[j] = NULL;
194 break;
195 }
196 }
197
198 pid = fork(); // create child process
199 if(pid < 0)
200 {
201 perror("fork failure");
202 return;
203 }
204
205 switch(how)
206 {
207 case 0: // general command
208 if(pid==0) // The child process executes the command entered by the user
209 {
210 if(!find_command(arg[0])) // Determine whether the command is executable
211 {
212 printf("%s: command not found\n", arg[0]);
213 exit(0);
214 }
215 execvp(arg[0], arg); // execvp starts executing the command
216 exit(0);
217 }
218 break;
219 case 1: // command contains output redirector
220 if(pid == 0)
221 {
222 if(!find_command(arg[0]))
223 {
224 printf("%s: command not found\n", arg[0]);
225 exit(0);
226 }
227 // Open or create a new output redirected file
228 int fd = open(file, O_RDWR | O_CREAT | O_TRUNC, 0644);
229 // Copy standard output to open file descriptor, i.e. replace standard output with file descriptor
230 dup2(fd, 1); // dup2(int oldfd, int newfd)
231 execvp (arg [0], arg); // execvp
232 exit(0);
233 }
234 break;
235 case 2: // command contains input redirector
236 if(pid == 0)
237 {
238 if(!find_command(arg[0]))
239 {
240 printf("%s: command not found\n", arg[0]);
241 exit(0);
242 }
243 int fd = open(file, O_RDONLY);
244 dup2(fd, 0);
245 execvp (arg [0], arg);
246 exit(0);
247 }
248 break;
249 case 3: // command contains pipe character
250 if(pid == 0) // child process
251 {
252 pid_t pid2;
253 int fd2;
254 if((pid2=fork()) < 0) // A new child process is being created in the current child process
255 {
256 perror("fork2 failure");
257 return;
258 }
259 if(pid2 == 0) // The newly created child process executes the command before the pipe character
260 {
261 if(!find_command(arg[0]))
262 {
263 printf("%s: command not found\n", arg[0]);
264 exit(0);
265 }
266 // Store the execution result of the command before the pipe character in fd2
267 fd2 = open("/tmp/youdontknowfile",
268 O_WRONLY | O_CREAT | O_TRUNC, 0644);
269 dup2(fd2, 1); // redirect stdout
270 execvp (arg [0], arg);
271 exit(0);
272 }
273 waitpid(pid2, NULL, 0); // wait for the command before the pipe character to return
274 if(!find_command(argnext[0]))
275 {
276 printf("%s: command not found\n", argnext[0]);
277 exit(0);
278 }
279 fd2 = open("/tmp/youdontknowfile", O_RDONLY);
280 dup2(fd2, 0); // define fd2 as standard input
281 execvp(argnext[0], argnext); // execute the command after the pipe character
282 exit(0);
283 }
284 //remove("/tmp/youdontknowfile");
285 //unlink("/tmp/youdontknowfile");
286 break;
287 default:
288 break;
289 }
290 // If there is a background running character in the command, the parent process returns directly without waiting for the child process to return
291 if(background == 1)
292 {
293 printf("[process id %d]\n", pid);
294 return;
295 }
296 waitpid(pid, NULL, 0); // waitpid parent process waits for child process to return
297 }
298
299 // Determine whether the command is executable and whether there is a corresponding executable file
300 int find_command(char *command)
301 {
302 DIR *dir;
303 struct dirent * ptr;
304 char *path[] = {"./", "/bin", "/usr/bin", NULL};
305 // When the command "./build" is entered, match the build command with the build file in the directory
306 if(strncmp(command, "./", 2) == 0)
307 command = command + 2;
308 int i = 0;
309 while(path[i] != NULL)
310 {
311 if((dir=opendir(path[i])) == NULL) // open directory
312 printf("cannot open /bin\n");
313 while((ptr=readdir(dir)) != NULL) // read list of files in directory
314 if(strcmp(ptr->d_name, command) == 0)
315 {
316 closedir(dir);
317 return 1;
318 }
319 closedir(dir);
320 i++;
321 }
322 return 0;
323 }