我正在用C创建一个shell,我需要帮助实现输入和输出重定向。
当尝试使用“>”创建文件时,会得到一个错误消息,称文件不存在。当我尝试执行类似ls>test.txt的操作时,它不会创建新文件。
我用提供给我的建议更新了代码,但是现在出现了不同的错误。但是,仍然没有为输出重定向创建新文件。
这是我的全部代码:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#define MAX_BUF 160
#define MAX_TOKS 100
int main(int argc, char **argv)
{
char *pos;
char *tok;
char *path;
char s[MAX_BUF];
char *toks[MAX_TOKS];
time_t rawtime;
struct tm *timeinfo;
static const char prompt[] = "msh> ";
FILE *infile;
int in;
int out;
int fd0;
int fd1;
in = 0;
out = 0;
/*
* process command line options
*/
if (argc > 2) {
fprintf(stderr, "msh: usage: msh [file]\n");
exit(EXIT_FAILURE);
}
if (argc == 2) {
/* read from script supplied on the command line */
infile = fopen(argv[1], "r");
if (infile == NULL)
{
fprintf(stderr, "msh: cannot open script '%s'.\n", argv[1]);
exit(EXIT_FAILURE);
}
} else {
infile = stdin;
}
while (1)
{
// prompt for input, if interactive input
if (infile == stdin) {
printf(prompt);
}
/*
* read a line of input and break it into tokens
*/
// read input
char *status = fgets(s, MAX_BUF-1, infile);
// exit if ^d or "exit" entered
if (status == NULL || strcmp(s, "exit\n") == 0) {
if (status == NULL && infile == stdin) {
printf("\n");
}
exit(EXIT_SUCCESS);
}
// remove any trailing newline
if ((pos = strchr(s, '\n')) != NULL) {
*pos = '\0';
}
// break input line into tokens
char *rest = s;
int i = 0;
while((tok = strtok_r(rest, " ", &rest)) != NULL && i < MAX_TOKS)
{
toks[i] = tok;
if(strcmp(tok, "<") == 0)
{
in = i + 1;
i--;
}
else if(strcmp(tok, ">")==0)
{
out = i + 1;
i--;
}
i++;
}
if (i == MAX_TOKS) {
fprintf(stderr, "msh: too many tokens");
exit(EXIT_FAILURE);
}
toks[i] = NULL;
/*
* process a command
*/
// do nothing if no tokens found in input
if (i == 0) {
continue;
}
// if a shell built-in command, then run it
if (strcmp(toks[0], "help") == 0) {
// help
printf("enter a Linux command, or 'exit' to quit\n");
continue;
}
if (strcmp(toks[0], "today") == 0) {
// today
time(&rawtime);
timeinfo = localtime(&rawtime);
printf("Current local time: %s", asctime(timeinfo));
continue;
}
if (strcmp(toks[0], "cd") == 0)
{
// cd
if (i == 1) {
path = getenv("HOME");
} else {
path = toks[1];
}
int cd_status = chdir(path);
if (cd_status != 0)
{
switch(cd_status)
{
case ENOENT:
printf("msh: cd: '%s' does not exist\n", path);
break;
case ENOTDIR:
printf("msh: cd: '%s' not a directory\n", path);
break;
default:
printf("msh: cd: bad path\n");
}
}
continue;
}
// not a built-in, so fork a process that will run the command
int rc = fork();
if (rc < 0)
{
fprintf(stderr, "msh: fork failed\n");
exit(1);
}
if (rc == 0)
{
if(in)
{
int fd0;
if((fd0 = open(toks[in], O_RDONLY, 0)) == -1)
{
perror(toks[in]);
exit(EXIT_FAILURE);
}
dup2(fd0, 0);
close(fd0);
}
if(out)
{
int fd1;
if((fd1 = open(toks[out], O_WRONLY | O_CREAT | O_TRUNC | O_CREAT,
S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
{
perror (toks[out]);
exit( EXIT_FAILURE);
}
dup2(fd1, 1);
close(fd1);
}
// child process: run the command indicated by toks[0]
execvp(toks[0], toks);
/* if execvp returns than an error occurred */
printf("msh: %s: %s\n", toks[0], strerror(errno));
exit(1);
}
else
{
// parent process: wait for child to terminate
wait(NULL);
}
}
}
最佳答案
乍一看,除了您的close
和dup2
在您的toks[in]
案例中不正常之外,没有任何明显的东西可以解释为什么重定向时不创建输出文件(例如cat somefile > newfile
)。然而,有许多微妙之处你没有检查。
例如,在调用open
和dup2
之前,需要检查在每种情况下对close
的调用是否成功。(否则,您试图重定向未打开的文件描述符)。简单的基本检查就可以了,例如。
if (in) {
int fd0;
if ((fd0 = open(toks[in], O_RDONLY)) == -1) {
perror (toks[in]);
exit (EXIT_FAILURE);
}
dup2(fd0, 0);
close(fd0);
}
if (out)
{
int fd1;
if ((fd1 = open(toks[out],
O_WRONLY | O_CREAT | O_TRUNC | O_CREAT,
S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1) {
perror (toks[out]);
exit (EXIT_FAILURE);
}
dup2(fd1, 1);
close(fd1);
}
(注意:我已经调整了将文件编写为
0644
(user'rw'
、group'r'
和world'r'
)的权限。您还应该检查dup2
的返回值,在迂腐的情况下应该检查close
)更大的问题在于如何在调用
toks
之前重新排列execvp
。使用dup2
或管道的原因是exec..
函数无法处理重定向(例如,它不知道如何处理'>'
或'<'
)。因此,通过将文件重定向到input case上的stdin
,或将stdout
(和/或stderr
)重定向到output case上的文件,您可以手动处理输入或输出到文件的重定向。在这两种情况下,必须在调用< filename
之前从> filename
中移除toks
或execvp
标记,否则将得到错误。如果您确保将
toks
中的每个指针设置为NULL
,并且您读取的内容不超过MAXTOKS - 1
(根据需要保留终止的NULL
),那么您可以遍历toks
移动指针,以确保您不会将< >
和filename
发送到execvp
。在索引<
处找到>
中的toks
或i
并确保存在toks[i+1]
文件名之后,如下所示: while (toks[i]) {
toks[idx] = toks[i+2];
i++;
}
然后将
toks
传递到execvp
不会生成错误(我怀疑这就是您正在经历的情况)还有一个你应该注意的角落案例。如果您的可执行文件对
atexit
或其他构造函数有任何已注册的调用,则这些引用不是对execvp
调用的一部分。因此,如果对execvp
的调用失败,则不能调用exit
(它可以调用任何POST退出函数中的未定义行为),因此正确的调用是“_exit
”,它不会尝试任何此类调用。最低限度的工作重定向如下所示。下面没有讨论解析和重定向的许多其他方面,但是对于基本的文件创建问题,它提供了一个框架,例如。
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
enum {ARGSIZE = 20, BUF_SIZE = 1024}; /* constants */
void execute (char **args);
int main (void) {
while (1) {
char line[BUF_SIZE] = "",
*args[ARGSIZE],
*delim = " \n",
*token;
int argIndex = 0;
for (int i = 0; i < ARGSIZE; i++) /* set all pointers NULL */
args[i] = NULL;
printf ("shell> "); /* prompt */
if (!fgets (line, BUF_SIZE, stdin)) {
fprintf (stderr, "Input canceled - EOF received.\n");
return 0;
}
if (*line == '\n') /* Enter alone - empty line */
continue;
for (token = strtok (line, delim); /* parse tokens */
token && argIndex + 1 < ARGSIZE;
token = strtok (NULL, delim)) {
args[argIndex++] = token;
}
if (!argIndex) continue; /* validate at least 1 arg */
if (strcmp (args[0], "quit") == 0 || strcmp (args[0], "exit") == 0)
break;
execute (args); /* call function to fork / execvp */
}
return 0;
}
void execute (char **args)
{
pid_t pid, status;
pid = fork ();
if (pid < 0) {
perror ("fork");
return;
}
else if (pid > 0) {
while (wait (&status) != pid)
continue;
}
else if (pid == 0) {
int idx = 0,
fd;
while (args[idx]) { /* parse args for '<' or '>' and filename */
if (*args[idx] == '>' && args[idx+1]) {
if ((fd = open (args[idx+1],
O_WRONLY | O_CREAT,
S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1) {
perror (args[idx+1]);
exit (EXIT_FAILURE);
}
dup2 (fd, 1);
dup2 (fd, 2);
close (fd);
while (args[idx]) {
args[idx] = args[idx+2];
idx++;
}
break;
}
else if (*args[idx] == '<' && args[idx+1]) {
if ((fd = open (args[idx+1], O_RDONLY)) == -1) {
perror (args[idx+1]);
exit (EXIT_FAILURE);
}
dup2 (fd, 0);
close (fd);
while (args[idx]) {
args[idx] = args[idx+2];
idx++;
}
break;
}
idx++;
}
if (execvp (args[0], args) == -1) {
perror ("execvp");
}
_exit (EXIT_FAILURE); /* must _exit after execvp return, otherwise */
} /* any atext calls invoke undefine behavior */
}
示例使用/输出
最低限度地工作
> filename
和< filename
,$ ./bin/execvp_wredirect
shell> ls -al tmp.txt
ls: cannot access 'tmp.txt': No such file or directory
shell> cat dog.txt
my dog has fleas
shell> cat dog.txt > tmp.txt
shell> ls -al tmp.txt
-rw-r--r-- 1 david david 17 Feb 25 01:52 tmp.txt
shell> cat < tmp.txt
my dog has fleas
shell> quit
让我知道这是否解决了错误问题。唯一的另一个创建问题是您在尝试创建文件时没有写权限。如果这不能解决问题,请将您的所有代码发布到MCVE中,这样我就可以确保问题不会在代码的其他区域产生。
在发布完整代码之后
您最大的问题是使用
strtok_r
而不删除文件名(或在调用NULL
之前设置它),以及在分配给execvp
和i + 1
的任务中使用i
而不是in
,例如。tok = strtok_r(rest, delim, &rest);
while(tok != NULL && i < MAX_TOKS)
{
toks[i] = tok;
if(strcmp(tok, "<") == 0)
{
in = i;
i--;
}
else if(strcmp(tok, ">")==0)
{
out = i;
i--;
}
i++;
tok = strtok_r(NULL, delim, &rest);
}
使用
out
时,将i + 1
或tok[in]
的索引设置为超出提示tok[out]
错误的文件名的索引。是其中一个!(或“id10t”)错误。。。(重写所有大写的引号)此外,在调用
Bad Address
之前,您必须将execvp
或tok[in]
设置为tok[out]
,因为您已经删除了NULL
和<
,并且文件描述符已经被复制,例如。 dup2(fd0, 0);
close(fd0);
toks[in] = NULL;
和
dup2(fd1, 1);
close(fd1);
toks[out] = NULL;
你还忘了重置循环变量,例如。
while (1)
{
in = out = 0; /* always reset loop variables */
for (int i = 0; i < MAX_TOKS; i++)
toks[i] = NULL; /* and NULL all pointers */
清理一下你手头的东西,你可以做如下事情:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/types.h> /* missing headers */
#include <sys/wait.h>
#define MAX_BUF 160
#define MAX_TOKS 100
int main(int argc, char **argv)
{
char *delim = " \n"; /* delimiters for strtok_r (including \n) */
// char *pos; /* no longer used */
char *tok;
char *path;
char s[MAX_BUF];
char *toks[MAX_TOKS];
time_t rawtime;
struct tm *timeinfo;
static const char prompt[] = "msh> ";
FILE *infile;
int in;
int out;
// int fd0; /* unused and shadowed declarations below */
// int fd1; /* always compile with -Wshadow */
in = 0;
out = 0;
/*
* process command line options
*/
if (argc > 2) {
fprintf(stderr, "msh: usage: msh [file]\n");
exit(EXIT_FAILURE);
}
if (argc == 2) {
/* read from script supplied on the command line */
infile = fopen(argv[1], "r");
if (infile == NULL) {
fprintf(stderr, "msh: cannot open script '%s'.\n", argv[1]);
exit(EXIT_FAILURE);
}
} else {
infile = stdin;
}
while (1)
{
in = out = 0; /* always reset loop variables */
for (int i = 0; i < MAX_TOKS; i++)
toks[i] = NULL;
// prompt for input, if interactive input
if (infile == stdin) {
printf(prompt);
}
/*
* read a line of input and break it into tokens
*/
// read input
char *status = fgets(s, MAX_BUF-1, infile);
// exit if ^d or "exit" entered
if (status == NULL || strcmp(s, "exit\n") == 0) {
if (status == NULL && infile == stdin) {
printf("\n");
}
exit(EXIT_SUCCESS);
}
// break input line into tokens
char *rest = s;
int i = 0;
tok = strtok_r(rest, delim, &rest);
while(tok != NULL && i < MAX_TOKS)
{
toks[i] = tok;
if(strcmp(tok, "<") == 0)
{
in = i; /* only i, not i + 1, you follow with i-- */
i--;
}
else if(strcmp(tok, ">")==0)
{
out = i; /* only i, not i + 1, you follow with i-- */
i--;
}
i++;
tok = strtok_r(NULL, delim, &rest);
}
if (i == MAX_TOKS) {
fprintf(stderr, "msh: too many tokens");
exit(EXIT_FAILURE);
}
toks[i] = NULL;
/*
* process a command
*/
// do nothing if no tokens found in input
if (i == 0) {
continue;
}
// if a shell built-in command, then run it
if (strcmp(toks[0], "help") == 0) {
// help
printf("enter a Linux command, or 'exit' to quit\n");
continue;
}
if (strcmp(toks[0], "today") == 0) {
// today
time(&rawtime);
timeinfo = localtime(&rawtime);
printf("Current local time: %s", asctime(timeinfo));
continue;
}
if (strcmp(toks[0], "cd") == 0)
{
// cd
if (i == 1) {
path = getenv("HOME");
} else {
path = toks[1];
}
int cd_status = chdir(path);
if (cd_status != 0)
{
switch(cd_status)
{
case ENOENT:
printf("msh: cd: '%s' does not exist\n", path);
break;
case ENOTDIR:
printf("msh: cd: '%s' not a directory\n", path);
break;
default:
printf("msh: cd: bad path\n");
}
}
continue;
}
// not a built-in, so fork a process that will run the command
pid_t rc = fork(), rcstatus; /* use type pid_t, not int */
if (rc < 0)
{
fprintf(stderr, "msh: fork failed\n");
exit(1);
}
if (rc == 0)
{
if(in)
{
int fd0;
if((fd0 = open(toks[in], O_RDONLY, 0)) == -1)
{
perror(toks[in]);
exit(EXIT_FAILURE);
}
dup2(fd0, 0);
close(fd0);
toks[in] = NULL;
}
if(out)
{
int fd1;
if((fd1 = open(toks[out], O_WRONLY | O_CREAT | O_TRUNC | O_CREAT,
S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
{
perror (toks[out]);
exit( EXIT_FAILURE);
}
dup2(fd1, 1);
close(fd1);
toks[out] = NULL;
}
// child process: run the command indicated by toks[0]
execvp(toks[0], toks);
/* if execvp returns than an error occurred */
printf("msh: %s: %s\n", toks[0], strerror(errno));
exit(1);
}
else
{
// parent process: wait for child to terminate
while (wait (&rcstatus) != rc)
continue;
}
}
}
您需要确认没有其他问题,但它肯定没有
>
的问题。
关于c - 用C创建外壳。如何实现输入和输出重定向?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/48970420/