[英]Creating a shell in C. How would I implement input and output redirection?
我正在用C創建外殼,我需要幫助來實現輸入和輸出重定向。
當我嘗試使用“>”創建文件時,我收到一條錯誤消息,指出該文件不存在。 當我嘗試做ls> test.txt之類的東西時; 它不會創建一個新文件。
我使用提供給我的建議更新了代碼,但是現在我遇到了不同的錯誤。 但是,仍然沒有為輸出重定向創建新文件。
這是我的完整代碼:
#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);
}
}
}
乍一看,除了close
和dup2
在toks[in]
出現問題外,沒有任何明顯的東西可以解釋為什么在重定向時不創建輸出文件(例如cat somefile > newfile
)。 但是,您沒有檢查很多細微之處。
例如,您需要在調用dup2
和close
之前檢查每種情況下的open
調用是否成功。 (否則,您嘗試重定向未打開的文件描述符)。 簡單的基本檢查就可以了,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:我已經調整了將文件寫為0644
的權限(用戶'rw'
,組'r'
和'r'
。您還應該檢查dup2
的返回值,並且在學究的情況下請close
)
更大的問題在於如何在調用execvp
之前重新排列toks
。 使用dup2
或管道的原因是exec..
函數無法處理重定向(例如,它不知道如何處理'>'
或'<'
)。 因此,通過將文件重定向到輸入案例上的stdin
或將stdout
(和/或stderr
)重定向到輸出案例上的文件,您正在手動處理輸入或輸出到文件的重定向。 無論哪種情況,都必須在調用execvp
之前從toks
刪除< filename
或> filename
令牌, toks
會出現錯誤。
如果確保將toks
每個指針都toks
為NULL
並且讀取的內容不超過MAXTOKS - 1
(根據需要保留終止NULL
),則可以遍歷toks
移動指針,以確保不將< >
和filename
發送給execvp
。 在索引i
toks
中找到<
或>
,並確保有toks[i+1]
文件名后,類似:
while (toks[i]) {
toks[idx] = toks[i+2];
i++;
}
然后將toks
傳遞給execvp
不會生成錯誤(我懷疑這是您遇到的問題)
您還應該注意另一個極端情況。 如果您的可執行文件具有對atexit
或其他析構函數的任何已注冊調用,則這些引用不是您對execvp
的調用的一部分。 因此,如果對execvp
的調用失敗,則無法調用exit
(它可以在對任何后退出函數的調用中調用未定義的行為),因此正確的調用是_exit
,它將不嘗試任何此類調用。
最小的工作重定向將類似於以下內容。 解析和重定向沒有很多其他方面在下面沒有解決,但是對於您的基本文件創建問題,它提供了一個框架,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 */
}
使用/輸出示例
最少使用> 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
讓我知道這是否解決了錯誤問題。 唯一的其他創建問題是您在嘗試創建文件的地方沒有寫權限。 如果這樣做不能解決問題,請在MCVE中發布所有代碼,以便確保在代碼的其他區域不會出現問題。
發布完整代碼后
最大的問題是使用strtok_r
而不刪除文件名(或在調用execvp
之前將其設置為NULL
),以及在分配in
和out
使用i + 1
而不是i
,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i + 1
,將tok[in]
或tok[out]
的索引設置為文件名后一個,提示出現Bad Address
錯誤。 這就是這些多哈之一! (或“ id10t”)錯誤...(改寫全大寫的引號)
此外,在調用execvp
之前,必須將tok[in]
或tok[out]
為NULL
因為您已刪除<
和>
並且文件描述符已被復制,例如
dup2(fd0, 0);
close(fd0);
toks[in] = NULL;
和
dup2(fd1, 1);
close(fd1);
toks[out] = NULL;
您還忘記了重置循環變量,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 */
清理一下您的工作后,可以執行以下操作:
#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;
}
}
}
您將需要驗證沒有其他問題,但是cat file1 > file2
當然沒有問題。
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.