linux execute shell script which echo something to a file [closed] - c

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I have a shell scripts ,which will execute something like below:
file:example.sh
#!/bin/sh
#some other code
echo "someconfig">config_file
I hope the config_file just contain someconfig ,but strange thing happen with config_file,It has a single 'c' in the first line. I found no printf('c') in the parent process who execute the example.sh
My process will call the linux c function to execute the script this way:
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/wait.h>
#include <sys/types.h>
int execute_shell(const char *shell)
{
pid_t pid;
int iRet = 0;
if((pid = fork()) < 0)
{
return -1;
}
else if (pid == 0)
{
execlp("sh", "sh", "-c", shell, (char*)0);
exit(127);
}
else
{
while (waitpid(pid, &iRet, 0) < 0) {
if (errno != EINTR) {
iRet = -1;
break;
}
}
}
if(WIFEXITED(iRet) == 0)
{
return -1;
}
if(WEXITSTATUS(iRet) != 0)
{
return -1;
}
return 0;
}
int main()
{
char shell_cmd[1024]="./example.sh";
if( execute_shell(shell_cmd) == -1 )
{
// handle error
}
/*other code blew,may be will write to stdout*/
return 0;
}
Sometimes the config file looks strange,not what the shell scripts echo.
I use the cmd to analysis the possibility:
strace -f ./fork
[pid 12235] open("config_file", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
[pid 12235] fcntl(1, F_GETFD) = 0
[pid 12235] fcntl(1, F_DUPFD, 10) = 10
[pid 12235] fcntl(1, F_GETFD) = 0
[pid 12235] fcntl(10, F_SETFD, FD_CLOEXEC) = 0
[pid 12235] dup2(3, 1) = 1
[pid 12235] close(3) = 0
[pid 12235] fstat(1, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
[pid 12235] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fbabb409000
[pid 12235] write(1, "someconfig\n", 11) = 11
[pid 12235] dup2(10, 1) = 1
[pid 12235] fcntl(10, F_GETFD) = 0x1 (flags FD_CLOEXEC)
[pid 12235] close(10) = 0
[pid 12235] rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0
[pid 12235] read(255, "", 52) = 0
[pid 12235] exit_group(0) = ?
[pid 12235] +++ exited with 0 +++
<... wait4 resumed> [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 12235
I do not understand what the meaning of function = num?
I will appreciate if someone analysis what the meaning of the strace output.
I suspect the parent and child write to the stdout,leading to strange output in config.
In my project we just use the linux c code execute a shell scripts which would echo someconfig to config_file,It run 6 months normally,but one day the config looks strange(two machine,with the same error,first line with c not what it echo to).
I just want to talk if there any possibility this happen,to have a direction to fix the problem.
After analysis the strace output ,child process execute some fd operation,wich make sure child and parent echo to differnet fd.So I think there is no possibility to make the config mess.

Answer 1

the following proposed code:
cleanly compiles
performs the desired functionality
properly handles any failures seen in the sub function
properly calls the function: execlp()
Here is what I fixed:
the call (and results) of execlp()
the handling of the call to fork() when it fails
the call to waitpid() when it returns an error.
added include statements for: <sys/types.h> and <sys/wait.h> for the function: waitpid()
replaced any calls to _exit() with exit() since most beginning programmers have no idea about the result of using _exit() rather than exit()
added place to handle the returned value from the call to: execute_shell()
and now, the proposed code:
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <errno.h>
int execute_shell(const char *shell)
{
pid_t pid;
pid = fork();
if( pid < 0 )
{
perror( "fork failed" );
return -1;
}
else if (pid == 0)
{ // child process
execlp("sh", "sh", "-c", shell, (char*)0);
perror( "execlp failed" );
return -2;
}
else
{ // parent process
int iRet;
if( waitpid(pid, &iRet, 0) == -1 )
{
return -3;
}
if (errno != EINTR)
{
return -4;
}
if(WIFEXITED(iRet) == 0)
{
return -5;
}
if(WEXITSTATUS(iRet) != 0)
{
return -6;
}
}
return 0;
}
int main( void )
{
char shell_cmd[1024]="./example.sh";
if( execute_shell(shell_cmd) )
{
// handle error
}
/*other code blew,may be will write to stdout*/
return 0;
}
I created a file: example.sh that contained:
#!/bin/sh
#some other code
echo "someconfig" > config_file
then ran the command:
chmod 777 example.sh
to make it executable, then ran the program I posted.
The result is nothing displayed on the terminal, however, now there is a new file: config_file that contains:
someconfig
I ran the code I posted several times and the file config_file contents did not change
I.E. no stray character 'c'
s
What have you not told us?