Implementing my own shell
I did not post the whole code to save your time,
briefly i used exec() to execute the command lines
my problem is clarified in a comment line below
thanks for any help
int main()
{
int pid[2];
char inbuf[10];
printf("\n\nSimple Shell using C\n");
char hostname[1024];
hostname[1023] = '\0';
gethostname(hostname, 1023);
char resolved_path[100];
realpath("./", resolved_path);
printf("Maram #%s :<%s>$ ", hostname,resolved_path); //MY PROBLEM: For cd command ex: cd Desktop/Folder.. does not go in this directory and the resolved path does not change
while(1){
printf("[my shell] :");
gets(inbuf);
if(fork()){
wait();
}else{
pip(inbuf, 0, 1);
}
}
return 0;
}
/// ALSO, how can I print a command not found if an entered command line is not correct?
Nowhere in this program do you call chdir() -- which is the only way to change your current process's directory (barring functional equivalents such as fchdir(); which still retain the fundamental restriction that they impact only the current process and not its parents).
Using an exec-family call to launch an external program which changes its own directory has no effect -- that program's directory is separate from your shell's. The shell must change its own directory without forking if you want to have any effect.
Related
I'm using D as a scripting language for Windows 7 console stuff to automate boring tasks. One of my scripts (open.exe) is supposed to allow me to open stuff from the command line without me having to specify which program I use (I have a configuration file with this stuff). Now, I use executeShell to do this, and call something like start [name of program I want to use] [name of input file]. If I do this directly from the shell, it returns immediately, but if I do it using my D script, it doesn't return until the program that it opens is closed. What should I do to allow it to return immediately?
For reference purposes, this is the business logic of my script (the main method just does some argument parsing for piping purposes):
immutable path = "some//path//going//to//config//file.conf";
void process(string input) {
string extension = split(input,".")[1]; //get file extension from input
auto config = File(path,"r"); auto found = false;
while (!config.eof()){
auto line = chomp(config.readln());
if (line[0]!='#') { //skip comment lines
auto divided = split(line, ":");
if (divided[0] == extension) {
found = true;
auto command = "start " ~ divided[1] ~ " " ~ input;
auto result = executeShell(command);
//test for error code and output if necessary
writeln(result.output);
}
}
}
if (!found)
writeln("ERROR: Don't know how to open " ~ input);
}
From the top of the std.process documentation:
Execute and wait for completion, collect output - executeShell
The Windows start program spawns a process and exits immediately. D's executeShell does something else. If you'd like to spawn another program, use the appropriate functions: spawnProcess or spawnShell.
I'm reading some code that uses fopen to open files for writing. The code needs to be able to close and rename these files from time to time (it's a rotating file logger). The author says that for this to happen the child processes must not inherit these FILE handles. (On Windows, that is; on Unix it's OK.) So the author writes a special subroutine that duplicates the handle as non-inheritable and closes the original handle:
if (!(log->file = fopen(log->path, mode)))
return ERROR;
#ifdef _WIN32
sf = _fileno(log->file);
sh = (HANDLE)_get_osfhandle(sf);
if (!DuplicateHandle(GetCurrentProcess(), sh, GetCurrentProcess(),
&th, 0, FALSE, DUPLICATE_SAME_ACCESS)) {
fclose(log->file);
return ERROR;
}
fclose(log->file);
flags = (*mode == 'a') ? _O_APPEND : 0;
tf = _open_osfhandle((intptr_t)th, _O_TEXT | flags);
if (!(log->file = _fdopen(tf, "at"))) {
_close(tf);
return ERROR;
}
#endif
Now, I'm also reading MSDN docs on fopen and see that their version of fopen has a Microsoft-specific flag that seems to do the same: the N flag:
N: Specifies that the file is not inherited by child processes.
Question: do I understand it correctly that I can get rid of that piece above and replace it (on Windows) with an additional N in the mode parameter?
Yes, you can.
fopen("myfile", "rbN") creates a non-inheritable file handle.
The N flag is not mentioned anywhere in Linux documentation for fopen, so the solution will be most probably not portable, but for MS VC it works fine.
So I've got a problem with my code and it's driving me nuts since I can't seem to figure out what's causing this. Basically, I'm trying to write to a file that I have open, using multiple child processes with fork(). Before I start forking, I can write to it just fine, but once I fork and THEN do an if statement to see if it's the child process, it won't write.
Basically, what I have is this:
FILE *output = NULL;
output = fopen(...); // Done successfully
fprintf(output, "This writes okay\n");
// Fork n processes
for (i = 0; i <= n; n++)
{
pid[i] = fork();
fprintf(output, "We can still write\n");
if (pid[i] == 0) // Child process
{
fprintf(output, "This won't write to output\n");
printf("I can still write and calculate stuff otherwise\n");
...
}
}
Can anyone figure out why it's not able to write after checking to see if it's a child process? There's definitely no error with forking from what I've seen.
Opening a file descriptor and using it in more than one process (for example in both a parent and child process) is a race condition and will most likely cause problems and unpredictable behavior.
I'm writing a fake shell, where I create a child process and then call execvp(). In the normal shell, when I enter an unknown command such as 'hello' it returns 'hello: Command not found.' However, when I pass hello into execvp(), it doesn't return any error by default and just continues running the rest of my program like nothing happened. What's the easiest way to find out if nothing was actually run? here's my code:
if(fork() == 0)
{
execvp(cmd, args);
}
else
{
int status = 0;
int corpse = wait(&status);
printf(Child %d exited with a status of %d\n", corpse, status);
}
I know that if corpse < 0, then it's an unknown command, but there are other conditions in my code not listed where I don't want to wait (such as if & is entered at the end of a command). Any suggestions?
All of the exec methods can return -1 if there was an error (errno is set appropriately). You aren't checking the result of execvp so if it fails, the rest of your program will continue executing. You could have something like this to prevent the rest of your program from executing:
if (execvp(cmd, args) == -1)
exit(EXIT_FAILURE);
You also want to check the result of fork() for <0.
You have two independent concerns.
1) is the return value of execvp. It shouldn't return. If it does there is a problem. Here's what I get execvp'ing a bad command. You don't want to wait if execvp fails. Always check the return values.
int res = execvp(argv[1], argv);
printf ("res is %i %s\n", res, strerror(errno));
// => res is -1 No such file or directory
2) The other concern is background processes and such. That's the job of a shell and you're going to need to figure out when your program should wait immediately and when you want to save the pid from fork and wait on it later.
I'm writing a tool that calls through to DTrace to trace the program that the user specifies.
If my tool uses dtrace -c to run the program as a subprocess of DTrace, not only can I not pass any arguments to the program, but the program runs with all the privileges of DTrace—that is, as root (I'm on Mac OS X). This makes certain things that should work break, and obviously makes a great many things that shouldn't work possible.
The other solution I know of is to start the program myself, pause it by sending it SIGSTOP, pass its PID to dtrace -p, then continue it by sending it SIGCONT. The problem is that either the program runs for a few seconds without being traced while DTrace gathers the symbol information or, if I sleep for a few seconds before continuing the process, DTrace complains that objc<pid>:<class>:<method>:entry matches no probes.
Is there a way that I can run the program under the user's account, not as root, but still have DTrace able to trace it from the beginning?
Something like sudo dtruss -f sudo -u <original username> <command> has worked for me, but I felt bad about it afterwards.
I filed a Radar bug about it and had it closed as a duplicate of #5108629.
Well, this is a bit old, but why not :-)..
I don't think there is a way to do this simply from command line, but as suggested, a simple launcher application, such as the following, would do it. The manual attaching could of course also be replaced with a few calls to libdtrace.
int main(int argc, char *argv[]) {
pid_t pid = fork();
if(pid == 0) {
setuid(123);
seteuid(123);
ptrace(PT_TRACE_ME, 0, NULL, 0);
execl("/bin/ls", "/bin/ls", NULL);
} else if(pid > 0) {
int status;
wait(&status);
printf("Process %d started. Attach now, and click enter.\n", pid);
getchar();
ptrace(PT_CONTINUE, pid, (caddr_t) 1, 0);
}
return 0;
}
This script takes the name of the executable (for an app this is the info.plist's CFBundleExecutable) you want to monitor to DTrace as a parameter (you can then launch the target app after this script is running):
string gTarget; /* the name of the target executable */
dtrace:::BEGIN
{
gTarget = $$1; /* get the target execname from 1st DTrace parameter */
/*
* Note: DTrace's execname is limited to 15 characters so if $$1 has more
* than 15 characters the simple string comparison "($$1 == execname)"
* will fail. We work around this by copying the parameter passed in $$1
* to gTarget and truncating that to 15 characters.
*/
gTarget[15] = 0; /* truncate to 15 bytes */
gTargetPID = -1; /* invalidate target pid */
}
/*
* capture target launch (success)
*/
proc:::exec-success
/
gTarget == execname
/
{
gTargetPID = pid;
}
/*
* detect when our target exits
*/
syscall::*exit:entry
/
pid == gTargetPID
/
{
gTargetPID = -1; /* invalidate target pid */
}
/*
* capture open arguments
*/
syscall::open*:entry
/
((pid == gTargetPID) || progenyof(gTargetPID))
/
{
self->arg0 = arg0;
self->arg1 = arg1;
}
/*
* track opens
*/
syscall::open*:return
/
((pid == gTargetPID) || progenyof(gTargetPID))
/
{
this->op_kind = ((self->arg1 & O_ACCMODE) == O_RDONLY) ? "READ" : "WRITE";
this->path0 = self->arg0 ? copyinstr(self->arg0) : "<nil>";
printf("open for %s: <%s> #%d",
this->op_kind,
this->path0,
arg0);
}
If the other answer doesn't work for you, can you run the program in gdb, break in main (or even earlier), get the pid, and start the script? I've tried that in the past and it seemed to work.
Create a launcher program that will wait for a signal of some sort (not necessarily a literal signal, just an indication that it's ready), then exec() your target. Now dtrace -p the launcher program, and once dtrace is up, let the launcher go.
dtruss has the -n option where you can specify name of process you want to trace, without starting it (Credit to latter part of #kenorb's answer at https://stackoverflow.com/a/11706251/970301). So something like the following should do it:
sudo dtruss -n "$program"
$program
There exists a tool darwin-debug that ships in Apple's CLT LLDB.framework which will spawn your program and pause it before it does anything. You then read the pid out of the unix socket you pass as an argument, and after attaching the debugger/dtrace you continue the process.
darwin-debug will exec itself into a child process <PROGRAM> that is
halted for debugging. It does this by using posix_spawn() along with
darwin specific posix_spawn flags that allows exec only (no fork), and
stop at the program entry point. Any program arguments <PROGRAM-ARG> are
passed on to the exec as the arguments for the new process. The current
environment will be passed to the new process unless the "--no-env"
option is used. A unix socket must be supplied using the
--unix-socket=<SOCKET> option so the calling program can handshake with
this process and get its process id.
See my answer on related question "How can get dtrace to run the traced command with non-root priviledges?" [sic].
Essentially, you can start a (non-root) background process which waits 1sec for DTrace to start up (sorry for race condition), and snoops the PID of that process.
sudo true && \
(sleep 1; cat /etc/hosts) &; \
sudo dtrace -n 'syscall:::entry /pid == $1/ {#[probefunc] = count();}' $! \
&& kill $!
Full explanation in linked answer.