I am tasked with writing a simple shell along with these three commands: "nl", "head", "chroot"(with no parameters). I've written nl.c and head.c but I don't know where to start with the chroot.c. I've read what chroot does, googled some documentation and to me as a beginner this is complicated.
Any advice on this matter?
chroot without an argument just prints an error message. You can use printf for that.
Otherwise, chroot calls chroot, chdir("/") and then executes a shell with one of the exec* functions.
Related
Hi have this line of code in a C program that I must understand as a whole, I understand everything except for the line below. I have been looking into the documentation at it seems to me that it is just as if user introduced the ls command in Ubuntu. However I am not sure about it and I do not know why there is a second ls, the -l option and the NULL. Any help is very much appreciated!
execlp ("ls","ls","-l", NULL);
I always use the -x (or debug flag) when it come to bash script, or shell scripts in general.
Now i'm curious to know, is there an equivalent, either using a specific compiler options, (i use gcc, but i don't mind any other compilers) or by using a specific code in my project?
Basically i just wanted a way to emulate what bash does (using the debug flag) which show which command/function was launched first, in order, and also show the output of said function, with additional errors message etc.But for C.
I'm aware of most debug option out there, especially considering the compiler, but i really wish i could do this in my C projects too.(especially the part where it show what is executed in order, like bash does with -x)
NB: There isn't any goal in this specific question beside the question itself, as i'm just curious if this exist, and thus don't have any need for it beside the actual knowledge acquired from said answered question.
Yes, you can mimic this behaviour with a debugger.
With GDB for instance you can write "Init Files" and "Command Files" in which you can write a simple loop:
break main
run
while 1
next
end
If you put a file named .gdbinit in the directory where you start gdb, this file will be executed or gdb will lead you on the way to configure it in order that it will be executed.
The other option is to pipe this file into your gdb-call:
gdb a.out < debug_me_like-x
Where the "debug_me_like-x" file is the one mentioned above.
As a reference for the "Command Files" have a look here.
i have a C program that opens an mp3 and extract the jpg artwork in the same folder. If i execute this program with no root privileges i get a crash. If i execute it with sudo it works normally.
Now, i need another C programs who launch the previous program when it needs a jpg artwork for the selected mp3.
I tried to call popen("./firstProgram test.mp3" , "r") function or system("/(absolute path)/firstProgram test.mp3") function by calling them even with sudo in the command or not and either with relative or absolute paths. But no version seems to work.
How can i launch the first program from the second one with success?
Thanks!
fork and then use execl
char sudo[]="/usr/bin/sudo";
char pbin[]="/usr/local/bin/puppet";
NOTICE("running puppet: %s %s",sudo,pbin);
errno=0;
execl(sudo,sudo,pbin,(char *)NULL);
/* we should never get as far as this */
obviously I recommend reading man execl for further info
Unix (Linux) systems have contained a C Programming Manual in them since possibly forever. Look in Section 2, "System Calls".
This Wikipedia Page explains the Unix Manual "sections"
It is section 2 of the manual you can read about "System Calls"
Try the command: man 2 setuid
This will give you the manual for the setuid() system call which I think is what you want.
That manual page will also list references to other related system calls that may be what you want.
Remember when compiling C programs and using system calls that do low-level hardware access, to use the -O2, or -O3 option to gcc. There is a mention of it in the manual.
Ultimately the setuid() system call makes a running process started by one user change the UID of that running process to be running as some other user. (For example, you may see the Apache running as "apache", even though it was started by root).
setuid(0) lets you be root.
I created a simple bash script called "myscript.h" I gave it a .h extensions for reasons that I won't disclose here. This bash script lives in "/var/ftp/something with spaces".
From the terminal, I can type in "/var/ftp/something with spaces/myscript.h" and the script works perfectly.
However, from within my C program, I type in
system("/var/ftp/something with spaces/myscript.h")
and it complains that "/var/ftp/something" is not found. I've changed my system call to the following with forward slashes:
system("/var/ftp/something\ with\ spaces/myscript.h")
However, it still complains that "/var/ftp/something" is not found. Assuming I can't change the directory names, how can I get around this?
Thanks!
To run a single script, you might avoid the system(3) library function (and use lower level system calls like fork(2), execve(2), waitpid(2)... which are used by the implementation of system(3)), or you could quote the script name when passing it to system(3).
For more details, read Advanced Linux Programming.
On Linux, system(3) is documented to fork a /bin/sh -c process. See sh(1p). And that POSIX shell has some quoting rules. You might use double-quotes and backslashes appropriately. So you would construct (and perhaps check) the string passed to system(3) (perhaps using asprintf(3) or snprintf(3) with care). Be aware that the C compiler also has (different) quoting conventions for string literals.
In general, you should avoid code injection (imagine a naughty user giving some a; rm -rf $HOME &; input as a "directory" name; you don't want to run system(3) on the weird "/var/ftp/a; rm -rf $HOME &;/myscript.h" string)
In your particular case, I recommend using fork(2), execve(2) (perhaps thru some carefully choosen exec(3) function), waitpid(2)... appropriately. This has the slight advantage to avoid depending on and running /bin/sh so could be slightly faster (by a millisecond).
Understand more the role of an Unix shell; for example, read about the various shell expansions in bash (they are similar to those mandated by POSIX sh) and be aware of globbing. See glob(7)
Note that you're adding quotes when running from the shell. You need to do the same here. Add quotes to the path name you're sending to system:
system("\"/var/ftp/something with spaces/myscript.h\"")
This should work with gcc version 5.4.0
system("\'\'/var/ftp/something\\ with\\ spaces/myscript.h\'\'");
Just put the filename inside single quotes
system("rm '/var/ftp/something with spaces/myscript.h'")
I am trying to compile a program which, when run, will be run in a predefined environment in c.
More specifically, I am looking to somehow include LD_PRELOAD=./lib.so so that when I run ./program it will run as if I had used the command LD_PRELOAD=./lib.so ./program
Does anyone know if this is possible?
Make the program set LS_PRELOAD and then run itself if LS_PRELOAD isn't set. Use execv() for that.