I am trying to run execlp with find ... -exec ..., and the find program consistently tells me:
find: missing argument to `-exec'
What could be wrong? When I run find with these arguments on my shell, it succeeds.
My function calls follow (after looking at related SO threads, I have tried several arrangements of the argmuments):
execlp("find","find","/home/me","-exec","/usr/bin/stat", "{}", "\\;",NULL);
execlp("find","find","/home/me","-exec","/usr/bin/stat", "'{}'", "\\;",NULL);
execlp("find","find","/home/me","-exec","/usr/bin/stat", "{}", "';'",NULL);
execlp("find","find","/home/me","-exec","/usr/bin/stat {} \\;",NULL);
When you execute the command from C, you don't need the \ before ;
Using this syntax should work
execlp("find","find","/home/me","-exec","/usr/bin/stat", "{}", ";",NULL);
When on the shell, ; marks the end of a command, and has to be escaped. execlp doesn't go through the shell to execute a command, it does it immediately.
Edit -- you actually only do one find so this part is not necessary
Moreover, the exec family replaces the current process with the requested command. So only the first execlp will be executed.
A solution is to fork() for each find (one by one, waiting for the child process to end, otherwise the output will be a mix of all results).
Related
Can someone please help me? In Perl, what is the difference between:
exec "command";
and
system("command");
and
print `command`;
Are there other ways to run shell commands too?
exec
executes a command and never returns.
It's like a return statement in a function.
If the command is not found exec returns false.
It never returns true, because if the command is found it never returns at all.
There is also no point in returning STDOUT, STDERR or exit status of the command.
You can find documentation about it in perlfunc,
because it is a function.
system
executes a command and your Perl script is continued after the command has finished.
The return value is the exit status of the command.
You can find documentation about it in perlfunc.
backticks
like system executes a command and your perl script is continued after the command has finished.
In contrary to system the return value is STDOUT of the command.
qx// is equivalent to backticks.
You can find documentation about it in perlop, because unlike system and execit is an operator.
Other ways
What is missing from the above is a way to execute a command asynchronously.
That means your perl script and your command run simultaneously.
This can be accomplished with open.
It allows you to read STDOUT/STDERR and write to STDIN of your command.
It is platform dependent though.
There are also several modules which can ease this tasks.
There is IPC::Open2 and IPC::Open3 and IPC::Run, as well as
Win32::Process::Create if you are on windows.
In general I use system, open, IPC::Open2, or IPC::Open3 depending on what I want to do. The qx// operator, while simple, is too constraining in its functionality to be very useful outside of quick hacks. I find open to much handier.
system: run a command and wait for it to return
Use system when you want to run a command, don't care about its output, and don't want the Perl script to do anything until the command finishes.
#doesn't spawn a shell, arguments are passed as they are
system("command", "arg1", "arg2", "arg3");
or
#spawns a shell, arguments are interpreted by the shell, use only if you
#want the shell to do globbing (e.g. *.txt) for you or you want to redirect
#output
system("command arg1 arg2 arg3");
qx// or ``: run a command and capture its STDOUT
Use qx// when you want to run a command, capture what it writes to STDOUT, and don't want the Perl script to do anything until the command finishes.
#arguments are always processed by the shell
#in list context it returns the output as a list of lines
my #lines = qx/command arg1 arg2 arg3/;
#in scalar context it returns the output as one string
my $output = qx/command arg1 arg2 arg3/;
exec: replace the current process with another process.
Use exec along with fork when you want to run a command, don't care about its output, and don't want to wait for it to return. system is really just
sub my_system {
die "could not fork\n" unless defined(my $pid = fork);
return waitpid $pid, 0 if $pid; #parent waits for child
exec #_; #replace child with new process
}
You may also want to read the waitpid and perlipc manuals.
open: run a process and create a pipe to its STDIN or STDERR
Use open when you want to write data to a process's STDIN or read data from a process's STDOUT (but not both at the same time).
#read from a gzip file as if it were a normal file
open my $read_fh, "-|", "gzip", "-d", $filename
or die "could not open $filename: $!";
#write to a gzip compressed file as if were a normal file
open my $write_fh, "|-", "gzip", $filename
or die "could not open $filename: $!";
IPC::Open2: run a process and create a pipe to both STDIN and STDOUT
Use IPC::Open2 when you need to read from and write to a process's STDIN and STDOUT.
use IPC::Open2;
open2 my $out, my $in, "/usr/bin/bc"
or die "could not run bc";
print $in "5+6\n";
my $answer = <$out>;
IPC::Open3: run a process and create a pipe to STDIN, STDOUT, and STDERR
use IPC::Open3 when you need to capture all three standard file handles of the process. I would write an example, but it works mostly the same way IPC::Open2 does, but with a slightly different order to the arguments and a third file handle.
Let me quote the manuals first:
perldoc exec():
The exec function executes a system command and never returns-- use system instead of exec if you want it to return
perldoc system():
Does exactly the same thing as exec LIST , except that a fork is done first, and the parent process waits for the child process to complete.
In contrast to exec and system, backticks don't give you the return value but the collected STDOUT.
perldoc `String`:
A string which is (possibly) interpolated and then executed as a system command with /bin/sh or its equivalent. Shell wildcards, pipes, and redirections will be honored. The collected standard output of the command is returned; standard error is unaffected.
Alternatives:
In more complex scenarios, where you want to fetch STDOUT, STDERR or the return code, you can use well known standard modules like IPC::Open2 and IPC::Open3.
Example:
use IPC::Open2;
my $pid = open2(\*CHLD_OUT, \*CHLD_IN, 'some', 'cmd', 'and', 'args');
waitpid( $pid, 0 );
my $child_exit_status = $? >> 8;
Finally, IPC::Run from the CPAN is also worth looking at…
What's the difference between Perl's backticks (`), system, and exec?
exec -> exec "command"; ,
system -> system("command"); and
backticks -> print `command`;
exec
exec executes a command and never resumes the Perl script. It's to a script like a return statement is to a function.
If the command is not found, exec returns false. It never returns true, because if the command is found, it never returns at all. There is also no point in returning STDOUT, STDERR or exit status of the command. You can find documentation about it in perlfunc, because it is a function.
E.g.:
#!/usr/bin/perl
print "Need to start exec command";
my $data2 = exec('ls');
print "Now END exec command";
print "Hello $data2\n\n";
In above code, there are three print statements, but due to exec leaving the script, only the first print statement is executed. Also, the exec command output is not being assigned to any variable.
Here, only you're only getting the output of the first print statement and of executing the ls command on standard out.
system
system executes a command and your Perl script is resumed after the command has finished. The return value is the exit status of the command. You can find documentation about it in perlfunc.
E.g.:
#!/usr/bin/perl
print "Need to start system command";
my $data2 = system('ls');
print "Now END system command";
print "Hello $data2\n\n";
In above code, there are three print statements. As the script is resumed after the system command, all three print statements are executed.
Also, the result of running system is assigned to data2, but the assigned value is 0 (the exit code from ls).
Here, you're getting the output of the first print statement, then that of the ls command, followed by the outputs of the final two print statements on standard out.
backticks (`)
Like system, enclosing a command in backticks executes that command and your Perl script is resumed after the command has finished. In contrast to system, the return value is STDOUT of the command. qx// is equivalent to backticks. You can find documentation about it in perlop, because unlike system and exec, it is an operator.
E.g.:
#!/usr/bin/perl
print "Need to start backticks command";
my $data2 = `ls`;
print "Now END system command";
print "Hello $data2\n\n";
In above code, there are three print statements and all three are being executed. The output of ls is not going to standard out directly, but assigned to the variable data2 and then printed by the final print statement.
The difference between 'exec' and 'system' is that exec replaces your current program with 'command' and NEVER returns to your program. system, on the other hand, forks and runs 'command' and returns you the exit status of 'command' when it is done running. The back tick runs 'command' and then returns a string representing its standard out (whatever it would have printed to the screen)
You can also use popen to run shell commands and I think that there is a shell module - 'use shell' that gives you transparent access to typical shell commands.
Hope that clarifies it for you.
I am implementing a minishell that will emulate a real bash shell. I am stacked with the execution of commands in background such as ls &.
My first approach was the following (which does not work)
char *execArgs[] = { "ls", "&", NULL };
execvp("ls", execArgs);
Then, I tried another way by modifying the parent process of the fork() and not waiting for the child in case it should be run in background. The problem here is that then it should print the list of jobs running on background in order to simulate the background bahaviour in a bash shell, but the command jobs is not executed correctly as a parameter of execvp().
My question is, is there any easier way to implement this background calls in C? In case there isn't, what does it fail in either of the options that I have mentioned?
The reasons your example fails is
The "&" as you have it coded is an argument to the "ls" program. When you enter the command at a shell prompt, the "&" is consumed by the shell and the "ls" program never see the backgrounding.
The "exec()" call terminates the current program (i.e., your mini-shell).
What you may want is
system("ls &");
Read a good Unix book. You will need to know fork(), exec(), wait() and more.
I'm coding a small shell that must execute my commands that I parse.
f is a char** like this: [ls][-la]
p is the same, used like this: [wc]
So I tried to pipe ls -la in wc.
My probleme is that when I execute "ls -la | wc && date", which works well for the pipe, my minishell get closed and it doesn't execute "date". I used the wait function to wait for it to finish but doesn't do anything. Looks like it's stuck and exit just after the 2nd execvp.
My arrays ends well by NULL.
ls -la | wc is well executed but I get back to bash after this.
I've tried with execlp and execl but I think this is not the probleme considering that I need options of my first argument (ls + -la).
Could you help me please ?
Thanks in advance :)
All forms of exec never return; they replace the currently running image with the indicated executable. The key word here is "replace".
The only circumstance in which the statement following a call to exec* gets executed is if the exec fails (for example, if it cannot find the executable).
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Writing a shell - how to execute commands
I've been tasked with writing a shell in C. So far I understand that execvp will try to run the program in arg1 with arg2 as parameters. Now it seems that doing this
execvp ("ls", args); //assume args is {"ls", ">", "awd.txt"}
is not equivalent to typing this in the console
ls > awd.txt
I realize that I need to use freopen to achieve the same results but I'm curious what execvp is doing.
The exec family of functions are ultimately a system call. System calls go straight into the kernel and typically perform a very specific service that only the kernel can do.
Redirection, on the other hand, is a shell feature.
So, when one types ls > awd.txt at a shell, the shell first does a fork(2), then it closes standard output in the child, then it opens awd.txt on file descriptor one so that it's the new standard output.
Then, and only then, the shell will make an exec-family system call.
In your case you simply passed the strings > and awd.txt to the exec system call, and from there to ls. BTW, be sure you terminate your execvp arg array with a null pointer.
Note: As you can see, the redirection operators are never seen by the executed program. Before Unix, directing output to a file had to be done by every program based on an option. More trivia: most programs never know they were redirected, but ironically, ls does check to see if its output is a tty, and if so, it does the multi-column formatted output thing.
It's executing ls with 2 arguments: > and awd.txt. It is equivalent to running:
'ls' '>' 'awd.txt'
You can pass your command directly to the shell:
char * const args[] = { "sh", "-c", "ls > awd.txt", NULL};
execvp("/bin/sh", args);
But it doesn't seems like a good idea.
I was implementing an echo command using the system() function. The argument for the echo command comes from a command line argument. But when used ';' in the argument it is showing the directory listing.
What should i do to avoid it? Is it because of command injection in my program?
update: code added from comment
#include<string.h>
#include<stdio.h>
#include<stdlib.h>
int main(int argc, char **argv) {
char cmd[50] = "echo ";
strcat(cmd,argv[1]);
system(cmd);
}
I could compile the code but while executing if i give the command line argument as eg: './a.out hello;ls ' then directory listing is happening.
Why are you trying to use a shell access (which is exactly what System() does), and than attempt to restrict it?
If you need for some reason to use 'echo', please build your own execve() parameters, and launch /bin/echo directly.. this way you can restrict the damage only to the tasks 'echo' can do.
When attempting to run your program with the command ./a.out hello;ls, you are actually providing the shell with two separate commands that it executes in sequence. First the shell runs a.out with the command line parameter "hello" in argv[1], which prints it out using echo. Then your program exits, and the shell runs the next command, ls, and displays the directory listing.
If you want to pass that string to the program as a command line parameter, you need to escape the special shell character ;, so the shell does not parse it before giving it to your program. To escape a character, precede it with a \.
Try running the command with ./a.out hello\;ls, and then using printf instead of echo.
[can't respond to other answers yet, so reposting the question]
"Is possible to get the argument with ';', without using '\' in the command line argument. Is possible for me to include a '\' from my program after getting argv?"
No, it is not possible. The interpretation of ";" is done by the shell before getting to your program, so unless you escape at the call, your program will never be aware of the ";". i.e.
PROG1 parms ; PROG2
will cause the shell (which is interpreting what you type) to do the following:
start PROG1 and pass it parms.
once PROG1 is done, start PROG2
There are a number of special characters which the shell will take over by default and your program will never see: * for wildcards, | for pipes, & for parallel execution, etc... None of these will be seen by the program being run, they just tell the shell to do special things.
Alternatively to using the "\", you can enclose your parameter in single or double quotes (which are different, but for your example will both work). i.e.:
./a.out "hello;ls"
./a.out 'hello;ls'
Note that these will work for the printf option, if you call "system" you are in effect telling C to start a shell to run what you are passing in, so the input will once again be subject to shell interpretation.
system() is very difficult to use in a secure manner. It's much easier to just use one of the exec* functions.