A command on shell which does not make system calls

piyush masrani · 07-24-2008, 04:53 AM

Hi,
I was going through some previous assignments and found out a question:

"Find out a command on a shell such that the command does not make a system call. Use strace to locate such a command."

Can please anybody help me to find such a command.
i tried many, but didnt get any. Also, for "strace cd" it gives output as command not found. Can anybody explain why?

Thanx and regards,

Piyush

colucix · 07-24-2008, 05:05 AM

strace accepts external commands as argument. cd is not a command, strictly speaking, but a shell built-in. You can verify this using:

Code:

type cd

to confirm this you can try

Code:

which cd

piyush masrani · 07-24-2008, 05:16 AM

thanx for the reply....
but what about a command without system call...it should work as
"strace command"
and should not produce any output(system calls)
how can i find such a command?

unSpawn · 07-24-2008, 06:02 AM

Quote:

Originally Posted by piyush masrani

but what about a command without system call...(...) how can i find such a command?

Since this is an assignment, maybe you should go back to the conceptual part and wonder what sys_calls are used for (in user mode)?

piyush masrani · 07-24-2008, 11:26 AM

sorry, i didnt get you...

matthewg42 · 07-24-2008, 12:06 PM

Surely every action uses system calls at some point...? Does the question mean, "find a command which does not invoke an external program"?

piyush masrani · 07-25-2008, 12:13 AM

ya...it means no external program should be invoked. To be more precise, strace should not display any system calls for the command.

matthewg42 · 07-25-2008, 02:06 AM

Well, every command requires at least one system call - reading the command from the input file handle.

Mr. C. · 07-25-2008, 02:21 AM

...and ultimately _exit().

jschiwal · 07-25-2008, 05:59 AM

The "time" function might help you with this assignment.

piyush masrani · 07-25-2008, 09:35 AM

Quote:

Originally Posted by matthewg42

Well, every command requires at least one system call - reading the command from the input file handle.

Quote:

Originally Posted by Mr. C.

...and ultimately _exit().

i agree with you..

how does time command help in here??

Mr. C. · 07-25-2008, 02:44 PM

Shell built-ins can't be strace'd because no process is exec'd.

strace false

You will always see the libc runtime initialization/shutdown, which makes several system calls.

matthewg42 · 07-26-2008, 07:54 AM

You can't strace internal commands directly because strace works by taking the name of a program and some arguments to it, and executes that program (creating a new process for it). A shell-internal is by definition not a program for which a new process is created - it is just a routine which runs in the existing shell process.

Here's one way to do it indirectly, by using strace on the bash process itself. I used cat instead of letting the shell read directly from the terminal, since a non-interactive shell will be created, and that doesn't produce a huge number of signal handling calls between commands. I also turn off echo to prevent the typed input and the strace output getting mixed up:

Code:

stty -echo
cat | strace bash > /dev/null

After the initial strace output from all the system calls used to create and initialise the new process, you can type in commands and see the system calls generated. For example, the command "echo hello world" produces this output:

Code:

read(0, "e", 1)                         = 1
read(0, "c", 1)                         = 1
read(0, "h", 1)                         = 1
read(0, "o", 1)                         = 1
read(0, " ", 1)                         = 1
read(0, "h", 1)                         = 1
read(0, "e", 1)                         = 1
read(0, "l", 1)                         = 1
read(0, "l", 1)                         = 1
read(0, "o", 1)                         = 1
read(0, " ", 1)                         = 1
read(0, "w", 1)                         = 1
read(0, "o", 1)                         = 1
read(0, "r", 1)                         = 1
read(0, "l", 1)                         = 1
read(0, "d", 1)                         = 1
read(0, "\n", 1)                        = 1
write(1, "hello world\n", 12)           = 12
rt_sigprocmask(SIG_BLOCK, NULL, [], 8)  = 0

As I said before, you will get read calls. This will always be the case, for any command, so strictly speaking the answer to the question is that all commands produce system calls. There are no exceptions. However, I expect the questioner didn't mean it quite like that.

In the case of echo, you get a write system call, and this will be the case with any builtin command which produces any output.

Commands which only change the internal state of the shell like alias or setting a variable will not do this, so that might be the answer which the questioner is looking for. However, even for these commands the shell always calls rt_sigprocmask. It's arguable if this is done as part of the command, or in a wrapper around the command.

In my opinion, the question is too vague to give a precise answer to it, but with th caveats I have mentioned here, I think you can cover all your bases.