CODEWITHSUNDEEP
linuxshellunixoperating-systemsystem-calls
Umair Khalid
Umair Khalid

Reputation: 579

Does Every Shell command in linux calls a system call at the back end

I have been asked to find a shell command that doesn't make any system call. I have searched a lot and finally ended up here asking that is there any command in linux shell or unix that doesn't call a system call at the back end.

Thanks.

Upvotes: 0

Views: 890

Answers (2)

DigitalRoss
DigitalRoss

Reputation: 146073

Yes and No

The Yes part...

There are a number of shell built-ins that handle control flow or definitions for the shell's macro processor programming language.

Only a few of the built-ins intrinsically need to make any system calls; in general they just change state inside the shell's memory image.

The No part...

You actually can't even just type Enter at the shell without having a number of system calls run. The shell might wait(2) to see if any children have terminated, it may check the time, it usually prints a prompt ... and of course it reads the next command.

Upvotes: 3

Dima Chubarov
Dima Chubarov

Reputation: 17169

Think of what a shell is, how does it run the commands.

The shell is an interpreter. Roughly speaking there is a single interpreter loop

do {
   get_next_command();
   switch(next_command){
   cmd_alias: ...
   cmd_break: ...
     ...
   }
} while (true);

Now think, which command just updates the internal structures of the shell process and which command needs to call the operating system to perform its function.

Upvotes: 1

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