UNIX for Dummies Questions & Answers

Hi all,

I am new to unix OS.
Commands(external commands) given by the user are examined by shell and later executed by kernel.
Now I want to know how the internal(built in) commands are executed.
Please clarify whether they are executed directly by shell or by kernel.

Thanks in advance,

It depends on the builtin. For example [ is either a keyword or a builtin depending on the shell you use. This simply sets up an "if... then" test, like /usr/bin/test provides.
[ $1 -gt 3 ] is evaluated completely in the shell code in user space. [ -d /somedir ] requires a system (kernel) call - stat.

/usr/bin/echo and the echo builtin both use the write system call.

"builtin" -- means the shell implements the call instead of calling an external program. It does not have anything to do with kernel/user space. It saves process creation overhead and makes the shell script run more efficiently.

Thanks for your reply. Got to know the difference.