Some commands that you type are
internal
,
built into
the shell.
For example, the
cd
command is built-in.
That is,
the shell interprets that command and changes your
current directory (
1.21
)
for you.
The
ls
command, on the other hand, is an
external
program stored in the file
/bin/ls
.
The shell doesn't start a separate process to run internal commands.
External commands require the shell to
fork
and
exec
(
1.11
)
a new
subprocess (
38.3
)
;
this takes some time, especially on a busy system.
(Article
7.4
shows an example where extra speed can be important.)
Reference : http://docstore.mik.ua/orelly/unix/upt/ch01_10.htm
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