echo: command or built-in?

colucix · 01-05-2008, 06:26 PM

Just out of curiosity, I wonder if the echo statement is a command or a shell built-in. Indeed, it is a shell built-in... but it has also its own executable. The type built-in confirms:

Code:

# type -a echo
echo is a shell builtin
echo is /bin/echo

and the Advanced Bash Scripting Guide, too

Quote:

This command is a shell builtin, and not the same as /bin/echo, although its behavior is similar.

Here are my questions: if the behavior is similar in what they differ? And when I put an echo statement (without path) in a BASH script which one is executed, the built-in or the command?

Or in other words: who wrote the source of the command /bin/echo and why?

harry edwards · 01-05-2008, 06:58 PM

I am guessing the reason is evolution as shells like the original sh did not have echo built in; hence, it was developed as a standalone tool., then, of course bash can along and rendered the command line echo redundant.

The /bin/echo utility is developed by GNU according /bin/echo --help

colucix · 01-06-2008, 05:51 AM

Thanks Harry, your guess sounds reasonable. I found a little history of the echo command through the various UNIX shells, here. It looks like the original Bourne shell written for Unix Version 7 didn't have echo as a built-in. Steve Bourne himself in the official documentation on 1978 states:

Quote:

echo is a standard UNIX command that prints its arguments, separated by blanks

Also, now I see that the shell uses its own built-in, since obviously it takes precedence over an external command. You gave me the correct suggestion to demonstrate: if I type

Code:

echo --help

it simply echoes the string "--help", whereas the external command /bin/echo prints out the usage.

Thank you again. Ciao,
Alex

druuna · 01-06-2008, 06:08 AM

Hi,

There's at least one more 'statement' that has a build-in and a binary version: pwd

They do not act the same in all situations (not sure if the same is true for echo).

Here's an example of pwd behaving different:

Quote:

$ mkdir foo
$ cd foo/
$ pwd
/home/druuna/foo

$ /bin/pwd
/home/druuna/foo

$ rm -rf ../foo/

$ pwd
/home/druuna/foo

$ /bin/pwd
/bin/pwd: couldn't find directory entry in `..' with matching i-node

Both have their use depending on what you want to do.

colucix · 01-06-2008, 06:39 AM

Hi druuna, it's correct. /bin/pwd as /bin/echo are supplied by the GNU coreutils. At least in this case the manual page states explicitly:

Quote:

NOTE: your shell may have its own version of pwd, which usually supersedes
the version described here.

Thanks for pointing this out.

xptools · 01-06-2008, 11:55 AM

Quote:

Originally Posted by colucix

Just out of curiosity, I wonder if the echo statement is a command or a shell built-in. . . . if the behavior is similar in what they differ?

One thing worthwhile to mention is that when using the shell built-in echo, you can avoid the "Argument list too long" errors

You can check the following articles as well.

HTH