Bash vs. Dash behavior with the command `echo -ne

问题:

I got different behaviors with the same command echo -ne "hello\n" with bash and with dash. See below :

$ bash -c 'echo -ne "hello\n"'
hello
$ dash -c 'echo -ne "hello\n"'
-ne hello

Why is that ? I don't understand at all…

My system :

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 12.04.5 LTS
Release:    12.04
Codename:   precise

回答1:

The POSIX specification for echo doesn't support any arguments. See it here.

And while the spec mentions -n it does so to say that it is not an option and is either an implementation defined case or to be treated as a string.

So dash is doing exactly that.

bash, on the other hand, has non-conforming behavior in a number of ways.

This is why the use of echo is generally discouraged in favor of the using printf which has a much better specification and much better portable behavior.

回答2:

While echo implementation in bash is not POSIX and Unix conformed by default, you can alter its behavior at run time or compile time.

At run time, with xpg_echo and in POSIX mode, bash echo became conformant:

$ env BASHOPTS=xpg_echo SHELLOPTS=posix bash -c 'echo -ne "hello\n"'
-ne hello

or:

$ env BASHOPTS=xpg_echo POSIXLY_CORRECT= bash -c 'echo -ne "hello\n"'
-ne hello

At compile time, you can pass --enable-xpg-echo-default and --enable-strict-posix-default options to configure script.