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Quentin
Quentin

Reputation: 1135

Bash vs. Dash behavior with the command `echo -ne "hello\n"`

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

Upvotes: 7

Views: 3813

Answers (3)

Chris Slycord
Chris Slycord

Reputation: 13

It's been stated in a previous answer that it's because dash doesn't use/understand the -n argument for echo, but this is not true.

Dash uses the -n argument to not print out a newline. But the example uses -ne. The following are all from dash version 0.5.12-2 in Debian:

$ echo -n string
string$
$ echo -n -e string
-e string$
$ echo -ne string
-ne string
$

As you can see:

In "echo -n string", it prints string with no newline.

In "echo -n -e string", it prints no newline because -n is an allowed argument that dash recognizes, and then it treats -e as a string because it's not a valid argument according to POSIX.

In "echo -ne string", it treats -ne as a string because it's not a valid argument according to POSIX (because of the "e" part).

Upvotes: 0

cuonglm
cuonglm

Reputation: 2806

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.

Upvotes: 2

Etan Reisner
Etan Reisner

Reputation: 80931

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.

Upvotes: 11

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