bug#20553: 'echo -e' does not escape backslash correctly

[Stephane Chazelas]

2015-05-11 17:36:50 -0600, Eric Blake:
> On 05/11/2015 04:14 PM, Pádraig Brady wrote:
> 
> >> echo -e "net use z: \\\\srv\\aqs /persistent:no /user:%USERNAME% 
> >> $BG_PASSWD\r"
> 
> 'echo -e' is non-portable.  POSIX recommends that you use printf
> instead, as the POSIX version of echo is supposed to behave as follows:
> 
> $ echo -e 'a\nb'
> -e a\nb
[...]

Strictly speaking without XSI (Unix conformance), the behaviour
of that command is unspecified because the arguments contain
backslash.

With XSI, the behaviour is specified but the expected output is:

-e a<newline>b<newline>

> You are relying on non-POSIX behavior for backslash interpolation.
> 
> > Note echo is not portable to other systems, and if that's required,
> 
> In fact, it's not even portable to bash:
> 
> $ shopt -s xpg_echo
> 
> tells bash to turn on POSIX rules for echo, invalidating any bash script
> that relies on 'echo -e'.

xpg_echo alone doesn't make bash's echo POSIX compliant. It just means
-e is implicit. It wouln't break scripts that use "echo -e",
just script that use "echo" without -e and expect escape
sequences not to be expanded.

To have a Unix (POSIX+XSI) conformant echo, you need both the
posix (set -o posix) and xpg_echo (shopt -s xpg_echo) options.

Those can also be enabled via the environment (BASHOPTS,
SHELLOPTS, POSIXLY_CORRECT) or at compile time. bash will also
enable "posix" when called as sh.

See
https://unix.stackexchange.com/questions/65803/why-is-printf-better-than-echo
for more info.

> 
> > printf(1) is a better option, though that will have different
> > quoting again due to the % chars etc.
> 
> % doesn't need quoting in shell.  But yes, printf is more portable.

% needs escaped (with %) in the format argument though, which is
probaby what Pádraig was refering to.

-- 
Stephane