On Thu, Sep 6, 2018, at 12:48, Eric Blake wrote: > On 09/06/2018 04:40 AM, Joshua Phillips wrote: > > Escape sequences don't work in single quotes: > > > > $ echo 'hello\world' > > hello\world > > $ echo 'hello\' > > Warning. Use of 'echo' and backslashes is non-portable. There are two > classical behaviors: > 1. backslashes are not special to echo unless you pass -e, so you also > have to have -n to elide a trailing newline (this is the behavior of > bash by default) > 2. backslashes ARE special by default, so you don't need -e; and \c > exists to elide a trailing newline, so you don't need -n (this is the > behavior of dash by default, and the behavior required by POSIX; bash > can also be configured to run in this mode via 'set -o posix; shopt -s > xpg_echo') > > > Which makes it surprising that double backslashes get converted to single > > backslashes: > > > > $ echo 'hello\\world' > > hello\world > > > > Is this intended behaviour? > > Yes. dash is obeying the POSIX-mandated behavior, and interpreting \ > sequences by default. Since \w is not a known sequence, dash cheats and > outputs \ as-is instead of giving you an error (although an error would > be friendlier at reminding you that \ is active-by-default in dash). > But since \\ is a known sequence, it gets interpreted by echo. > > > Bash behaves as I would have expected. > > Rather, bash in its default mode does what you are used to, but violated > POSIX. Bash in the mode that I mentioned above (set -o posix; shopt -s > xpg_echo) behaves like dash. > > -- > Eric Blake, Principal Software Engineer > Red Hat, Inc. +1-919-301-3266 > Virtualization: qemu.org | libvirt.org
Thank you very much for your explanation. I did not realise it was the "echo" built-in itself further interpreting the backslash sequences - I thought it was the shell passing just a single backslash to the echo command. Trying with /bin/echo produces the two backslashes I expected, so that makes perfect sense, thanks. I'll note well that "echo" is not a good command to use to test shell quoting!
