UNIX for Dummies Questions & Answers

That is indeed possible in bash en ksh93, and it is faster too, but in POSIX this can only be done with cat..

Yes. It is unfortunate POSIX doesn't include this ksh syntax. I also just noticed bash implements it a slightly different way, i.e. requires quotes where ksh doesn't.

Code:

$ cat "a b"
data
$ cat s
file="a b"
echo $(<$file)
$ ksh88 s
data
$ ksh93 s
data
$ bash s
s: line 2: $file: ambiguous redirect

As expected, other POSIX and non POSIX shells have various behaviors:

Code:

$ /usr/xpg4/bin/sh s
data
$ dash s

$ sh s
s: syntax error at line 2: `(' unexpected
$ pdksh s
data
$ zsh s
data

Interesting. This appears to apply to any redirect:

Code:

cat < $file

In bash this gives an ambiguous redirect
In ksh this works.

The bash version is more consistent with how things would usually work in shell, but IMO the ksh way is more logical in the sense that a variable reference can only ever mean one thing in this case, since a redirect to/from two files will never make sense (nor be allowed) anyway.

So I guess there is yet another reason to always use double quotes around a variable reference: portability.

Code:

cat < "$file"