Conditionals in BASH

[Luciano Rocha]

On Wed, Apr 09, 2008 at 06:29:53PM +0000, Matthew Saltzman wrote:
> The conditionals && and || in bash are short-circuit, so that once a
> term is evaluated that determines the success or failure of the entire
> expression, evaluation of the remaining alternatives is skipped.
> 
> In a test expression (enclosed in [ ... ]), are the conditionals -a and
> -o also short-circuit?  The O'Reilly BASH book seems to suggest that
> they are not, but the description seems to me to be ambiguous.

No, they're not. However, [[ ... && ... || ... ]] are.

cf help [; help [[ and help test, and a test:

$ strace -f -e file sh -c '[ -e /tmp/a -a -e /tmp/b ]' 2>&1 | tail -2
stat64("/tmp/a", 0xbfaed638)            = -1 ENOENT (No such file or directory)
stat64("/tmp/b", 0xbfaed628)            = -1 ENOENT (No such file or directory)

$ touch /tmp/a
$ strace -f -e file sh -c '[ -e /tmp/a -o -e /tmp/b ]' 2>&1 | tail -2
stat64("/tmp/a", {st_mode=S_IFREG|0664, st_size=0, ...}) = 0
stat64("/tmp/b", 0xbfc53788)            = -1 ENOENT (No such file or directory)

I usually do [ .. ] && [ .. ] instead.

$ rm -f /tmp/a
$ strace -f -e file sh -c '[ -e /tmp/a ] && [ -e /tmp/b ]' 2>&1 | tail -2
open("/usr/lib/locale/LC/LC_TIME", O_RDONLY) = -1 ENOENT (No such file or directory)
stat64("/tmp/a", 0xbf9a12e8)            = -1 ENOENT (No such file or directory)

-- 
lfr
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