-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Thanks for your great explanation. I suspected that what was happening was happening because the script ended up comparing nothing to 1. When I saw the other examples in this thread, I realized that it might be because of what you describe. When I changed my script to run the command, and then check $? in the if statement on the next line, things worked as i expected. Yes, as I said, the fragment I provided was to illustrate the problem. A script that expects /bin/true to return 1 wouldn't be very useful after all. Thanks again to everyone for your help. Greg On Thu, Mar 29, 2012 at 09:00:04PM -0600, D. Curtis Willoughby wrote: > Hi Greg > > Others have told you other ways to skin the cat. I will try to explain > why what you did did what it did, and suggest a way to make what > you did work. > > I have to say, your script is a little strange, but I assume you did it to > illustrate something which you are doing in another script. > > The true command produces no output on standard out, but only > returns an exit status of zero. > > Since its exit status is zero, the "echo $?" after the || is > not evaluated. (not executed) > > Thus there is no output from the command(s) between the grave accent (`) > characters. > > When the shell evaluates the null string standing alone, it is > treated as nothing at all, and thus the first non-blank character > after the open bracket ([) is the equal sign (=). Your error message > is a complaint from the shell that an equal sign is not allowed > as the first character inside an open bracket. > > When I run your script fragment, i get: > > unary operator expected > > which means the same thing. > > If you want the shell to see that null string as a separate string, > you must enclose it in quotation marks like this: > > if [ "`/bin/true || echo $?`" = 1 ] ; then > echo "returned 1" > fi > > which does in fact produce no output. > > Of course, if you want the script to tell the truth, you should > do this: > > if [ "`/bin/true || echo $?`" = 1 ] ; then > echo "returned 0" i.e. true > else > echo "returned 1" i.e. false > fi > > The zero from the true command is in fact not equal to 1. > > I hope this is more informative than confusing. > > D. Curtis Willoughby > - -- web site: http://www.romuald.net.eu.org gpg public key: http://www.romuald.net.eu.org/pubkey.asc skype: gregn1 (authorization required, add me to your contacts list first) - -- Free domains: http://www.eu.org/ or mail dns-manager at EU.org -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) iEYEARECAAYFAk91WEUACgkQ7s9z/XlyUyDxsQCfQlCMBa05sza60p0a5wYCzkPr mLEAmQHrQSJ0ZCE2mizKI+1f4yJfUeXv =xxLF -----END PGP SIGNATURE-----
