Bug with 'test' built-in.
From: [EMAIL PROTECTED] To: bug-bash@gnu.org Subject: Bug with 'test' built-in. Configuration Information [Automatically generated, do not change]: Machine: i686 OS: linux-gnu Compiler: gcc Compilation CFLAGS: -DPROGRAM='bash' -DCONF_HOSTTYPE='i686' -DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='i686-pc-linux-gnu' -DCONF_VENDOR='pc' -DSHELL -DHAVE_CONFIG_H -D_GNU_SOURCE -I. -I. -I./include -I./lib -O2 -march=i386 -mcpu=i686 uname output: Linux ragtop 2.4.18-3 #1 Thu Apr 18 07:32:41 EDT 2002 i686 unknown Machine Type: i686-pc-linux-gnu Bash Version: 2.05a Patch Level: 0 Release Status: release Description: There is a problem using '!' (boolean negation) with the -a flag of the 'test' built-in command. The following construct _always_ evaluates true, regardless of whether the file exists or not. if [ ! -a file ]; then echo This line always prints no matter what. else echo This line never prints. fi Using -a w/o the negation works fine. Repeat-By: [Describe the sequence of events that causes the problem to occur.] Fix: [Description of how to fix the problem. If you don't know a fix for the problem, don't include this section.] ___ Bug-bash mailing list Bug-bash@gnu.org http://lists.gnu.org/mailman/listinfo/bug-bash
Re: Bug with 'test' built-in.
John Wenker [EMAIL PROTECTED] wrote: The following construct _always_ evaluates true, regardless of whether the file exists or not. if [ ! -a file ]; then echo This line always prints no matter what. else echo This line never prints. fi -a can act as a prefix unary operator, or an infix binary and operator. When it appears as the second of three arguments (not counting ]) in a test, it's treated as a binary operator, even if the first argument is !. So in this case, ! is test, and as a single string it's only tested for non-emptiness, and so is considered true. Likewise for file. Since both subtests are true, the combined and test is also true. There are a few different ways to get the behavior you want. You could use [ ! \( -a file \) ] or ! [ -a file ], although those are not portable to some other shells. (The behavior of a test is generally more predictable when there are fewer arguments, so I'd go with the second of those two, since it pulls the ! outside the test and leaves only two arguments for the test to examine.) You could also remove ! and switch your then and else clauses, which works with any shell. paul ___ Bug-bash mailing list Bug-bash@gnu.org http://lists.gnu.org/mailman/listinfo/bug-bash
Re: Bug with 'test' built-in.
Paul Jarc wrote: John Wenker [EMAIL PROTECTED] wrote: The following construct _always_ evaluates true, regardless of whether the file exists or not. if [ ! -a file ]; then echo This line always prints no matter what. else echo This line never prints. fi -a can act as a prefix unary operator, or an infix binary and operator. When it appears as the second of three arguments (not counting ]) in a test, it's treated as a binary operator, even if the first argument is !. So in this case, ! is test, and as a single string it's only tested for non-emptiness, and so is considered true. Likewise for file. Since both subtests are true, the combined and test is also true. There are a few different ways to get the behavior you want. You could use [ ! \( -a file \) ] or ! [ -a file ], although those are not portable to some other shells. (The behavior of a test is generally more predictable when there are fewer arguments, so I'd go with the second of those two, since it pulls the ! outside the test and leaves only two arguments for the test to examine.) You could also remove ! and switch your then and else clauses, which works with any shell. I *know* '! [ -a file ]' is not portable. I tried to use it in some script, somewhere, at some time, and it was sometimes treated as history expansion. Switching clauses may be the only safe way. -- Matthew vIMprove your life! Now on version 7! ___ Bug-bash mailing list Bug-bash@gnu.org http://lists.gnu.org/mailman/listinfo/bug-bash
Re: Bug with 'test' built-in.
mwoehlke [EMAIL PROTECTED] wrote: I *know* '! [ -a file ]' is not portable. I tried to use it in some script, somewhere, at some time, and it was sometimes treated as history expansion. Quoting the ! would take care of that particular problem, but there are some older shells, like Solaris /bin/sh, where the ! command doesn't exist at all, so if this script will have to run in anything but bash, it'd still be good to use one of the other methods. paul ___ Bug-bash mailing list Bug-bash@gnu.org http://lists.gnu.org/mailman/listinfo/bug-bash
