davidsen@steinmetz.ge.com (William E. Davidsen Jr) (05/11/88)
I had a user find a good (?) one the other day. She has a shell script
which did something like:
if [ "$1" = "x" ]; then do something; fi
The value passed in as the first argument was "-d". Now, the construct
"-d =" should test for a directory named "=", followed by a barf on the
operator "x". This script in fact compared strings on a number of
systems, and blew up on only a few. I believe that it should fail, but
would like some opinions on this.
I fixed it by inverting the test, but it really looks as though some
shells assume a precedence of operators in the shell. A good reason
never to put ANY shell variable in the first position of an = test.
--
bill davidsen (wedu@ge-crd.arpa)
{uunet | philabs | seismo}!steinmetz!crdos1!davidsen
"Stupidity, like virtue, is its own reward" -mebarmar@think.COM (Barry Margolin) (05/11/88)
In article <10778@steinmetz.ge.com> davidsen@steinmetz.ge.com (William E. Davidsen Jr) writes: > if [ "$1" = "x" ]; then do something; fi >A good reason >never to put ANY shell variable in the first position of an = test. Another common convention is to put an extra character onto the front of both parts of the test, e.g. if [ "x$1" = "xx" ]; ... That way, if $1 starts with a hyphen, this becomes something like if [ "x-d" = "xx" ] Barry Margolin Thinking Machines Corp. barmar@think.com uunet!think!barmar
lew@gsg.UUCP (Paul Lew) (05/12/88)
> I had a user find a good (?) one the other day. She has a shell script > which did something like: > if [ "$1" = "x" ]; then do something; fi > > The value passed in as the first argument was "-d". Now, the construct > "-d =" ... on BSD 4.* system, you will find out '[' is actually a link to '/bin/test' and it is NOT part of the shell. Test expect a lot of flags; -r, -w, -f, -d, -s, -t -z -n, etc (see man page for test). If you need string comparison, always prefix by other characters like: if [ "_$1" = "_$x" ]; then ... (system V sh has test built-in, probably interprete '[' to be 'test' like BSD?) -- Paul Lew {oliveb,harvard,decvax}!gsg!lew (UUCP) General Systems Group, 5 Manor Parkway, Salem, NH 03079 (603) 893-1000