[CLUE-Tech] shell programming

[marcus hall]

On Wed, Dec 11, 2002 at 03:26:36AM -0700, Dave Price wrote:

> When you 'su' your $USER variable does not change, but your $UID
> variable does.  (root's UID is 0 {zero})
...
> 
> if [ $USER = root ] 
...
> if [ $UID = 0 ]


Of course, if this script does anything where it *really* matters what
the user is, don't rely on a variable from the environment, as it is
trivial to set it before invoking the script.  You could use $(id -un)
to compare against root, or $(id -u) to compare against 0.

Also, especially if you have not set the variable in your script, it is
generally unsafe to write a comparison that way.  It's too easy to get
a syntax error if the variable contains unexpected text.  For instance,
if USER was null, the test becomes:

if [ = root ]

which gives an error "[: =: unary operator expected".  It is better to
enclose the $USER in double quotes:

if [ "$USER" = root ]

However, this could still be confused if $USER contained an unexpected
value like "-a", since then the test becomes:

if [ -a = root ]

It seems that recent implementations parse this much better, as trying
this out I correctly get a false return from the test, but in previous
shells you would get an error message about a missing argument before
the -a.  That is why you will frequently see tests written like:

if [ "X$USER" = Xroot ]

The leading X added to the 1st argument prevents it from inadvertently
appearing as a special argument that test recognizes.

marcus hall
marcus at tuells.org