Very Computer

Hi,

I am a little confused if the following shell testing clauses are the
same, or, if not, what would be the benefit for using one over the other?

Using /bin/sh,

[[ -f /tmp/myfile ]] && echo "cool, file exists!"

or

[ -f /tmp/myfile ]   && echo "cool, file exists!"

I've always used the second form, but in HP-UX's /sbin/init.d/* init scripts,
most lines use the first form, so, is there any good vs bad thing to use one
of them instead of the other?

Thanks!  Please send a copy of your reply via email... many thanks!

Jack Duan

--

 UNIX System Administrator/Specialized Consultant
^------------------------------------------------------------------^

Quote:> Hi,

> I am a little confused if the following shell testing clauses are the
> same, or, if not, what would be the benefit for using one over the other?

> Using /bin/sh,
> [[ -f /tmp/myfile ]] && echo "cool, file exists!"
> or
> [ -f /tmp/myfile ]   && echo "cool, file exists!"

> I've always used the second form, but in HP-UX's /sbin/init.d/* init scripts,
> most lines use the first form, so, is there any good vs bad thing to use one
> of them instead of the other?

The [[ ... ]] construct will not work in a bourne shell script.  The
bourne shell has no internal testing capability, and one has to rely
on the Unix test command which can be invoked as either:
   test expression
   [ expression ]

The Korn shell does have the ability, and of course it can also invoke
Unix programs just like any other shell.  The [[ expression ]]
construct invokes the KSH builtin testing feature, which is obviously
much faster than executing an external program.

The default shell under HPUX is ksh, not sh, so in scripts where the
#! is ommitted from the first line, ksh is invoked to interpret it.
In short, the init files you looked at were Korn shell scripts.

\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\

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My "UNIX in a Nutshell" book says (qouting)

[[expression]]

Korn shell only. Same as "test expression" or "[ expression ]", except that
[[ ]] allows additional operators. Word splitting and filename expansion are
disabled. Additional operators
&& logical AND of test expressions
|| logical OR of test expressions

Quote:> First string is lexically "greater than" the second

< First string is lexically "less than" the second

Quote:> The default shell under HPUX is ksh, not sh, so in scripts where the
> #! is ommitted from the first line, ksh is invoked to interpret it.
> In short, the init files you looked at were Korn shell scripts.

The default for root (and the one that is used for those startup
scripts) *is* actually "sh", but this sh is the POSIX-shell
(which is nothing else than a POSIX-blessed Korn shell, under HP-UX).

S.

Note that these are *instead* of -a and -o - you can't use -a or -o in [[]].

Quote:>> First string is lexically "greater than" the second
>< First string is lexically "less than" the second

Also, you can use parentheses for controlling the order of evaluation,
and (the most common reason for me to use [[]] instead of []), = does
matching using ksh patterns, e.g.  [[ $j = */* ]]

        John
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