Mailinglist Archive: opensuse (1786 mails)

[David Haller <dnh@xxxxxxxxxxxx>]

Hello,

On Thu, 10 Nov 2011, Ed Greshko wrote:
> On 11/10/2011 10:25 AM, George OLson wrote:
> > I am trying to set up ssh, and I find different command line examples
> > on the internet to help in my research. As I am learning CLI and
> > scripting, can someone explain the following to me please?
> >
> > In a given directory, I typed in the following line.
> >
> > if [ ! -f authorized_keys ]; then touch authorized_keys ; chmod 600
> > authorized_keys ; fi
> >
> > It created the file authorized_keys and gave it the time stamp and
> > changed the permission. I got that part.
> >
> > The only thing I don't understand is the very first boolean clause
> > after the "if".
> >
> > What exactly does the exclamation mark signify?
> >
> > What also is the "-f" option?
>
> http://www.linuxtutorialblog.com/post/tutorial-conditions-in-bash-scripting-if-statements
>
> is one of the many places you'll be directed to if you use an
> appropriate google search.

That stuff is _SO_ WRONG!

The syntax is:

if: if COMMANDS; then COMMANDS; [ elif COMMANDS; then COMMANDS; ]... [ else 
COMMANDS; ] fi

Note: COMMANDS. Not(!) EXPRESSION.

'if [ .. ]; then' is just a weird way to write

'if /usr/bin/[ ... ]; then'
or
'if builtin [ ... ]; then'

which both are weird variants of '/bin/test' / 'builtin test' with the
extra quirk to gobble up ']' as the last argument. And it's only an
argument if it's seperated from the rest by a space etc., or else a ]
will be part of the argument before it.

In short: NEVER EVER EVER use [ .. ]. Use 'test'.

The EXPRESSION comes with 'test':

$ help test
test: test [expr]
    Evaluate conditional expression.
[..]
      -f FILE        True if file exists and is a regular file.
[..]
      ! EXPR         True if expr is false.

OTOH: [[ ... ]], (( ... )), $( ... ), $(( ... )), $[ .. ], _are_ bash
syntax and NOT to be confused with [ .. ].

$ help [[
[[ ... ]]: [[ expression ]]
    Execute conditional command.
$ help '(('
(( ... )): (( expression ))
    Evaluate arithmetic expression.

==== man bash ====
   Arithmetic Expansion
              $((expression))
====

-dnh, too tired now to elaborate properly.

-- 
"Cynical" is a term invented by optimists to describe realists.
    -- Gregory Benford
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