[Linux-Discussion] Re: what does the variable $1 mean in Bourne shell?
- From: ianezz@xxxxxxxxxx
- To: linux-discussion@xxxxxxxxxxxxx
- Date: Wed, 19 Dec 2001 10:34:56 +0100
David Bruce Jr, aggrappandosi alla tastiera per non cadere, ha scritto:
> is that correct? sorta like a 'place holder'
Well, if you look at the bottom of it, every name of a variable is a
placeholder for its value, except for when one assigns values to
them... :-)
Basically, every time the shell finds ``$'' followed by a word
or by some special characters, it replaces (``expands'') that with the
value of a variable. There are some exceptions to this rule:
- when $something is inside single quotes, i.e.
something=1234
echo '$something'
- when the ``$'' is quoted with a backslash
something=1234
echo \$something
and then there are some special variables having special
values. Usually you don't assign to them, but their value changes
according to their context. I.e.
$1 ... $9 contains the parameters (arguments?) that one passed to your
script on the command line.
$? contains the numeric exit code of the last command executed by the shell
(0 = ok or true, everything else means an error of some sort or false).
See also later in this message.
$RANDOM gives a different random value each time you use it.
other variables have special meaning to the shell:
$PATH contains the list of all directories searched by the shell for
commands when you specify them on the command line or in a script
without giving a path (i.e. ``ls'' vs. ``/bin/ls'').
$PS1, $PS2 and $PS3 specify the prompts used by the shell in various
situations. The one you normally see is the one in $PS1. If you assign
values to them, the prompt changes.
> man pages are almost gibberish
Man pages are reference pages, but reading the whole bash manpage
at least once usually is well worth the time :-)
> lemme see, so if:
> ! Expands to the process ID of the most recently executed background
> (asynchronous) command.
>
> then $! means a variable assigned to the most recently backgrounded command?
Exactly. The value of ``$!'' is the process id (a numeric value
uniquely identifying a process in the system) of the most recent
process started in background by that shell (i.e. specifying ``&'' as
the last char on the command line).
> I'm trying to figure out our class project:
>
>
> Write a shell script that does X, but first make sure the file exists
>
> I'm working on the 'does the file exist or not' part.
Well, you have to understand some things first:
every process always returns a numeric exit code when it terminates. EVERY
process. Even if you kill it.
There is a convention that a process giving an exit code of ``0'' (zero)
means ``everything is OK'', or TRUE, and other values mean ``something
went wrong'' or FALSE.
Example:
if you execute
ls /usr/bin
``ls'' lists the contents of /usr/bin, and then gives an exit code
of zero to say that everyting is ok.
On the other hand, if you execute
ls /foobarbazasdfasdf
``ls'' complains that the ``/foobarbazasdfasdf'' directory doesn't
exist, and gives an exit code different from zero (i.e. 1, 2, or so).
You can see the exit code of the last excuted command with
echo $?
because ``$?'' is a variable that contains just that.
Try
ls /usr/bin
echo $?
and then
ls /foobarbazasdfasdf
echo $?
and compare the results.
----------------------------------------------------------------------
Then you have to understand how ``if ... then ... fi'' works. Its
syntax is:
if command parameter parameter ...; then
...
else
...
fi
the shell see the ``if'' statement, and executes ``command'', and if
its exit value is 0 (which by convention means TRUE) it executes what
is immediately after ``then'', otherwise it executes what is
immediately after ``else'' (if there is an ``else'' at all).
So, for example, you could write:
if ls -l /foobarbaz; then
echo "Ok"
else
echo "Directory /foobarbaz doesn't exists"
fi
----------------------------------------------------------------------
Then you have to understand how ``['' and the ``test'' command are related
The
if [ expression ]; then
...
fi
is the same as
if test expression; then
...
fi
Or, in other words, ``['' is just another name for the ``test''
command, and it looks nicer in shell scripts.
----------------------------------------------------------------------
The ``test'' command can perform several tests on values, files, etc.,
and of course you can use variables on its command line.
In particular, ``test'' can check for the existance of a file using
the ``-f'' option. So:
if [ -f "$file" ]; then
...
else
echo "$file not found"
fi
or, if you prefer:
if test -f "$file"; then
...
else
echo "$file not found"
fi
----------------------------------------------------------------------
> if [ $? != 0 ]
this means:
``if the exit code given by the last command is not equal to zero''
or, in other words
``if the last executed command gave an error of some sort''
the ``!='' operator is taken from the C language, and means "not
equal".
--
UNIX diapers by Pannolini USPTO 2039887 http://www.uspto.gov
Matteo Ianeselli ianezz AT sodalia.it (+39) 0461 316452
Visita il LinuxTrent: http://www.linuxtrent.it
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