[linux-help] Re: Dollar Arguments

[Bruce Bales <bbales@xxxxxxx>]

On Saturday 01 July 2006 15:25, Tom Hull wrote:
> Bruce Bales wrote:
> > I can find what is contained in $HOME or $PATH by using the command echo
> > $HOME or echo $PATH.  But I often run into other arguments preceded with
> > a "$" which I can't decipher with echo; as in the yum repos where it uses
> > $basearch and fc$releasever.  I know basearch is probably i386 or i586
> > and fc$releasever is fc5, but often I don't know what is hidden and echo
> > usually doesn't help.  google seems to ignore the "$".
> >
> > Is there a way to retrieve this information or a file where it is stored?
> > bruce
>
> sounds like those are just ordinary shell variables. somewhere before they
> are used there should be a statement that sets them; e.g.:
>
>    basearch=i386
>
> or
>
>    basearch=`uname -m`
>
> which can also be written
>
>    basearch=$(uname -a)
>
> unless there's an export statement, those variables only exist within
> the scope of the /bin/sh program that runs the script. an export statement
> looks like:
>
>    export basearch
>
> exported variables are inherited by programs run by the shell, including
> other shell programs. those variables are accessible through the
> environment; see getenv(3)
>
> you'll also see references to shell variables written with braces, e.g.:
>
>    fc${releasever}
>
> there are several variants on this which are useful for shell programmers.
>
> some shell variables are set up by your login shell and exported, so they
> are effectively everywhere and easy to check as you note above -- $HOME
> and $PATH are two. you can get a list of all active shell variables by
> running:
>
>    set
>
> to find out what's going on inside a shell script, you can run the whole
> shell script with execution trace on:
>
>    sh -x -c "command"
>
> if you only want to debug a small part of the script, you can edit the
> script to turn execution trace on/off by surrounding the part of interest
> with these commands:
>
>    set -x
>
>    set +x

Should this be sh -x and sh +x ??

>
> the -x flag will have already expanded any shell variables. the -v flag
> does a similar trace, but as the command lines are read, so the variable
> names are present there. you can combine these.

Thanks Tom.  The set command gives me much of what I have been wondering about 
for years.  It listed 71 variables.  

Looking at /etc/yum, where the $basearch and $releasever are used didn't show 
any assignments.  I tried to grep for them and found they are used a lot, but 
not assigned here.

[bruce@blacky etc]$ grep -Hr "basearch" yum* |wc
     59     240    6026
[bruce@blacky etc]$ grep -Hr "basearch=" yum* |wc
      0       0       0
[bruce@blacky etc]$ grep -Hr "releasever" yum* |wc
     70     270    7070
[bruce@blacky etc]$ grep -Hr "releasever=" yum* |wc
      0       0       0
[bruce@blacky etc]$
(I also did a visual search and found no assignments.)  I'll try to dig into 
some of the python code associated with yum.

Since these variables are used in a configure file and not a shell script I 
can't see how to use the sh -x -c .
bruce

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