Help with awk

[David G. Miller]

Les Mikesell <lesmikesell at gmail.com> wrote:

> aragonx at dcsnow.com wrote:
>> > I bet awk has it's own user list but before I go and subscribe to another
>> > one, I figured I would ask here first.  I have the following awk code that
>> > is trying to set a variable of the shell it is running in.  It doesn't
>> > work and I've searched high and low to see how to get it to do so.
>> > 
>> > The reason I don't just use DOMAIN= 'awk blah blah blah' is I want to set
>> > more than one shell variable for each record.  But I have to get this
>> > working first.   :( 
>> > 
>> > #!/bin/bash
>> > 
>> > awk '{FS=":"}{system ("DOMAIN=" $1); }
>> >         {system ("DOMAIN="$1)};
>> >         {system ("echo \"$DOMAIN\"")};
>> > ' /etc/passwd
>> > 
>> > Any help would be appreciated.
>>     
>
> This isn't an awk question, it's a 'how unix works' question.  A child 
> process can't change anything in it's parent's environment.  Your awk 
> program will be a child of the shell running it; the system statements 
> run yet another shell as a child under awk.  All of those processes have 
> their environment strings in protected memory (inherited shared but 
> copy-on-write).
>
>
> The var=`command ..` will work, or you can do all of the work that needs 
> the setting in the lowest level subprocess that has the right values.
>
> -- Les Mikesell lesmikesell at gmail.com
An alternative is to have the awk script output the shell commands you 
want.  That is, change your {system ("DOMAIN="...} to a print 
statement.  You then execute the command created by your awk script in 
the calling shell's environment.  Something along the lines of:

foo = `awk '{FS=":"}{printf "export DOMAIN=%s\n", $1}' < /etc/passwd`
`echo $foo`

After the above, $DOMAIN should have the value you want for the *last 
entry* in /etc/passwd.  Not sure if this is what you really want.  That 
may be:

foo=`awk 'BEGIN {FS=":"} $1 ~ /'$USER'/ {printf "export DOMAIN=%s\n", 
$1}' /etc/passwd`
`echo $foo`

You'll find understanding quoting and order of execution is really 
useful when writing this kind of shell script. ;-)


Cheers,
Dave

-- 
Politics, n. Strife of interests masquerading as a contest of principles.
-- Ambrose Bierce