UNIX for Dummies Questions & Answers

I have a file called vm.cfg .

content of file.

Code:

acpi = 1
apic = 1
builder = 'hvm'

How would i write a script which will add
boot = 'd' at 3rd line.

For example.
after running the script the file should be like below.

Code:

acpi = 1
apic = 1
boot = 'd'
builder = 'hvm'

I dont want to edit the file if boot = 'd' is already present in the file.

Code:

V="'d'"; awk -v v=$V 'NR==3 && $0 !~ /^boot/ {print "boot =",v}1' vm.cfg > tmpfile
mv tmpfile vm.cfg

thanks zaxxon for solving my problem . I would highly appreciate if you explain the logic behind the code.

Defining the single quotes outside awk since I was unable to escape them inside; double quotes around the single quotes to preserve them as text.

Code:

V="'d'"

Handing over the shell variable $V to awk with the name v.

Code:

awk -v v=$V

If NumberRecords (ie. the line number) equals 3

Code:

'NR==3

logical and

Code:

&&

At the beginning of the lline of input is not the regular pattern ^boot (^ stands in this case for the beginning of the line)

Code:

$0 !~ /^boot/

Then do following command, ie. printing the string "boot =" and the value of the variable v

Code:

{print "boot =",v}

Also for all other lines awk finds, it shall just do it's default behaviour, which is printing. Shortest form to write is just a single number, in this case a 1.

Code:

1'

And this is just the input file and the redirection after parsing into a new file

Code:

vm.cfg > tmpfile

So in the case the patter is not found (boot...) in row 3, it will just do the 1, which is print.
If it is not found in row 3, it will print it.