Ygtjki

I'm trying to figure out how I can use AWK to subtract lines. For example, imagine the input file is:

30
20

The output would be:

10

Now, as a test I am trying to calculate the "Used" memory column from:

$ cat /proc/meminfo

So at the moment I have written this:

$ grep -P 'MemTotal|MemFree' /proc/meminfo | 
 -- Here comes the calculation using AWK

I have tried the following:

$ grep -P 'MemTotal|MemFree' /proc/meminfo | 
 awk 'print $2' | awk '$0-ss=$0 END print s' 

But this just gives me the last row of data.

I've found a working solution, but I doubt it's the most optimal one. All my coding experience tells me that hard coding the amount of rows is terrible :P

$ grep -P 'MemTotal|MemFree' /proc/meminfo | 
 awk 'print $2' | awk 'NR == 1s=$0 NR == 2 s=s-$0 END print s'

You can also do this using awk, paste, and bc. I find this approach easier to remember, the syntax of awk always requires me to look things up to confirm.

NOTE: This approach has the advantage of being able to contend with multiple lines of output, subtracting the 2nd, 3rd, 4th, etc. numbers from the 1st.

$ grep -P 'MemTotal|MemFree' /proc/meminfo | 
 awk 'print $2' | paste -sd- - | bc
7513404

Details

The above uses awk to select the column that contains the numbers we want to subtract.

$ grep -P 'MemTotal|MemFree' /proc/meminfo | 
 awk 'print $2'
7969084
408432

We then use paste to combine these 2 values values and add the minus sign in between them.

$ grep -P 'MemTotal|MemFree' /proc/meminfo | 
 awk 'print $2'| paste -sd- -
7969084-346660

When we pass this to bc it performs the calculation.

$ grep -P 'MemTotal|MemFree' /proc/meminfo | 
 awk 'print $2'| paste -sd- - | bc
7513404

The purely awk solution, no redundant cat or grep commands:

awk '/MemTotal/ TOT=$2 /MemFree/ FREE=$2 END printf("%d kB Usedn", TOT-FREE)' /proc/meminfo

I see awk_FTW beat me to it but I though formatting the output could be nice.

Try this:

grep -P 'MemTotal|MemFree' /proc/meminfo | 
awk 'NR==1s=$2;nexts-=$2ENDprint s'