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


The output would be:


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-s}{s=$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 == 1{s=$0} NR == 2 {s=s-$0} END {print s}'
  • What would you want done with N rows? Should the final result be line1_$2 - line2_$2 - lineN-$2? Do you only want to subtract the first two consecutive rows?
    – terdon
    Mar 2, 2014 at 16:55

3 Answers 3


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


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}'

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- -

When we pass this to bc it performs the calculation.

$ grep -P 'MemTotal|MemFree' /proc/meminfo | \
    awk '{print $2}'| paste -sd- - | bc
  • @terdon - thanks I was making that edit when you did it 8-)
    – slm
    Mar 2, 2014 at 17:00

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

awk '/MemTotal/ {TOT=$2} /MemFree/ {FREE=$2} END {printf("%d kB Used\n", TOT-FREE)}' /proc/meminfo

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

  • Excellent! I love it. Thanks a lot
    – arif
    Sep 3, 2020 at 15:30

Try this:

grep -P 'MemTotal|MemFree' /proc/meminfo | \
awk 'NR==1{s=$2;next}{s-=$2}END{print s}'

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