how not process the first line using awk?

I want to change the ps virtual memory size output from KiB to Mib and add sign to it, but neither want to change the header（first line），nor remove it from the result.

example
from

PID COMMAND            VSZ
    9 bash              6304
  537 ps                7476

to

PID COMMAND            VSZ
    9 bash              6MB
  537 ps                7MB

1. the default implicit print only happens if no action is provided
2. (...) is not the same as {...}

NR!=1 ($2=$2"MB")

• whole line is treated as a single pattern
• concatenate $2 and "MB" and assign to $2
• concatenate 1 and result of parenthesised expression (ie. value of $2)
• test if NR is not equal to that

As NR is always a number and "MB" is not, the test will always succeed. (ie. the pattern matches)

No action has been provided, so the default print is done.

{ if(NR==1){ print } else {$2=$2"mb"} }

• whole line is treated as single action
• no pattern provided, so this action is performed for every record
• record is (explicitly) printed if NR equals 1
• otherwise value of $2 is updated - there is no default print

Probably you want something like:

NR!=1 { $2 = int($2/1024) "MB" }
      { print }

or equivalently:

NR!=1 { $2 = int($2/1024) "MB" } 1

I try out and found the answer ps -o pid,vsz| awk 'NR ==1 {print} NR>1 ($2=int($2/1024)"MB") '

and I have no idea why this work.

input00='PID COMMAND            VSZ
    9 bash              6304
  537 ps                7476'

printf '%s\n' "$input00" | gcat -b
printf '%s\n' "$input00" | 

> mawk 'NR==!_ || sub(__ = "[0-9]+[[:space:]]*$",
> int(substr(___ = $+,
> match(_, __), RLENGTH) / 4^5) "MB")^' FS='\n' | gcat -b

     1	PID COMMAND            VSZ
     2	    9 bash              6304
     3	  537 ps                7476 

>

     1	PID COMMAND            VSZ
     2	    9 bash              6MB
     3	  537 ps                7MB

This approach preserves the random combination of spaces and tabs from the input, other than trailing ones.

this performs mere integer truncation instead of proper rounding. adjust accordingly if needed.

but if you don't care to preserve all the spaces and tabs, then it's simply :

> mawk 'NR==1 ? NF=NF : $NF = int($NF/4^5) "MB"'

 1	PID COMMAND VSZ
 2	9 bash 6MB
 3	537 ps 7MB