Sure.

Code:

NR == FNR

The above expression returns true only while processing the first input file (file2). So, the corresponding actions are:

Code:

{ 
  sub(/,/, "")
  _[$1] = $2 
  next 
  }

Strip the thousands separator, populate an associative array _with $1 as keys, $2 as values, so given the input it will be:

Code:

key -> "/u01" value -> 1500
key -> "/u05" value -> 500
key -> "/u10" value -> 2500

The next statement forces awk to stop processing the current record so no further rules/actions will be executed for this record. This means that the next actions won't process the first input file.

Code:

FNR == 1

FNR is the number of records that have been read so far from the current input file (the second input file in this case). So, while processing the first record of the second input file:

Code:

{ 
  printf "%4s %5s %8s %8s %14s\n", 
  "FS", "Total", "Used+Adj", "Free-Adj", "(Used+Adj)/Total"
  while (++i < 46) printf "-"
  print ""
  next
  }

Print the header columns and go to the next record.

Code:

$1 in _ { 
  $3 += _[$1]
  $4 -= _[$1] 
  $5 = $3/$2*100 
  }

The expression key in array returns true if the indicated key exists in the indicated array. For those records do the following:
- add the value _[$1] of the corresponding key $1 to the third column
- subtract the value _[$1] of the corresponding key $1 from the fourth column
- calculate the value of the fifth column

Code:

{
  printf "%4s %5d %8d %8d %14d%\n",
  $1, $2, $3, $4, $5
  }

Print the new values.


Hope this helps.