Hi,

Want to know what does it returns.

$ awk '$1 == /Jan/' inv

$ awk '$1 = /Jan/' inv --- What does this one signify.
1 13 25 15 115
1 21 36 64 620

$ awk '$1 ~ /Jan/' inv
Jan  13  25  15 115
Jan  21  36  64 620

Dont know... You did not show us what was is the file inv for a start...
You also di not mention your OS (awk differ depending of OS you know...)

I am using CYGWIN_NT-5.1

$ cat inv

Jan  13  25  15 115
Feb  15  32  24 226
Mar  15  24  34 228
Apr  31  52  63 420
May  16  34  29 208
Jun  31  42  75 492
Jul  24  34  67 436
Aug  15  34  47 316
Sep  13  55  37 277
Oct  29  54  68 525
Nov  20  87  82 577
Dec  17  35  61 401

Jan  21  36  64 620
Feb  26  58  80 652
Mar  24  75  70 495
Apr  21  70  74 514

In the second awk, you are assigning the outcome of /Jan/ (1 if true, 0 if false) to field 1.., in the first one, you are comparing 0 or 1 to the value of field 1..
What you are probably looking for is:

awk '$1=="Jan"' inv

awk '$1==m' m=Jan inv