Pattern is a logical condition that would be invalid, such as values that are negative and should not be, or greater than they should be. Action is what to do about it. Usually the action is to print out a message.

-Dn

debug mode. Print messages on the standard error about what row is doing. When n is specified, the detail and quantity of messages increases as n increases. This is handy for debugging conditional expressions by examining the resulting awk input.

-Fc

use c (instead of TAB) as the input column separation character.

-f file

look in file for the conditional expressions instead of specifying them as a command line argument.

$ cat inventory
Item    Onhand  Cost    Value   Description
----    ------  ----    -----   --------------
1       3       50      150     rubber gloves
2       100     X       500     test tubes
3       -5      80      -400    clamps
4       23      19      437     plates
5       -99     24      -2376   cleaning cloth

There are several invalid values in this table. To check that no Onhand items are less than zero:

$ cat inventory |\\
validate 'Onhand < 0 { print "negative Onhand line " NR }'
Item    Onhand  Cost    Value   Description
----    ------  ----    -----   --------------
1       3       50      150     rubber gloves
2       100     X       500     test tubes
negative Onhand line 5
3       -5      80      -400    clamps
4       23      19      437     plates
negative Onhand line 7
5       -99     24      -2376   cleaning cloth
5       -99     24      -2376   cleaning cloth

NR is a built-in awk variable which refers to the current line in the file. We could subtract 2 from NR to get the logical row number. A more complex example involves many tests and messages, executed as one validation file.

$ cat valfile
{if (Onhand < 0 ) print "negative Onhand in row " NR-2;
if (Value < 0 ) print "negative Value in row " NR-2;
if (Cost ~ /[A-Za-z]/) print "letter in Cost in row " NR-2}

$ validate -f valfile < inventory
Item    Onhand  Cost    Value   Description
----    ------  ----    -----   --------------
1       3       50      150     rubber gloves
letter in Cost in row 2
2       100     X       500     test tubes
negative Onhand in row 3
negative Value in row 3
3       -5      80      -400    clamps
4       23      19      437     plates
negative Onhand in row 5
negative Value in row 5
5       -99     24      -2376   cleaning cloth