Explain this duplicate line removing, order retain

Question

I learned a really handy way to remove duplicate lines retaining the order from Remove duplicates without sorting file - BASH.

Say, if you have the following file,

$cat file
a
a
b
b
a
c

you can use the following to remove the duplicate lines:

$awk '!x[$1]++' file
a
b
c

How does this work in terms of precedence of operations?

Answer 1

In AWK arrays are associative, so the first column or first field of each line, $1, is used as an index for the array x.

Answer 2

The expression is parsed as

!(x[$(1)]++)

So, from the inside out, it's:

• Take field 1 of the current input line, $(1) (note that $ is an operator in AWK, unlike in Perl).
• Index x with the value of field 1; if x is an unbound variable, bind it to a new associative array.
• Post-increment x[$(1)]; a rule similar to the one in C applies, so the value of the expression is that of x[$(1)] prior to the increment, which will be zero if x[$(1)] has not yet been assigned a value.
• Negate the value of the previous, which will yield truth when x[$(1)] is zero.
• Actually do the increment so that x[$(1)] gets a non-zero value. So, the next time, x[$(1)] for the same value of $(1) will return 1.

This expression is then evaluated for every line in the input and determines whether the implied default action of awk should be executed, which is to echo the line to stdout.