UNIX for Dummies Questions & Answers

I found this on an awk site and would like to know what it does:

Code:

/CARS/{x="";next} {if(x)print x;x=$0} END{if(x)print x}'

Does it mean if it finds the word cars it skips that line and then prints the next one?

Each line that contains CARS is skipped. It'll display x once it comes across a line without CARS, yes. In the end it prints all the lines without CARS, but lagging behind a line (thus the need for the END statement to catch up).

equivalent and quicker: awk '!/CARS/'

Not quite, it also skips the line above the line with the match and it also skips empty lines..

How awk basically works:

Any awk program has 3 parts, all of which are optional:

Code:

awk '
BEGIN {
      ...
}

{
      ...
}

END   {
      ...
}' /path/to/inputfile

The program part marked "BEGIN" is executed before the first line is read. The part marked "END" is executed after the last line is read. The (unnamed) part in the middle is executed for every line of the input file.

Most times the part in the middle consists of rules which specify a certain pattern of lines along with one or more actions specifying what to do with these. Such a rule could be:

Code:

/^ABC/ {
        number++;
        }

This means: whenever a line starts with "ABC" ("^" means "line start") increment a variable named "number". Put in the END-part a statement printing this variable and you have a counter for a certain sort of lines (before you ask: yes, "grep -c" would do the same).

I suggest Dale Doughertys "sed & awk" from O'Reilly if i managed to peak your interest.

I hope this helps.

bakunin

That's not correct.
Have the following test file:

Code:

line 1
line 2
line 3
CARS
line 5

0
line 8
line 9

You'll see that it skips NULL lines (e.g. line 6 being empty and line 7 being zero).
Also, it does a delayed print: prints the value from the previous line.
When it meets CARS it "NULLs" the saved line (x="") and skips the current line (next).
In the END section the last line is printed (because it was only stored yet).