awk

Question 1

AWK

Print first field in every line

Answer 1

awk ‘{print $1}’ file

Question 2

AWK

Print first field in every line where fields are
separated by a “:”

Answer 2

awk -F: ‘{print $1}’ file
or
awk -F “:” ‘{print $1}’ file

-F is an awk variable specifying the field separator

Question 3

AWK

Change the second field in every line to ‘Hi’

Answer 3

awk ‘{$2=”Hi” ; print $0}’ file

Multiple commands separated by “;”

Question 4

AWK

Apply the awk script in ‘awkscript’ to the file named ‘testfile’ using delimiter :

Answer 4

awk -F: -f awkscript testfile

-f argument specifies file containing awk script

Question 5

AWK

Print a line “Header” followed by all of the lines in the file

Answer 5

awk ‘BEGIN {print “Header”} {print $0}’ file

BEGIN is a filter which only returns true before the first line. BEGIN blocks can be used for initialization of variables etc.

Note that the following would also work:

awk ‘{print $0} BEGIN {print “Header”}’ file

Question 6

AWK

How do you specify a command that should run after all lines have been processed?

Answer 6

awk ‘END { command }’ file

Note that you can have other pattern/action pairs.

An awk program contains multiple pattern/action pairs. The pattern can be a regex etc or BEGIN/END. If no pattern is supplied for an action then it applies to all rows.

Question 7

AWK

There are two ways to set variables. What are they?

Answer 7

As params to the awk command itself.
awk -F “:” ‘{print %0}’ file

Or in the BEGIN command.
awk ‘BEGIN {FS = “:”} {print %0}’ file

Note that the names of the command line argument and the variable do not necessarily match. In the above, the -F command line argument and the FS variable both represent the Field Separator variable.

Also, not all variables have a command line equivalent.

Question 8

AWK

Each line of the input is by default a ‘record’. How do you change this to use a different delimiter for records?

Answer 8

The RS variable sets the Record Separator, which is the Newline character by default. Here is a script which uses records separated by ‘;’ and fields separated by “,”. Note that NR represents the record number currently being processed.

awk ‘BEGIN {RS = “;” ; FS = “,”} {print NR “) “ $1 “=” $2}’ file

This script will process a single line, where records are separated by “;”. Each record consists of two fields, separated by “,”. It will print, for example “1) a = b” for each record.

Example:
if file contains “a,b;c,d;e,f”
Then the output will be:
1) a=b
2) c=d
3) e=f

Question 9

AWK

Which variable controls what is ouput between arguments passed to’print’?

Answer 9

OFS

Output Field Separator

Question 10

AWK

Which variable controls what is output between records?

Answer 10

ORS

Output Record Separator

Question 11

AWK

Which variable indicates how many fields are in the current record?

Answer 11

NF

Num Field

Question 12

Which variable indicates the number of the current record?

Answer 12

NR

Num Record

Question 13

AWK

How would you process data where each record consists of a number of fields, each on its own line, and the records are separated by a blank line?

eg

Joe Blow
554-456-3422

Bob Skulk
643-644-6789

Answer 13

awk ‘BEGIN {FS=”\n”; RS=””} {print $1 “ph=” $1}’ file

RS=”” says records are separated by empty line.
FS=”\n” says fields separated by newline.

Question 14

AWK

How do you create your own variables?

Answer 14

Initialize them in the BEGIN block. Can update and use in other blocks.

e.g.
awk ‘BEGIN {myvar = “Hi there”} {print myvar, $0}’ file

Question 15

AWK

Print the second field of each line if the first field is > 30

Answer 15

awk ‘{if ($1 > 30) print $2}’ testfile

Question 16

AWK

Put multiple statements inside an ‘if’ block.

Answer 16

Put the ; separated statements inside a {…}
For example:
awk ‘{if($1 > 30) {x = $1 * NR ; print x}}’ file

If the first field is > 30 then print the field value * record number