8 Powerful Awk Built-in Variables – FS, OFS, RS, ORS, NR, NF, FILENAME, FNR

This article is part of the on-going Awk Tutorial Examples series. Awk has several powerful built-in variables. There are two types of built-in variables in Awk.

Variable which defines values which can be changed such as field separator and record separator. Variable which can be used for processing and reports such as Number of records, number of fields.

1. Awk FS Example: Input field separator variable.

Awk reads and parses each line from input based on whitespace character by default and set the variables $1,$2 and etc. Awk FS variable is used to set the field separator for each record. Awk FS can be set to any single character or regular expression. You can use input field separator using one of the following two options:

Using -F command line option. Awk FS can be set like normal variable.

Syntax: $ awk -F 'FS' 'commands' inputfilename (or) $ awk 'BEGIN{FS="FS";}'

Awk FS is any single character or regular expression which you want to use as a input field separator.

is any single character or regular expression which you want to use as a input field separator. Awk FS can be changed any number of times, it retains its values until it is explicitly changed. If you want to change the field separator, its better to change before you read the line. So that change affects the line what you read.

Here is an awk FS example to read the /etc/passwd file which has “:” as field delimiter.

$ cat etc_passwd.awk BEGIN{ FS=":"; print "Name\tUserID\tGroupID\tHomeDirectory"; } { print $1"\t"$3"\t"$4"\t"$6; } END { print NR,"Records Processed"; }

$awk -f etc_passwd.awk /etc/passwd Name UserID GroupID HomeDirectory gnats 41 41 /var/lib/gnats libuuid 100 101 /var/lib/libuuid syslog 101 102 /home/syslog hplip 103 7 /var/run/hplip avahi 105 111 /var/run/avahi-daemon saned 110 116 /home/saned pulse 111 117 /var/run/pulse gdm 112 119 /var/lib/gdm 8 Records Processed

2. Awk OFS Example: Output Field Separator Variable

Awk OFS is an output equivalent of awk FS variable. By default awk OFS is a single space character. Following is an awk OFS example.

$ awk -F':' '{print $3,$4;}' /etc/passwd 41 41 100 101 101 102 103 7 105 111 110 116 111 117 112 119

Concatenator in the print statement “,” concatenates two parameters with a space which is the value of awk OFS by default. So, Awk OFS value will be inserted between fields in the output as shown below.

$ awk -F':' 'BEGIN{OFS="=";} {print $3,$4;}' /etc/passwd 41=41 100=101 101=102 103=7 105=111 110=116 111=117 112=119

3. Awk RS Example: Record Separator variable

Awk RS defines a line. Awk reads line by line by default.

Let us take students marks are stored in a file, each records are separated by double new line, and each fields are separated by a new line character.

$cat student.txt Jones 2143 78 84 77 Gondrol 2321 56 58 45 RinRao 2122 38 37 65 Edwin 2537 78 67 45 Dayan 2415 30 47 20

Now the below Awk script prints the Student name and Rollno from the above input file.

$cat student.awk BEGIN { RS="



"; FS="

"; } { print $1,$2; } $ awk -f student.awk student.txt Jones 2143 Gondrol 2321 RinRao 2122 Edwin 2537 Dayan 2415

In the script student.awk, it reads each student detail as a single record,because awk RS has been assigned to double new line character and each line in a record is a field, since FS is newline character.

4. Awk ORS Example: Output Record Separator Variable

Awk ORS is an Output equivalent of RS. Each record in the output will be printed with this delimiter. Following is an awk ORS example:

$ awk 'BEGIN{ORS="=";} {print;}' student-marks Jones 2143 78 84 77=Gondrol 2321 56 58 45=RinRao 2122 38 37 65=Edwin 2537 78 67 45=Dayan 2415 30 47 20=

In the above script,each records in the file student-marks file is delimited by the character “=”.

5. Awk NR Example: Number of Records Variable

Awk NR gives you the total number of records being processed or line number. In the following awk NR example, NR variable has line number, in the END section awk NR tells you the total number of records in a file.

$ awk '{print "Processing Record - ",NR;}END {print NR, "Students Records are processed";}' student-marks Processing Record - 1 Processing Record - 2 Processing Record - 3 Processing Record - 4 Processing Record - 5 5 Students Records are processed

6. Awk NF Example: Number of Fields in a record

Awk NF gives you the total number of fields in a record. Awk NF will be very useful for validating whether all the fields are exist in a record.

Let us take in the student-marks file, Test3 score is missing for to students as shown below.

$cat student-marks Jones 2143 78 84 77 Gondrol 2321 56 58 45 RinRao 2122 38 37 Edwin 2537 78 67 45 Dayan 2415 30 47

The following Awk script, prints Record(line) number, and number of fields in that record. So It will be very simple to find out that Test3 score is missing.

$ awk '{print NR,"->",NF}' student-marks 1 -> 5 2 -> 5 3 -> 4 4 -> 5 5 -> 4

7. Awk FILENAME Example: Name of the current input file

FILENAME variable gives the name of the file being read. Awk can accept number of input files to process.

$ awk '{print FILENAME}' student-marks student-marks student-marks student-marks student-marks student-marks

In the above example, it prints the FILENAME i.e student-marks for each record of the input file.

8. Awk FNR Example: Number of Records relative to the current input file

When awk reads from the multiple input file, awk NR variable will give the total number of records relative to all the input file. Awk FNR will give you number of records for each input file.

$ awk '{print FILENAME, FNR;}' student-marks bookdetails student-marks 1 student-marks 2 student-marks 3 student-marks 4 student-marks 5 bookdetails 1 bookdetails 2 bookdetails 3 bookdetails 4 bookdetails 5

In the above example, instead of awk FNR, if you use awk NR, for the file bookdetails the you will get from 6 to 10 for each record.

Recommended Reading

Sed and Awk 101 Hacks, by Ramesh Natarajan. I spend several hours a day on UNIX / Linux environment dealing with text files (data, config, and log files). I use Sed and Awk for all my my text manipulation work. Based on my Sed and Awk experience, I’ve written Sed and Awk 101 Hacks eBook that contains 101 practical examples on various advanced features of Sed and Awk that will enhance your UNIX / Linux life. Even if you’ve been using Sed and Awk for several years and have not read this book, please do yourself a favor and read this book. You’ll be amazed with the capabilities of Sed and Awk utilities.

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