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In this guide, you’ll learn how to leverage awk’s built-in variables to parse and manipulate text data efficiently. We’ll cover:
  • Positional variables ($1, $2, …)
  • NR (Number of Records)
  • NF (Number of Fields)
  • $NF (Last Field in the Current Record)
  • FILENAME
Throughout this article, we’ll use a sample file, size.txt, generated from df -h: 
df -h > size.txt

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1. Positional Variables

By default, awk splits each input line on whitespace. You can print specific fields using $1, $2, $3, and so on: 
awk '{ print $1, $2, $3 }' abc.txt
  • $1 → first column
  • $2 → second column
  • $3 → third column
For example, given abc.txt: 
abc def ghi
jkl mno pqr
stu vwx yz

awk '{ print $1 }' abc.txt
# Output:
abc
jkl
stu

awk '{ print $2 }' abc.txt
# Output:
def
mno
vwx

awk '{ print $3 }' abc.txt
# Output:
ghi
pqr
yz

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2. NR: Number of Records

NR tracks the current record (line) number. This is useful for adding line numbers or filtering specific lines: 
awk '{ print NR, $1, $4 }' size.txt
Produces: 
1 Filesystem Avail
2 udev       1.8G
3 tmpfs      356M
4 /dev/sda1  9.3G
...
To print only the 8th line’s available space: 
awk 'NR == 8 { print $4 }' size.txt
# Output:
360G

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3. NF: Number of Fields

NF contains how many fields are in the current record. It helps you spot inconsistencies in your data: 
awk '{ print NF }' size.txt

7
6
6
6
6
6
6
6
6
The header line has 7 fields because “Mounted on” is split into two separate fields.

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4. Combining NR and NF

Print both the record number and its field count: 
awk '{ print NR, NF }' size.txt

1 7
2 6
3 6
4 6
...
Concatenate text with values: 
awk '{ print "Line", NR, "has", NF, "fields" }' size.txt

Line 1 has 7 fields
Line 2 has 6 fields
...

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5. $NF: The Last Field

Use $NF to refer directly to the last field of each record: 
awk '{ print $NF }' size.txt

on
/dev
/run
/
...

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6. FILENAME: Current Filename

FILENAME holds the name of the file being processed (empty when reading from stdin): 
awk '{ print FILENAME, $1 }' size.txt

size.txt Filesystem
size.txt udev
size.txt tmpfs
...
When you pipe data into awk, FILENAME is empty: 
df -h | awk '{ print FILENAME, $1 }'
# Output:
 Filesystem
 udev
 tmpfs
 ...

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7. Summary Table of Built-in Variables

VariableDescriptionExample
$1, $2Positional fields (first, second, etc.)awk '{ print $2 }' file.txt
NRCurrent record (line) numberawk '{ print NR }' file.txt
NFNumber of fields in the current recordawk '{ print NF }' file.txt
$NFThe last field in the current recordawk '{ print $NF }' file.txt
FILENAMEName of the file being processed (or empty)awk '{ print FILENAME }' file.txt

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8. Custom Field Separators

If your data uses a delimiter other than whitespace, set the -F option: 
awk -F, '{ print $1, $NF }' data.csv
Always quote the -F argument when it contains special characters, e.g., awk -F'|' '...' file.txt.

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