Mastering Delimiters with awk and OFS

Introduction

awk is a powerful command for efficient text processing, but one of the points where beginners tend to stumble is "handling of delimiters."

In particular, the concept of ofs is related not only to splitting input but also to output formatting, and there are many cases where people use it without a clear understanding.

This article carefully explains how awk handles columns and when ofs takes effect, and summarizes advanced techniques that can be used in practical work.

Reference: GNU awk

How awk Splits Columns

Creating the File

cat << 'EOF' > input.txt
apple orange banana
dog cat mouse
EOF

Command

awk '{print $1, $2, $3}' input.txt

Output

apple orange banana
dog cat mouse

Command

awk 'BEGIN{OFS=","} {print $1, $2, $3}' input.txt

Output

apple,orange,banana
dog,cat,mouse

How It Works

Item	Description
FS	Input field separator (default is whitespace)
OFS	Output field separator (default is whitespace)
$1, $2...	Retrieve data on a per-field basis
print	Outputs with OFS inserted between fields

Explanation

awk splits input using FS and joins output using OFS.
By changing OFS, you can freely control the delimiter.

Default OFS and the Behavior of “Single Space”

Creating the File

cat << 'EOF' > input.txt
A B C
1 2 3
EOF

Command

awk '{print $1, $2, $3}' input.txt

Output

A B C
1 2 3

Command

awk 'BEGIN{OFS=" "} {print $1, $2, $3}' input.txt

Output

A B C
1 2 3

Command

awk 'BEGIN{OFS=","} {print $1, $2, $3}' input.txt

Output

A,B,C
1,2,3

How It Works

Item	Description
OFS	Output Field Separator
Default value	Single space " "
Behavior of print $1, $2	OFS is automatically inserted between fields
OFS=" "	Same as default (explicitly specified)
OFS=","	Output is comma-separated

Explanation

By default, awk uses a "single space" for OFS, so the output is space-delimited even without specifying anything. By changing OFS, you can flexibly control the output format.

Conditions Under Which OFS Takes Effect

Creating the File

cat << 'EOF' > input.txt
a b c
1 2 3
EOF

Command

awk '{print $1, $2, $3}' input.txt

Output

a b c
1 2 3

Command

awk 'BEGIN{OFS=","} {print $1, $2, $3}' input.txt

Output

a,b,c
1,2,3

Command

awk '{print $1 "-" $2 "-" $3}' input.txt

Output

a-b-c
1-2-3

How It Works

Condition	OFS Effect	Description
print $1, $2 with comma separation	Affected	OFS is used as the separator between fields
BEGIN{OFS=","} specified	Enabled	Changes the delimiter character at output time
String concatenation like $1 "-" $2	Not affected	Concatenation is treated as a single string, so OFS is not used

Explanation

OFS is applied only "when multiple fields are output with comma separation in print."
In the case of string concatenation, the result becomes a single value, so OFS is not involved.

How to Specify Major Delimiter Characters

Creating the File

cat << 'EOF' > input.txt
apple orange banana
cat dog mouse
red blue green
EOF

Command

awk '{print $1, $2, $3}' input.txt

Output

apple orange banana
cat dog mouse
red blue green

Command

awk 'BEGIN{OFS=","} {print $1, $2, $3}' input.txt

Output

apple,orange,banana
cat,dog,mouse
red,blue,green

How It Works

Item	Description
OFS	Output Field Separator
Default	Space
How to set	BEGIN{OFS="delimiter"}
Scope of effect	Applied between items comma-separated in print
Note	Not applied when fields are listed without commas like $1 $2

Explanation

OFS is a variable for changing the output delimiter when fields are comma-separated in print. It is commonly used when converting to CSV format and similar formats.

Setting Multibyte Characters or Symbols as Delimiters

Creating the File

cat << 'EOF' > input.txt
apple banana cherry
dog elephant fox
EOF

Command

awk 'BEGIN{OFS="★"} {print $1,$2,$3}' input.txt

Output

apple★banana★cherry
dog★elephant★fox

Command

awk 'BEGIN{OFS="あいう"} {print $1,$2,$3}' input.txt

Output

appleあいうbananaあいうcherry
dogあいうelephantあいうfox

How It Works

Element	Description
OFS	Output Field Separator
BEGIN	A block executed only once before input processing
$1,$2,$3	Each field (space-separated)
print	Joins fields with OFS and outputs
Multibyte characters	Japanese and symbols can also be set in OFS

Explanation

OFS specifies the delimiter at output time, and multibyte characters such as Japanese and symbols can be used without issue. This enables flexible formatted output.

Technique for Rebuilding $0 to Forcibly Apply OFS

Creating the File

cat << 'EOF' > input.txt
apple orange banana
dog cat mouse
EOF

Command

awk 'BEGIN{OFS=","} {$1=$1; print}' input.txt

Output

apple,orange,banana
dog,cat,mouse

How It Works

Element	Description
OFS	Output Field Separator
$1=$1	Trigger that reassigns a field to force $0 to be rebuilt
$0	The entire line (OFS is applied when rebuilt)
print	Outputs the rebuilt $0

Explanation

By using $1=$1, awk rebuilds the line, and OFS is forcibly applied at that point.
This allows the output format to be unified regardless of the input delimiter.

Output Control Through NF (Number of Fields) Manipulation and Its Relationship with OFS

Creating the File

cat << 'EOF' > input.txt
apple orange banana
cat dog
red blue green yellow
EOF

Command

awk '{print $1, $2}' input.txt

Output

apple orange
cat dog
red blue

Command

awk 'BEGIN{OFS=","} {print $1, $2}' input.txt

Output

apple,orange
cat,dog
red,blue

Command

awk '{NF=1; print $0}' input.txt

Output

apple
cat
red

Command

awk 'BEGIN{OFS="-"} {NF=2; print $0}' input.txt

Output

apple-orange
cat-dog
red-blue

How It Works

Element	Description	Behavior
NF	Number of fields (columns)	Changing the value changes the number of fields included in output
$1, $2	Each field	Outputs only the specified columns
OFS	Output field separator	Controls the delimiter character at print time
NF=1	Field reduction	Keeps only column 1 and removes the rest
NF=2	Field adjustment	Limits to 2 columns and rebuilds
print $0	Entire line	The content after NF change is reconstructed and output

Explanation

When NF is rewritten, awk reconstructs the record, and OFS is used as the delimiter at that point.
In other words, NF manipulation and OFS work together to determine the output format.

Flexible Line Break Control Combined with ORS (Output Record Separator)

Creating the File

cat << 'EOF' > input.txt
apple orange banana
cat dog elephant
red blue green
EOF

Command

awk 'BEGIN{ORS=" | "} {for(i=1;i<=NF;i++) print $i}' input.txt

Output

apple | orange | banana | cat | dog | elephant | red | blue | green | 

Command

awk 'BEGIN{OFS=", "; ORS="\n---\n"} {$1=$1; print}' input.txt

Output

apple, orange, banana
---
cat, dog, elephant
---
red, blue, green
---

How It Works

Element	Role	Description
ORS	Output Record Separator	Changes the delimiter per print, controlling line breaks and arbitrary strings
OFS	Output Field Separator	Specifies the delimiter character between fields
NF	Number of fields	Used to handle all fields in loop processing
$1=$1	Rebuild trigger	Re-outputs the entire line reflecting OFS

Explanation

Using ORS allows flexible control of the output format including line breaks, and combining it with OFS greatly increases formatting freedom.
It works especially powerfully for log formatting and one-liner processing.

Fixed Format and Dynamic Delimiters

Creating the File

cat << 'EOF' > input.txt
apple 100 red
banana 200 yellow
grape 300 purple
EOF

Command

awk 'BEGIN{OFS=","} {print $1, $2, $3}' input.txt

Output

apple,100,red
banana,200,yellow
grape,300,purple

Command

awk 'BEGIN{OFS=" | "} {OFS="-"; print $1, $2, $3}' input.txt

Output

apple-100-red
banana-200-yellow
grape-300-purple

How It Works

Element	Description
awk	Text processing tool
BEGIN{}	Executed only once before processing begins
OFS	Output Field Separator
$1,$2,$3	Specifies each column (field)
print	Outputs the specified fields
Dynamic delimiter	Output format can be flexibly changed by modifying OFS

Explanation

By setting OFS, you can freely change the delimiter character at output time.
This makes it easy to generate different formats from the same data.

Field Joining by Setting an Empty String

Creating the File

cat << 'EOF' > input.txt
A 1
B 2
C 3
EOF

Command

awk 'BEGIN{OFS=""} {print $1, $2}' input.txt

Output

A1
B2
C3

How It Works

Element	Description
OFS	Output Field Separator
OFS=""	Sets the delimiter to empty
print $1, $2	Fields are originally joined by OFS between them
Result	The delimiter disappears and fields are concatenated

Explanation

By setting OFS to an empty string, the delimiter between fields disappears and they are concatenated. This is a fundamental technique for output control in awk.

CSV Format Data Conversion Flow

Creating the File

cat << 'EOF' > input.txt
name age city
Alice 25 Tokyo
Bob 30 Osaka
EOF

Command

awk 'BEGIN {OFS=","} {print $1, $2, $3}' input.txt

Output

name,age,city
Alice,25,Tokyo
Bob,30,Osaka

Command

awk 'BEGIN {OFS=","} NR>1 {print $1, $2, $3}' input.txt

Output

Alice,25,Tokyo
Bob,30,Osaka

How It Works

Item	Description
awk	Text processing tool
OFS	Output Field Separator
BEGIN	Executed only once before processing
NR	Line number (Number of Records)
print	Joins fields with OFS and outputs

Explanation

By using awk's OFS, you can easily convert space-delimited data to CSV.
The key point is to specify OFS in BEGIN.

Practical Summary for Mastering awk’s OFS

By correctly understanding the relationship between awk and ofs, you can use it as an advanced data formatting tool, going one step beyond a simple text processing tool.

You may be confused by the differences in behavior at first, but by grasping the key points introduced in this article, you will be able to consistently obtain the intended output.