This is useful:

  • when you want to exclude text on a line which does not match
  • when there are multiple matches on a line and you want to show each match on a separate line, or

See also thread.

Help

 -o, --only-matching

macOS:

Prints only the matching part of the lines.

Linux:

Show only nonempty parts of lines that match

Usage

$ grep -o FILE PATTERN

Examples

Partial

Given file:

  • go.mod 
      # ...
    
  go 1.16
    
  # ...

If you do a plain grep, you’ll get the entire line. In color mode, only the number will be highlighted like go **1.16**.

go 1.16
    
    github.com/cpuguy83/go-md2man/v2 v2.0.1 // indirect
    github.com/urfave/cli/v2 v2.3.0

Also, that match is too broad on the other lines.

And using groups doesn’t work. With with -o, the entire line is shown.

$ egrep -o go.mod '(\d\.\d+)
go 1.16

But you can chain grep commands together.

$ grep '^go' go.mod | egrep -o '(\d\.\d+)'
1.16

Multiple matches

With the flag:

$ echo 'abcd abcdef' | grep -o bc
bc
bc

Without:

$ echo 'abcd abcdef' | grep bc
abcd abcdef