How to concatenate multiple lines of output to one line?

Question

Use tr '\n' ' ' to translate all newline characters to spaces:

$ grep pattern file | tr '\n' ' '

Note: grep reads files, cat concatenates files. Don't cat file | grep!

Edit:

tr can only handle single character translations. You could use awk to change the output record separator like:

$ grep pattern file | awk '{print}' ORS='" '

This would transform:

one
two 
three

to:

one" two" three" 

Piping output to xargs will concatenate each line of output to a single line with spaces:

grep pattern file | xargs

Or any command, eg. ls | xargs. The default limit of xargs output is ~4096 characters, but can be increased with eg. xargs -s 8192.

grep xargs

In bash echo without quotes remove carriage returns, tabs and multiple spaces

echo $(cat file)

This could be what you want

cat file | grep pattern | paste -sd' '

As to your edit, I'm not sure what it means, perhaps this?

cat file | grep pattern | paste -sd'~' | sed -e 's/~/" "/g'

(this assumes that ~ does not occur in file)

This is an example which produces output separate by commas. You can replace the comma by whatever separator you need.

cat <<EOD | xargs | sed 's/ /,/g'
> 1
> 2
> 3
> 4
> 5
> EOD

produces:

1,2,3,4,5