The -s option is one of the useful, but lesser known feature of the paste command. It helps you to serialize input file contents to a single output line.

$ cat colors.txt

$ paste -sd, colors.txt

If multiple files are passed, serialization of each file is displayed on separate output lines.

$ paste -sd: <(seq 3) <(seq 5 9)

The advantage of using paste instead of other options like tr, awk, etc is that you do not have to worry about trailing delimiters, newlines, etc. For example:

# issue 1: trailing comma
# issue 2: no newline at the end
$ <colors.txt tr '\n' ','

# correcting the above two issues
$ <colors.txt tr '\n' ',' | sed 's/,$/\n/'

Here's an equivalent awk solution for single file input. While slower and complicated compared to the paste solution, you get more flexibility since awk is a programming language. For example, it is pretty easy to use multicharacter output delimiter.

$ awk -v ORS= 'NR>1{print ","} 1; END{print "\n"}' colors.txt

$ awk -v ORS= 'NR>1{print " : "} 1; END{print "\n"}' colors.txt
blue : white : orange

Video demo:

info See paste command chapter from my Command line text processing with GNU Coreutils ebook for more details.

info See my CLI text processing with GNU awk ebook if you are interested in learning about the awk command.