With perl:

perl -pe 's/\n/, / unless eof' IP

Add the -i option to edit the file inplace. Or -i.orig to edit it in place but keep the original as IP.orig.

With the zsh shell, you can also load those IPs into an array with:

ips=( ${(f)"$(<IP)"} )

And then write them back joined with commas with:

print -r -- ${(j[, ])ips} > IP

With perl:

perl -pe 's/\n/, / unless eof' IP

Add the -i option to edit the file inplace. Or -i.orig to edit it in place but keep the original as IP.orig.

With the zsh shell, you can also load those IPs into an array with:

ips=( ${(f)"$(<IP)"} )

And then write them back joined with commas with:

print -r -- ${(j[, ])ips} > IP

Contrary to the perl one which processes one line at a time, that one loads the file whole in memory.

Two other differences:

• empty lines are discarded
• a newline character is always added at the end even if the original file didn't contain any like for an empty file or a file whose last line was not properly delimited.

With perl:

perl -pe 's/\n/, / unless eof' IP

Add the -i option to edit the file inplace. Or -i.orig to edit it in place but keep the original as IP.orig.

With perl:

perl -pe 's/\n/, / unless eof' IP

Add the -i option to edit the file inplace. Or -i.orig to edit it in place but keep the original as IP.orig.

With the zsh shell, you can also load those IPs into an array with:

ips=( ${(f)"$(<IP)"} )

And then write them back joined with commas with:

print -r -- ${(j[, ])ips} > IP