tr

wikipedia:tr translate characters

• https://man7.org/linux/man-pages/man1/tr.1p.html

Concatenate lines on a file:

• cat file | tr '\n' ' '
• cat file | xargs

Concatenate/merge every 2 lines:

cat /path/tofile.txt | paste -d " "  - -

cat file | tr '\n' 
tr: when not truncating set1, string2 must be non-empty

Related commands

• awk
• paste
• cut

See also

• cat, tac, more, less, tail, mtail, echo, stdin, tr, column, paste, truncate, logrotate, xzcat pv, progress, sort, ncat, virt-cat, awk, join, col, fold, tee, sponge, nfs-cat, journalctl, ccat, icat, EOF, lolcat
• file, File system, directory, touch, mkdir, ls, ln truncate, fallocate, split, stat, inode, File descriptor, superblock, block size (blockdev), fuser, lsof, scrub, chattr, ulimit (nofiles), cp, mv, file (command), mkfifo, chmod

https://stackoverflow.com/a/9605450