tr

wikipedia:tr (Unix) 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

tr --help
tr: illegal option -- -
usage: tr [-Ccsu] string1 string2
       tr [-Ccu] -d string1
       tr [-Ccu] -s string1
       tr [-Ccu] -ds string1 string2

Related commands

• awk
• paste
• cut
• column

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