These commands will work with most (if not all) distributions of Linux as well as most (?) implementations of Unix.
Communication
ftp – File Transfer Protocol.
login – Sign on.
rlogin – Sign on to remote system.
rsh – Run shell or single command on remote system.
talk -Exchange messages interactively with other terminals.
telnet – Connect to another system.
tftp – Trivial file transfer protocol.
Comparisons
cmp – Compare two files, byte by byte.
comm – Compare items in two sorted files.
diff – Compare two files, line by line.
diff3 – Compare three files.
File Management
cat – Concatenate files or display them.
chfn -Change user information for finger, email, etc.
cksum – Compute checksum.
chmod -Change access modes on files.
chsh – Change login shell.
cp – Copy files.
csplit -Break files at specific locations.
dd -Copy files in raw disk form.
file – Determine a file’s type.
head – Show the first few lines of a file.
less – Display files by screenful.
ln – Create filename aliases.
ls – List files or directories.
merge – Merge changes from different files.
mkdir -Create a directory.
more – Display files by screenful.
mv – Move or rename files or directories.
newgrp – Change current group.
pwd – Print working directory.
rcp – Copy files to remote system.
rm -Remove files.
rmdir -Remove directories.
split – Split files evenly.
tail – Show the last few lines of a file.
wc – Count lines, words, and characters.