Once I had to rescue a SunOS system and edit /etc/vfstab but I only had ed to my disposal. At that point I had never touched ed before, so that was a bit of a learning exercise, for sure.
In a computer lab I hung out in back in college...
We used rcp to keep passwords in sync. Add the account on the main machine, rcp the password file to the other machine. sudo rcp /etc/password other:/etc/passwd was muscle memory.
One day, someone was getting added to the groups file to be able to work in the server web project. sudo rcp /etc/group other:/etc/passwd
Ooops. Couldn't log in to fix it.
"Is anyone logged into the other machine?" (someone said yes). "Type while 1 sync" ... (ok) ... And we flipped the power switch and brought it up in single user mode (since the password file was invalid). Next, need to establish a minimal /etc/passwd ... emacs /etc/passwd (nope) vi /etc/passwd (nope - invalid terminal 300h not in termcap). "Uhm... cat > /etc/passwd ?" (possible, but a PITA when there is a typo in transcription)
I was a wizard on a lpmud. "I know ed".
And we got a minimal password file restored while reading the hashed values over (no way where we going to have root::0:0:... as the file even for a second) and then rcp'ed the proper /etc/passwd and /etc/group file over to the other machine.
