On 05-Mar-20 6:57, Doug McIlroy wrote: Tail acquired a -r option between 3BSD [1] and 4BSD [2]. I remember using that option on SunOS in 1990 as part of a prank we played on a friend at the university. On the Sun 3 workstations we were using at the time, one could enter the monitor/debugger program by pressing L1-A. By remotely logging into a workstation and running a shell loop, one could ensure that when the monitor was entered the active program would be that shell. It was then easy to modify the uid field for the active process (the loop-running shell) and set it to zero. After exiting the monitor, a subshell launched from that shell would have full root privileges. All we had to do was wait for the friend to lock his workstation when taking a break in order to obtain root privileges on his workstation and then change to his uid in order to modify his files via NFS on the university's Gould file server. Based on this capability, I wrote the following script that would rename all our friend's files and directories to words from the dictionary. The script also created (via tail -r) another script that would undo this change. #!/bin/sh TMP=/tmp DIR=$1 FILES=$TMP/f.$$ WORDS=$TMP/w.$$ CMD=$TMP/c.$$ REV=$TMP/r.$$ trap '' 0 1 2 3 15 find $DIR -depth -print >$FILES head -`wc -l <$FILES|sed 's/[ ]*//'` /usr/dict/words >$WORDS paste $FILES $WORDS | sed -e ' /^\. /d s/\(.*\)\/\(.*\) \(.*\)/mv \1\/\2 \1\/\3/ ' >$CMD rm $FILES $WORDS tail -r $CMD | sed -e ' s/mv \(.*\) \(.*\)/mv \2 \1/ ' >$REV sh <$CMD rm $CMD Unfortunately, it turned out that tail -r had a limit on the number of lines it could reverse. Although the script and its undo worked fine on a test set of a small number of files, when run on our friend's directory it created a faulty undo script. Our friend ended up graduating with files named "abaca" and "abacinate". [1] https://dspinellis.github.io/manview/?src=https%3A%2F%2Fraw.githubuserconte… [2] https://dspinellis.github.io/manview/?src=https%3A%2F%2Fraw.githubuserconte… -- Diomidis Spinellis Free edX MOOC on Unix Tools: Data, Software, and Production Engineering https://www.spinellis.gr/unix?tuhs20200306