The Tiny Unix And Linux Shell Crash Course For Beginners

Hey there,

Today's been a long day, filled with fun and gainful employment. Unfortunately, nothing I did today was the least bit interesting to write about ;)

So, in the mean time - in between time - I came up with a little something to help a few new guys get used to the shell (bash in this instance, but most of this will work in any shell... except the c shell - nothing is compatible with the c shell except tcsh and ...the c shell ;)

The setup is simple for this crash course, and (we'll consider ourselves the teachers) you should let the student know that they're in for a fun little piece of problem solving. Maybe, if I had the time to give this some more thought, it could be a pretty decent (although probably unusually cruel ;) interview tactic. If you have the time (and the stomach) to do that to some poor bastard that's looking to make a buck; go for it. I'm a pretty harsh interview, but I don't know if this is something I'd do to someone who didn't already know that their future employment was ensured (pending the outcome of drug testing, of course. Quite frankly, if you don't have what it takes to do your job while you're loaded to the gills, I don't think you'll not be able to see right through my fragile mask of sanity ;)

Anyway. Load up your mark's .profile (or .bashrc, etc) with this little collection of shell aliases and see how long it takes them to break out and get a normal shell back. Plenty of folks can do it without thinking. Anyone who can't will learn a little something.

And, please remember, this isn't about judgement; it's about pushing someone (even yourself) to think outside the box (or whatever object they're figuratively trapped in ;). We all need to be able to think in that abstract space every once in a while. Why not make it fun :)

Cheers,

PS1="youAreHere/ "
export PS1
alias --='-'
alias ash='echo youAreHere/ '
alias bash='echo youAreHere/ '
alias cat='perl -l'
alias cd='cd .'
alias cp='echo cp'
alias csh='echo youAreHere/ '
alias df='echo /dev/dump 100% 100% 0% /tmp/jailfs/hoosegow/you'
alias echo='echo '
alias emacs='ed'
alias env='vmstat'
alias ex='ed'
alias exec='echo cannot fork'
alias exit='echo are you sure?'
alias id='echo user\(me\) group\(sadly the same\)'
alias joe='ed'
alias jsh='echo youAreHere/ '
alias kill='echo all dead'
alias ksh='echo youAreHere/ '
alias less='more ---x'
alias logout='echo are you sure?'
alias ls='echo .'
alias mkdir='echo making directory'
alias more='less </dev/null'
alias mv='echo stay'
alias netstat='cat /dev/random'
alias ping='ping /dev/null'
alias prompt='echo youAreHere/'
alias ps='echo you 501 501 0 Apr 1 ? 0:00 /usr/bin/vicks -vaporub'
alias pwd='echo you are here'
alias rm='echo can\'\''t find'
alias rmdir='removing directory'
alias set='iostat'
alias sh='echo youAreHere/ '
alias su='echo cannot su to'
alias sudo='exec'
alias touch='echo please don\'\''t touch'
alias unalias='echo no aliases found named'
alias vi='ed'
alias vim='ed'
alias w='echo x, y and z'
alias who='echo what?'
alias zsh='echo youAreHere/ '
# alias alias="sleep 5"

# Uncommenting that last line is optional ;)

, Mike

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Back To Basics: Avoiding Recursive Alias Disasters On Linux And Unix

Hey there,

Today, we're going to step into the wayback machine and take a look at something very basic that, if ignored or forgotten, can potentially have devastating side-effects. And, I'm not referring to the way I used to drink in college ;) This has more to do with an op-ed piece we ran a while back about paying attention to the small things. In fact, it has exactly to do with that.

The topic for today's blast of hot air is "infinite recursion." Specifically, we'll be looking at a kind of insidious way of getting stung by that issue through the use of shell aliases. They seem harmless, and they do make it so you can, for instance, type "sit-stay" instead of "while :;do print -n "*";sleep 15;done" when you want to walk away from your terminal and not disconnect your SSH session, but they do have a down side, which is fairly easy to exploit. In my experience, these cpu-killers happen most often "by accident" rather than with any malicious intent. Nevertheless, their very existence cries out for caution (Tomorrow I might write some fiction to get all these fifty cent phrases out of my system. Although some folks may contend that I've done enough of that on this blog already ;)

The infinite recursion problem with aliases closely parallels a very simple exploit that can be run in a simple shell script. The ends are the same, and the means are closely related. For instance, if you create a shell script called "ls," with the contents:

#!/bin/sh

ls;sleep 1500000000

and manage to get that in a user's PATH before the real /bin/ls (or /usr/bin/ls), it'll ratchet up the number of open processes and filehandles very quickly. If let sit, it will take down any machine of any size eventually. This end can actually be accomplished even more simply than that, but we're not trying to encourage reckless behaviour; just rubbernecking a little ;)

The problem you can get into with simple shell aliases is when you have an alias for a command (like "ls") that actually already exists. I will usually make my aliases unique (like "sit-stay," above, which substitutes for an asterisk-printing while loop), but not everyone does. And, sometimes, an alias that is unique can become the opposite if you change OS's. For instance "ll" doesn't exist on Solaris, but it does on HP-UX.

It's very important that, when you create an alias which is named the same as a system binary (or shell built-in) that you compensate for that fact. It's actually made very easy to protect yourself, from yourself, by pretty much every shell I've ever worked with. At the most basic level, you can instruct your shell to not even attempt alias expansion of whatever you type on the command line by simply enclosing it in quotes (single or double should both work - If not, let me know what shell you're using. It would be interesting to have a list of shells that make this distinction). So, while:

host # cd /tmp

would (and I'm oversimplifying this by a preposterous degree) cause the shell to check, in order, if "cd" was an alias, a shell built-in, a function, an actual binary in our PATH or an erroneous entry (stopping at whatever point returns true first), typing:

host # "cd" /tmp

would remove the "alias check" from the equation. Problem solved and/or potential for damage neutralized. Just like the simple shell script above, if you created an alias for "cd" (which might differ from this shell to yours) like this one:

host # alias cd="cd $@"

you could have a problem on your hands. Lord knows why you'd want to do this, since you'd still be typing "cd wherever" either way, but I'm just being obtuse to make a point ;) In this situation, every time someone types something like:

host # cd /my/home/dir

The shell will determine that "cd" is an alias before it processes the command and, while it's processing the alias it will see that the actual command (to which the alias refers) contains an alias, which it will process, etc, etc, and, before you know it, system resources will become depleted to the point that no new processes can be forked. The machine, however, will be completely forked ;)

The preferred way (or so they tell me) is to encapsulate your alias in a function of the type _NAME (for the command NAME) (even though you, theoretically, only have to make sure your alias has the "real" command in quotes):

function _cd {
 "cd" $@
}
alias cd="_cd"

If you're lazy, like me, you'll just do this, instead:

alias cd='"cd" $@'

And, of course, if you want to have your alias/function print out where you've cd'ed to, you'll need to keep in mind that the return code (errno) for cd will be overwritten when you do the print or echo. This is easy enough to handle by grabbing the value of $?, and we use it in a lot of our scripts:

function _cd {
 "cd $@"
 return=$?
 echo $PWD
        return $return
}
alias cd="_cd"

and you shouldn't have to worry about recursive calling of the "cd" (or any) command ever again. Assuming you always follow these precautions when dreaming up your aliases :)

Cheers,

, Mike

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