Find out EXACTLY what shell is being used

[for.i.am.root]

Hi All:

So I was browsing the web today and came across a post where an individual was interested in finding out what shell they are using. There were several responses: echo $0, ps -p $$, echo $SHELL, however, none of them hit the mark (IMHO). If you were, for example, using Ubuntu 9.04 /bin/sh is a symlink to /bin/dash. If you ran echo $0 or ps -p $$ while using /bin/sh (/bin/dash) they would both report /bin/sh as the shell, not /bin/dash (the real interpretor). Furthermore if you ran echo $SHELL while using /bin/sh it would report /bin/bash (the login shell).

I wrote a little snippet of code to try to figure out what shell is actually being used, however, have hit a little bit of a thunker.

CODE:
var=`ps -p $$ | sed 's/[0-9]*//g' | sed -re 's/^.+\///g' | sed -re 's/://g' | sed -re 's/[A-Z]*//g' | sed -re 's/ //g'`
echo $var | grep "/" > /dev/null
if ! [ $? -eq 0 ]; then fullvar=`echo "/bin/"$var | sed -re 's/ //g'`; fi
readlink $fullvar > /dev/null
if ! [ $? -eq 0 ]
then echo $fullvar" is not a link. It must be the REAL shell interpretor."
else
fullvar=`readlink -f $fullvar` > /dev/null
var=`echo $var | sed -re 's/ //g'`
echo $fullvar" is the REAL shell interpretor, not /bin/"$var
fi

Obviously the first thing that stands out is how sloppy it is.
The second thing that stands out is that if the interpretor is outside of /bin the output would be wrong.
The third is using multiple symlinks to "fool" the script.

Any ideas on how to "fix" it?

Or better yet is there a one liner to figure out the current shell interpretor?

Thanks!

[ArgusVision]

Nothing against Mr "root",but since we're in the "Absolute Beginner Talk" Forum. I'd like to reiterate the warning not to run code/scripts unless you know what it does.

Again, let me say this is just a generic warning, not any implication against root.

EDIT: @ for.i.am.root, If I can make a suggestion, you might get better answers in the "Programming Talk" forum. (I think they cover scripting as well...)
http://ubuntuforums.org/forumdisplay.php?f=39

[for.i.am.root]

Hi Vision:

Based on the grammar and punctuation used, your post seems more like a rebuke than a suggestion.

For those that are interested:

ps - report a snapshot of the current process
sed - stream editor for filtering and transforming text
readlink - display value of a symbolic link

The rest is (hopefully) self explanatory.

If a mod finds that this post is better suited in the "Programming Talk" forum, I would have no objections to it being moved.

Thanks!

[uRock]

If a mod finds that this post is better suited in the "Programming Talk" forum, I would have no objections to it being moved.

Thanks!

Done!

[ArgusVision]

Please accept my apologies. It's dificult to express tone in type.
I simply think that you'll find more people with scripting and programming experience viewing your post there than in here.
The use of quotes is to indicate that it is an exact name being used for that forum.

[for.i.am.root]

Hi uRock:

You Rock! Thanks!

Hi Vision:

Thanks for the suggestion!

[cyberslayer]

You might want to check out pwd -P.

[saulgoode]

Or better yet is there a one liner to figure out the current shell interpretor?

The following will print the full path wrapped in quotes.

stat -c%N $(which `ps -o comm= $$`)|awk -F' ' '{print $NF}'

Note that the $(command) construct is not available in the original Bourne shell (sh) -- if it is not supported in your shell(s) then you will have to separate things out a bit more (backticks can't be nested).

Additionally, 'stat' appears to only perform one level of dereferencing of symbolic links.

[for.i.am.root]

Hi saulgoode:

stat -c%N $(which `ps -o comm= $$`)|awk -F' ' '{print $NF}'

Thanks!

That worked really well.

jay@DARKSTAREEE:~$ stat -c%N $(which `ps -o comm= $$`)|awk -F' ' '{print $NF}'
`/bin/bash'
jay@DARKSTAREEE:~$ /bin/dash
$ stat -c%N $(which `ps -o comm= $$`)|awk -F' ' '{print $NF}'
`/bin/dash'
$ /bin/sh
sh-3.2$ stat -c%N $(which `ps -o comm= $$`)|awk -F' ' '{print $NF}'
`bash'
sh-3.2$ exit
exit
$ exit
exit
jay@DARKSTAREEE:~$

For those that are interested:

stat - display file or file system status
-c --format=FORMAT
use the specified FORMAT instead of the default; output a new‐line after each use of FORMAT
%N
Quoted file name with dereference if symbolic link

[spjackson]

Hi saulgoode:

stat -c%N $(which `ps -o comm= $$`)|awk -F' ' '{print $NF}'

Thanks!

That worked really well.

We have a bit of chicken and egg situation here. The syntax works for those shells for which it works, and doesn't work for other shells where this is the wrong syntax, csh for instance.

Also, what if /bin/sh was a hard link to dash? It isn't, but what if it was? ( /bin/uncompress and /bin/gunzip are hard links to the same file for example, so it's conceivable that shells could do this in a future release.)

Why doesn't using shebang solve the problem and tell you everything you need to know?