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Tidy up and correct the new example

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wip-fakessh-exit-status
David Wilson 7 years ago
parent eb2aef866b
commit b580ada89f
1 changed files with 71 additions and 74 deletions
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145
docs/examples.rst
Unescape Escape View File

@ -12,7 +12,7 @@ an ``if`` statement as root on a file server behind a bastion host:
.. code-block:: bash
ssh bastion "
if \"$PROD\";
if [ \"$PROD\" ];
then
ssh fileserver sudo su -c \"
if grep -qs /dev/sdb1 /proc/mounts;
@ -27,31 +27,28 @@ an ``if`` statement as root on a file server behind a bastion host:
sudo touch /var/run/start_backup;
"
Chances are high this is familiar territory, we've all seen it, and many of us
working in infrastructure have almost certainly even written it. At first
glance, ignoring that annoying quoting, it looks perfectly fine: well
structured, neatly indented, and the purpose of the snippet seems clear. But I
have some questions:
Chances are high this is familiar territory, we've all seen it, and those
working in infrastructure have almost certainly written it. At first glance,
ignoring that annoying quoting, it looks perfectly fine: well structured,
neatly indented, and the purpose of the snippet seems clear.
1. At first glance, can you say if ``"/media/Main Backup Volume"`` is quoted
correctly?
1. At first glance, is ``"/media/Main Backup Volume"`` quoted correctly?
2. How will the ``if`` statement behave if there is a problem with the machine,
and, say, the ``/bin/grep`` binary is absent?
3. Ignoring quoting, can you spot any other syntax problems?
4. If you cutpaste this snippet from its original script into an interactive
shell, will it behave the same as before?
3. Ignoring quoting, are there any other syntax problems?
4. If this snippet is pasted snippet from its original script into an
interactive shell, will it behave the same as before?
5. Can you think offhand of differences in how the arguments to ``sudo
...`` and ``ssh fileserver ...`` are parsed?
6. In which context will the ``*`` glob be expanded, if it is expanded at all?
7. What will the exit status of ``ssh bastion`` be if ``ssh fileserver`` fails?
Innocent But Deadly
~~~~~~~~~~~~~~~~~~~
And now some answers:
1. No, the quoting used is nonsense! At best, ``mount`` will receive 3
arguments. At worst, the snippet will not parse at all.
1. The quoting used is nonsense! At best, ``mount`` will receive 3 arguments.
At worst, the snippet will not parse at all.
2. The ``if`` statement will treat a missing ``grep`` binary (exit status 127)
the same as if ``/dev/sdb1`` was not mounted at all (exit status 1). Unless
the program executing this script is parsing ``stderr`` output, the failure
@ -59,19 +56,23 @@ And now some answers:
``rm`` was executed, it got wiped.
3. There is at least one more syntax error present: a semicolon missing after
the ``umount`` command.
4. Depending in which environment the ``PROD`` variable is set, either it will
always evaluate to false, because it was set by the bastion host, or it
will do the right thing, because it was set by the script host.
5. If you cutpaste the snippet into an interactive shell, the apparently quoted
"!" character in the ``echo`` command will be interepreted as a history
4. If you paste the snippet into an interactive shell, the apparently quoted
"!" character in the ``echo`` command will be interpreted as a history
expansion.
6. Nobody knows this one! ``sudo`` preserved the remainder of the argument
vector as-is, whereas ``ssh`` **concatenates** each part into a single
string that is passed to the login shell.
7. As for where the glob is expanded, the answer is I have absolutely no idea
5. ``sudo`` preserves the remainder of the argument vector as-is, while
``ssh`` **concatenates** each part into a single string that is passed to
the login shell. While quotes appearing within arguments are preserved by
``sudo``, without additional effort, pairs of quotes are effectively
stripped by ``ssh``.
6. As for where the glob is expanded, the answer is I have absolutely no idea
without running the code, which might wipe out the backups!
7. If the ``ssh fileserver`` command fails, the exit status of ``ssh bastion``
will continue to indicate success.
8. Depending in which environment the ``PROD`` variable is set, either it will
always evaluate to false, because it was set by the bastion host, or it
will do the right thing, because it was set by the script host.
Golly, we've managed to hit at least 7 potentially mission-critical gotchas in
Golly, we've managed to hit at least 8 potentially mission-critical gotchas in
only 14 lines of code, and they are just those I can count! Welcome to the
reality of "programming" in shell.
@ -79,63 +80,60 @@ In the end, superficial legibility counted for nothing, it's 4AM, you've been
paged, the network is down and your boss is angry.
The Madness That Is Shell Quoting
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Shell Quoting Madness
~~~~~~~~~~~~~~~~~~~~~
Let's assume on first approach that we really want to handle those quoting
issues. I wrote a little Python 3 script based around the
:py:func:`shlex.quote` function to help construct, to the best of my knowledge,
the quoting required for each stage of the command:
::
>>> c1 = q('if grep -qs /dev/sdb1 /proc/mounts ; then echo "sdb1 already mounted!"; unmount /dev/sdb1; fi; mount /dev/sdb1 "/media/Main Backup Volume"')
>>> c2 = qq('bash', '-c', c1)
>>> c3 = qq('sudo', 'su', '-c', c2)
>>> c4 = qq('bash', '-c', 'if $PROD; then ssh 'diskserver', c3)
>>> c5 = 'if "$PROD"; then %s; fi; sudo touch /var/run/start_backup' % (c4,)
>>> c6 = qq('ssh', 'bastion', qq('bash', '-c', c5))
>>> print(c6)
And now, the output:
issues. I wrote a little Python script based around the :py:func:`shlex.quote`
function to construct, to the best of my knowledge, the quoting required for
each stage:
.. code-block:: bash
ssh bastion 'bash -c '"'"'if "$PROD"; then ssh diskserver '"'"'"'"'"'"'"
'"'sudo su -c '"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'bash
-c '"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"
'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'
"'"'"'"'"'"'"'"'"''"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'
"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"
'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'
"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"
'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'
"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"
'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'if grep
-qs /dev/sdb1 /proc/mounts ; then echo "sdb1 already mounted!"; unmount
/dev/sdb1; fi; mount /dev/sdb1 "/media/Main Backup Volume"'"'"'"'"'"'"'"'
"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"
'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'
"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"
'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'
"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"
'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'
"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"''"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'
"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"
'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"''"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'
"'"'"'"'"'"'"'"'"'"''"'"'"'"'"'"'"'"'; fi; sudo touch /var/run/start_backup'"'"''
Even with Python handling all the heavy lifting of correctly quoting each layer
of shell, and producing the monstrosity that is a syntactically correct result,
ssh bastion '
if [ "$PROD" ];
then
ssh fileserver sudo su -c '"'"'
if grep -qs /dev/sdb1 /proc/mounts;
then
echo "sdb1 already mounted!";
umount /dev/sdb1
fi;
rm -rf "/media/Main Backup Volume"/*;
mount /dev/sdb1 "/media/Main Backup Volume"
'"'"';
fi;
sudo touch /var/run/start_backup
'
Even with Python handling the heavy lifting of quoting each shell layer, and
and even if we fixed the aforementioned minor disk-wiping issue, I am still not
100% confident that I know precisely the argument handling rules for all of
``su``, ``sudo``, ``ssh``, and ``bash``.
Finally, if any of the login shells involved may not be set to ``bash``, we
must introduce additional layers of quoting, in order to explicitly invoke
``bash`` at each stage, causing an explosion in quoting:
.. code-block:: bash
ssh bastion 'bash -c '"'"'if [ "$PROD" ]; then ssh fileserver bash -c '"'"'
"'"'"'"'"'"'sudo su -c '"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"
'bash -c '"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"
'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'
"'"'"'"'"'"'"'"'"'"'if grep -qs /dev/sdb1 /proc/mounts; then echo "sdb1 alr
eady mounted!"; umount /dev/sdb1 fi; rm -rf "/media/Main Backup Volume"/*;
mount /dev/sdb1 "/media/Main Backup Volume"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"
'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'
"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"''"'"'"'"'"'"'"'"'"'"'
"'"'"'"'"'"'"'"'"'"'"'"'"'"'"'"''"'"'"'"'"'"'"'"'; fi; sudo touch /var/run/
start_backup'"'"''
There Is Hope
~~~~~~~~~~~~~
We could instead express the above using Mitogen directly in Python code:
We could instead express the above using Mitogen:
::
@ -163,13 +161,12 @@ We could instead express the above using Mitogen directly in Python code:
bastion_sudo.call(run, 'touch', '/var/run/start_backup')
And now, a few more questions:
* Can you tell in which context the ``PROD`` variable must be defined?
* Can you tell on which machine each step executed?
* Can you see any escaping issues?
* In which context must the ``PROD`` variable be defined?
* On which machine is each step executed?
* Are there any escaping issues?
* What will happen if the ``grep`` binary is missing?
* What will happen if any step fails?
* What will happen if any login shell is not ``bash``?
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