Will iPython replace Bash?

Roberto Polli - roberto.polli@babel.it
Will iPython replace Bash?
s/bash/ipython/g
EuroPython 2013, 3th
July - Firenze
Babel Srl P.zza S. Benedetto da Norcia, 33 0040, Pomezia (RM) – www.babel.it

What? Who? Why?
How ipython can speed up your daily bash work, improve
reusability and your python confidence...without entirely
replacing bash.
Roberto Polli - Community Manager @ Babel.it. Loves writing in C,
Java and Python. Red Hat Certified Engineer and Virtualization
Administrator.
Babel – Proud sponsor of this talk ;) Delivers large mail
infrastructures based on Open Source software for Italian ISP and
PA. Contributes to various FLOSS.

Agenda - 1
Bash Rocks
Simple bash patterns
Python vs Bash: strenght and limits
Can python save the day?
SysAdmins can gently learn python!

Agenda - 2
Real life scenarios
●
using fields
●
parsing /proc/ files
●
reusing stuff
●
flaskize
●
test
You can manage it faster ;)

Python vs Bash
–Bash is "The" shell
–Python is the widespreading system
language
–A reunion?
# bash
for f in ${files[@]}; do
name=${f%.*}; ext=${f##*.}
mv "$f" "${name,,}.${ext}
done
# python
for f in files:
name, ext = f.splitext()
shutil.move(f,name.lower()+"."+ext)

Bash rocks - 1
Job Control
Simple pipelines and streams
# zipinfo -t just_files.zip | xargs rm --
Clean I/O redirection
# kinit user <<< 'secret' # kerberos login
# script.py 2>py.err < /etc/hosts # redirect stdin
for x in $(seq 1 10); do
sleep $x &
done
jobs -l;
wait %1

Bash rocks - 2
Braces and Globbing
# du -ms .[a-z]*
# touch {01..20}.out
...bash3 behaves differently from bash2
History and readline substitution
# mv foo.out foo-$(date -I).out
# !!:gs/foo/bar # same as above with "bar"
Long commands
# fc;

Wishes - 1
Safe system scripts
# touch -- -rf; rm * #ouch!
Readability
# mv "$file" "${file,,}"
# [ x$1 != x ] || exit
# ${WTF[@]}
Reusability
# . /etc/init.d/functions
# are you sure
# bash scripts
# are short?
RM=$(which rm)
? ${RM:?Missing rm}
WTF=(world taekwondo fed)
for file in ${A[@]}; do
$RM -- "$file";
done

Wishes - 2
Code consistency
●
bash3 != bash4
One language
●
perl
●
awk, gawk, mawk
●
grep, egrep, fgrep
●
sed, gsed,
●
gnuplot, bc
#bash4
if [[ $file =~ ^p ]]
then
strace $file |&
grep inputrc
fi
# bash3
if echo "$file" |
egrep -q "^p"
then
strace 2>&1 $file |
grep inputrc
fi

Interactive Python
●
CPython delivers an interactive interpreter
●
Limited Readline and History support
●
Requires parenthesis and string quoting
●
Batteries included (math, telnetlib, shutil, ...)
$ telnet localhost 80
telnet: command not found
$ python -m telnetlib 0 80
Present by default
in almost
all Linux distros!

Interactive Python
Cpython shell can autocomplete!
import readline as rl
import rlcompleter
rl.parse_and_bind('tab: complete')
...but you can use iPython!

iPython: interactivity += 1
Easy to install
●
pip install ipython
Various flavours!
●
vanilla#ipython
●
math# ipython --pylab
●
shell# ipython --profile=[py]sh
Customizable
~/.ipython/profile_XX/ipython_config.py
Customize:
prompt
spacing
functions
shotcuts
modules
...

iPython
/bin/sh passthru:
●
ls -ld ~/python; # run via sh
●
ll /tmp; # alias pointing to ls -l
Capture both stdout and stderr
●
ret = !cat /etc/hosts #run via sh
●
ret = !ls /MISSING
Get exit status
●
print(_exit_code, "resembles $?")
ipython uses
os.system
# output is
# in a flexible
# list (SList)
type(ret)
_exit_code may
not be exactly
as expected

iPython features
Use SList as:
●
list or string
●
w or w/o newlines
Various Magics
%autocall
%edit
%debug
Automagic string-ify
and parenthesis
#ret = !cat /etc/resolv.conf
[ "search babel.it",
"nameserver 127.0.0.1" ]
#ret.s == ' '.join(ret)
True
#ret.nlstr == 'n'.join(ret)
True
#, ret.grep babel.it

iPython expansion
Pass python stuff to the shell
# import os
# ret = !cat /proc/{os.getpid()}/status
Consider the following steps during expansion
1. ipython parses the line and expands {contents}
2. prepare the command to be executed to the shell
3. fork/exec the shell
4. the shell interprets the command!
# from os import getpid as PID
# ret = !cat /proc/{PID()}/* # guess what?

Parsing system files
Get ipython Memory usage
# ret = !cat /proc/{os.getpid()}/status
Replace AWK
# ret.fields(0) # awk '{print $1;}'
# ret.fields(2,1,3) # awk '{print $2, $1, $3;}'
# ret.fields(*range(1,10)) # print 2..9th fields
# ret.fields(-1) # awk '{print $NF;}'
VmPeak: 27308 kB
VmSize: 27244 kB
VmHWM: 7080 kB
VmRSS: 7072 kB

Unpack data
SList.fields() just splits
by white-space
fieldize() is more
generic!
iPython profiles loads
custom files under
./startup/ directory.
# 99-fieldize.py
# save under the ./startup/
# of the rofile directory:
# ~/.ipython/profile_xx/
def fieldize(ret, sep=":"):
"""Let's be generic"""
return dict([
map(str.strip,
x.split(sep,1))
for x in ret ])

monitoring.ipy
#!/usr/bin/ipython --profile=pysh
# Important: save as .ipy to run with ipython
from time import sleep
from os import getpid
fields = '{VmSize},{VmRSS},{VmSwap}'
while sleep(1) == None:
ret = !grep ^Vm /proc/{getpid()}/status
d = fieldize(ret)
print(fields.format(**d))

$PITFALLS - 1
Beware of bash expansion/substitution
ret = !echo /proc/$$/cmdline
1.ipython replaces $fn with its
python value - eg. "cwd"
2.then uses os.system
3.fork() happens before bash
expansion
4.the shell interprets $$ as the
pid of the current process (the
forked one!)

$PITFALLS - 1
Expand Early and in Python
unless you know what you're doing!
GOOD: ! echo /proc/{os.getpid()}/cmdline
EVIL: ! echo /proc/$$/cmdline

$PITFALLS - 2
os.system|subprocess use /bin/shell
# this = !{echo does; echo >&2 not; } |& grep work
We can work it out ;)
edit _process_common.py
add the `executable` argument to
subprocess.Popen (
executable= '/bin/bash',
shell=True, ...)
IMHO: Don't you trust os.environ.get('SHELL')? Don't trust
os.system too!
quick &
dirty

Plotting system data
A sysadmin must plot...
# from matplotlib import pyplot
Collect data...
#ret = !ping -t 100 -w 100 foo.com
#ret = ret.fields(7).grep('time')
Mangle it... (get just millisecs)
#ret = [ x.split("=")[1]
for x in ret ]
And show!
# pyplot.hist(map(float, ret))
# pyplot.show()

Flaskize
Scripting in python you'll collect a
lot of useful snippets in a very
short time.
%history #use history magic
Flask is a web microframework
you can use to convert them in
webservices!
Can't use the !command syntax
with Flask...(can I?)
"""Things are easy even without
the !command stuff
"""
from flask import Flask
import simplejson
from fieldize import fieldize
app = Flask(__name__)
@app.route('/r/mem.view/<pid>')
def monitor(pid):
"""Return some process info"""
int(pid) # trigger ValueError
fpath = '/proc/%s/status' % pid
# a file is iterable ;)
with open(fpath) as ret
d = fieldize(fpath, ":")
return simplejson.dumps(d)
app.run(debug=True)

Nosetests: speed up your tests!
Three simple steps
●
put your test files in ./test/
●
run #nosetests ./test/
●
nose discovers and runs
them all
...really! That's all!
"""Test is easy with nose.
Docstrings are used in reports!
"""
from telnetlib import Telnet
def mping(ip):
# ICMP requires root privileges ;)
cmd = '/bin/ping -w 2 -t 2 '+ip
return os.system(cmd)
def test_ping_gw():
"ping default gw"
assert mping('192.168.1.254')==0
def test_ping_remote():
"ping remote host"
assert mping('10.0.11.1')==0
def test_tcp_remote():
"port 80 remote"
port, timeout = 80, 1
Telnet('10.0.11.1', port, timeout).close()

Bonus tracks: Ovirt and 389ds
iPython auto-complete helps learning new API
●
Ovirt open-source virtualization infrastructure
●
389 LDAP directory server
#!sudo pip install ovirt-engine-sdk
from ovirtsdk.api import API
client = API(**{'url':.., 'username':..})
for vm in client.vm.list():
name = vm.get_name()
try: vm.stop() # PEP8 who? ;)
except: log.exception("Error stopping " + name)

Bonus tracks: Ovirt and 389ds
389 API are under development: feedback is important!
#git clone https://github.com/ioggstream/dsadmin
from dsadmin import DSAdmin
# connect to the server
ds = DSAdmin(**{'binddn':..., 'bindpw': ...})
# configure 389 to use SSL
ds.setupSSL()
# weird name? iPython will auto-complete!
ds.getReplStatus()
Development releases includes method name refactoring!
ds.replica.status()
ds.replica.agreements()

Will iPython replace Bash?
Not in the near future ;)
Can lead to a different use of bash
Compete with weakly interactive
toos like perl, awk, sed
It is a tool that you shouldn't miss!

Thank You!
roberto.polli@babel.it
Code
http://ipython.org/
http://flask.pocoo.org/
http://nose.readthedocs.org/
https://github.com/ioggstream/dsadmin/
Babel
http://www.babel.it
http://vaunaspada.babel.it/blog

Will iPython replace Bash?

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