Posts Tagged shell script

A little one-liner: rename to lowercase recursively

Here is my little one-liner, because I used it today and I find it fun:

for f in `find .` ; do mv $f `echo $f | tr '[A-Z]' '[a-z]'` ; done 

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netcat as a logging tcp proxy

I felt I needed to write an article about netcat, so here is it !
Netcat is an incredibly usefull tool, that allows you to play with tcp connection easily from the shell.
Basically, as it name implies, it’s just cat over the network, but what its name doesn’t tell you is that it also can act as a socket listener.
So let’s play with pipes, here is one of my favourite use of netcat:

mkfifo proxypipe
cat proxypipe | nc -l -p 80 | tee -a inflow | nc localhost 81 | tee -a outflow 1>proxypipe

This command will redirect traffic from localhost:80 to localhost:81, in the inflow file you while find the incoming http request, in the outfile, you will find the http response from the server.
Similarly, you can do this:

cat proxypipe | nc -l 80 | tee -a inflow | sed 's/^Host.*/Host:' |  nc 80 | tee -a outflow >proxypipe

This will allow your browser to point to google using http://localhost .
Anyway, this is my favourite but netcat has thounds of other uses, have a look at it !
It can be usefull for file transfers (gzip|nc) , performance measurement (dd|gzip), protocol debugging (replaying requests), security testing (nc does port scan) …

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Installing redmine 0.8 on intrepid (ubuntu 8.10)

I’ve successfully insalled redmine pretty much easily but I needed to find out what packages to install with apt, which one with gem, which version …
Here is my magic receipe to install it all:

apt-get update 
apt-get install subversion mysql-server rubygems rake pwgen
# next line generates a password for the database
export PASSWORD=`pwgen -nc 8 1`
gem install -v=2.1.2 rails
cd /opt/
svn export redmine-0.8
cd redmine-0.8/
cat <<EOF >> config/database.yml
  adapter: mysql
  socket: /var/run/mysqld/mysqld.sock 
  database: redmine
  host: localhost
  username: redmine
  password: $PASSWORD
  encoding: utf8

rake db:migrate RAILS_ENV="production"
rake redmine:load_default_data RAILS_ENV="production"
apt-get remove pwgen subversion
RAILS_ENV="production" ./script/server  

And that’s it ! Redmine is running on port 3000.
I did this on an EC2 instance and it works like a charm (ami-7cfd1a15).
Maybe next article will discuss running redmine in mongrel or apache, and creating an init script for having redmine running on boot !

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Extract a table from a mysqldump

Today I encountered a problem: I needed to restore a single table from a database mysqldump.
Usually you cat the-mysqldump.sql |mysql the_database so you’re only able to restore the full database. I didn’t find any mysqldump option to extract a single table from a full database dump, so I’ve come up with this (minimal) shell script:


  grepstr="/*!40000 ALTER TABLE \`$TABLE\`"
  lines=`grep -n "$grepstr" $DUMPFILE |cut -d":" -f1`
  lines=`echo $lines|sed 's/\ /,/' `
  sed -n "$lines p" $DUMPFILE

extract_table $1 $2

Use it like this:
./ table-to-extract dumfile-for-extract |mysql the_database (use the |mysql after you have checked the content).

Be carefull, this script is minimalistic:

  • It doesn’t check if the file exist and is really a mysqldump file
  • It doesn’t check if the table to extract exists
  • It doesn’t work if disable-keys is set to false in mysqldump
  • It doesn’t have a usage() function

If some people request it, I’ll write all these features, but as usual, I wanted to come up with a solution I could already use one hour a ago, and I’m spending time to write this script, let’s do it the faster I can ! 🙂

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Selecting a range of lines within a file

Let’s say you want to extract a part of a file, for example from line 12 to 20.
I’ve come up with two solutions:

  • head -n20 |tail -n8
    You take the n’th first line where n is the last line you want, then you go backward by the total line number you want to have, that is: 20-12=8
  • A nicer solution which is straightforward (use the right tools guys !):
    sed -n '12,20p'
    You need the -n option, so that the input is not printed to the output, than give sed an expression (within quotes), the expression is the first line, a coma, the last line, and the “p” instruction which means print.
    This solution doesn’t need you to calculate the number of lines you will get, I find it nicer !

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