Posts Tagged ‘bash’

Can't fork?

Saturday, May 29th, 2010

Can't fork but need to see what's going on? Hint: a box that can't fork can often `exec'.

Here are a pair of slick bash functions that can be lifesavers in dire situations:

`ls':

$ myls() { while [ $# -ne 0 ] ; do echo "$1" ; shift ; done ; }
$ myls /etc/s*
/etc/services
/etc/shells
/etc/syslog.conf

`cat':

$ mycat() { while IFS="" read l ; do echo "$l" ; done < $1 ; }
$ mycat /etc/shells

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Posted in one-liners, tips and tricks | No Comments »

Auto-iptables off IPs with high connection counts

Saturday, August 29th, 2009

via Paul (lovepig.org):

netstat -npa --inet | grep :80 | sed 's/:/ /g' | awk '{print $6}' | sort | uniq -c | sort -n | while read line; do one=`echo $line | awk '{print $1}'`; two=`echo $line | awk '{print $2}'`; if [ $one -gt 100 ];
then iptables -I INPUT -s $two -j DROP; fi; done; iptables-save | grep -P '^-A INPUT' | sort | uniq -c | sort -n | while read line; do oneIp=`echo $line | awk '{print $1}'`; twoIp=`echo $line | awk '{print $5}'`; if [ $oneIp -gt 1 ]; then iptables -D INPUT -s $twoIp -j DROP; fi; done

This one-liner is quite effective when tossed into a file and run as a cronjob once per minute. Any IP with more than 100 concurrent connections — which, quite honestly, is far more than any one IP should ever have on a standard webserver — will be blocked via iptables. This script as a cronjob is extremely effective dealing with small-to-midsize DDoSes (too much traffic for Apache/whatever service to handle, but not saturating the pipe).

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Posted in one-liners | 2 Comments »

Combining text files as columns

Friday, June 19th, 2009

To combine two (or more) text files as individual columns in the same file, such as:

file1:

foo
foo1
foo2
foo3

file2:

foobar
foobar1
foobar2
foobar3

into:

foo foobar
foo1 foobar1
foo2 foobar2
foo3 foobar3

rather than using an ugly combination of sed and awk, you can use the `paste' command:

paste file1 file2

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Posted in one-liners | No Comments »

Extend bash functionality

Saturday, April 11th, 2009
# make bash autocomplete with up arrow
bind '"\e[A":history-search-backward'
bind '"\e[B":history-search-forward'

# make tab cycle through commands instead of listing
bind '"\t":menu-complete'

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Posted in howto, tips and tricks | No Comments »

Intercepting "command not found" in bash

Friday, March 6th, 2009

On Debian, bash is patched with an interesting new function: command_not_found_handle. This intercepts exit code 127 ("command not found") and allows you to do neat things. Debian uses it to pass it through the apt database, letting you know if a command you tried to invoke is not available, but can be found in the apt repos, and how to install it. Pretty spiffy.

This, of course, can be modified. Where I work, we use numbers to identify servers. I have a script that grabs login credentials from our internal systems and auto-logs me into servers based on their number. For example, I'd run `connect 12345′ to connect to server 12345.

By adding the following to my .bashrc:

function command_not_found_handle {
    /home/kale/bin/command-not-found $1
}

And creating the following script, with regex in place to only care about numbers, placed in /home/kale/bin/command-not-found:

#!/bin/bash

MYBOOL=`echo $1 | awk '$1 ~ /^[0-9]+-*[0-9]*$/ {print $0}' | wc -l | awk '{print $1}'`

if [ "$MYBOOL" == "1″ ]; then
        /home/kale/bin/connect $1
else
        exit 127
fi

(where `connect' is the path to my connect script, previously written)

This now allows me to do this awesome deal:

kale@bastion:~$ 12345
Connecting to server 12345…
root@12345:~#
 

If you didn't catch it, I don't need to specify a command — just the argument. As there's no application in my $PATH named `12345′, it falls through to the command_not_found_handle function, which then launches my connect script.

Who needs commands? I just saved hundreds of wasted seconds per night on typing "connect"!

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Posted in howto | 2 Comments »