Linux Hacks

Here is a goto list of linux hacks that I find useful in my day to day. I will keep on updating it as I gather more. Please feel free to respond back in comments in case you want me to add some more

Random Password Generator

Following two scripts generate a random string, a sub set of which, or the whole string can be used as an password. Give it a try.

date +%s | sha256sum | base64 | head -c 32 ; echo

Removing trailing spaces

echo "   lol  " | xargs

Xargs will do the trimming for you. It's one command/program, no parameters, returns the trimmed string, as easy as that! Note: this doesn't remove the internal spaces so "foo bar" stays the same. It does NOT become "foobar".

Breaking a file into smaller chunks

# By Size
split -b 100M -d -a 3 forever.log.bck forever

# By number of Lines (input and output are just two directories)
split -l 2500 input/result.txt output/result.txt

Search and replace a string in all files in my present directory

sed -i -- 's/STR1/STR2/g' *

This replaces all occurances of STR1 with STR2 in all files in my current directory

Delete logs older than X days

find /path/to/files* -mtime +X -exec rm {} \;

Total number of lines in all the files in current directory

find . -type f -exec cat {} + | wc -l

Install FTP on rhel or centos

yum install vsftpd -y
yum install ftp -y
service vsftpd start

Install jq

jq is a really cool tool if you work with json's a lot. Its like sed for json. You can use it to slice and filter and map and transform structured data with the same ease that sed, awk, grep and friends let you play with text.

Installing it is an breeze too

wget -O jq https://github.com/stedolan/jq/releases/download/jq-1.5/jq-linux64
chmod +x ./jq
cp jq /usr/bin
rm jq

Using tcpdump

Although a whole seperate blog could be ritten about tcpdump and its usage. Here is a very short, quick and dirty SOP to get it up and running Please note that sudo access is required for this

Install tcpdump

yum install tcpdump

Script to capture network traffic on a port and write a Wireshark-compatible pcap file

# Use tcpdump to capture network traffic and write a Wireshark-compatible pcap file:
tcpdump -s 0 port 443 -w mypcapfile.pcap

# Terminate the capture with a ctl-c

# gzip the file:
gzip mypcapfile.pcap

Capture traffic on particular interface

tcpdump -i eth0

Display Available Interfaces

tcpdump -D

Read Captured Packets File

tcpdump -r mypcapfile.pcap

Capture Packets from source IP

tcpdump -i eth0 src 172.176.98.1

Capture Packets from destination IP

tcpdump -i eth0 dst 172.176.98.1

Adding a Script to auto start on reboot/startup

At times, you need a way to make a script auto start at the time of reboot's ans startup. There are multiple ways to accomplish this.

However the one I find most effective is :

First create a init.d script for your operation that supports start|stop|restart operations. For example, here is a sample init.d script to start/stop ldap

#!/bin/sh
# ldap          Init script for running the ldap client daemon
#
# Author:       Akshay Sinha <akshaysin@gmail.com>
#
# chkconfig: - 63 37
#
# description: Start's Stops ldap.
# processname: ldap
# config: /opt/IBM/ldap/V6.3.1/sbin/ibmslapd

#
# Below is the chkconfig syntax for auto startup at different run levels
# Note runlevel 1 2 and 3, 69 is the Start order and 68 is the Stop order
# Make sure these are unique by looking into /etc/rc.d/*
# Also below is the description which is necessary.
#
# chkconfig: 123 63 37
# description: Description of the Service
#
# Below is the source function library
#
. /etc/init.d/functions
#
#
# Below is the Script to start stop ldap services
#
case "$1" in
start)
printf "Starting service : LDAP\n"
curl --output /dev/null --silent --head --fail ldap://localhost:389
if [[ $? -eq 0 ]]; then
    printf "LDAP already running. Doing nothing\n"
else
    /opt/IBM/ldap/V6.3.1/sbin/ibmslapd -I isamldap
fi
;;
stop)
printf "Stopping service : LDAP\n"
curl --output /dev/null --silent --head --fail ldap://localhost:389
if [[ $? -eq 0  ]]; then
    /opt/IBM/ldap/V6.3.1/sbin/ibmslapd -I isamldap -k
    printf "LDAP Stopped gracefully\n"
else
    printf "LDAP Already stopped. Doing nothing\n"
fi
;;
restart)
printf "Restarting service : LDAP\n"
curl --output /dev/null --silent --head --fail ldap://localhost:389
if [[ $? -eq 0 ]]; then
    /opt/IBM/ldap/V6.3.1/sbin/ibmslapd -I isamldap -k
    printf "LDAP Stopped gracefully. Starting it now\n"
    sleep 2
    /opt/IBM/ldap/V6.3.1/sbin/ibmslapd -I isamldap
else
    printf "LDAP Already stopped. Starting it now\n"
    /opt/IBM/ldap/V6.3.1/sbin/ibmslapd -I isamldap
fi
;;
*)
printf "Usage: $0 {start|stop|restart}"
exit 1
;;
esac

Next run the chkconfig --list command to list all current run levels for different programs
Run chkconfig --list ldap
Run chkconfig ldap on
Run chkconfig --list ldap again

That's it.