Advanced Linux Shell Techniques (with sample commands)

change one character in a file: tr "\r" "\n" < in > out

execute arguments from a file: find /home -name 'weblog_config.pl' -exec {} ; -print

execute commands from arguments of a file: awk -F : '{print "chown -R " $1 " " $6}' < /etc/passwd | /bin/sh

building a file from another file:
#! /bin/sh
(for i in cat domainlist; do
echo "zone "$i" {"
echo
done) > $1 slave.conf

booting up single user rescue: kernelname single root=/dev/hdc1 init=/bin/sh

remounting a broken partition read-only: mount -o ro,remount /dev/hdc1 /

iterative command with delay: while true; do ps; sleep 1; done

mass text replace within files: perl -pi -e 's/foo/bar/g;' filenames*

test a webserver after telnet to port 80:
GET / HTTP/1.1
HOST: localhost
\n
\n b

manual query of an SQL server:
/c databasename
/d
/d tablename
select * from tablename:
pg-dump databasename

port-forward an SSH tunnel from a remote host:
ssh -f [email protected] 2222:localhost:80 -N
lynx http://localhost:2222
this forwards remote hosts httpd port (80) to local host on port 2222 for web viewing
-f == go into background, -N == don't execute a command on the remote host

reverse port-forward an SSH tunnel to a remote host:
ssh -R 10000:localhost:22 sysadmin@remotehost
anyone@anyhost# ssh sysadmin@remotehost(then->)sysadmin@remotehost# ssh -p 10000 (you come back to localhost from outside)

full IP forwarding over SSH using tunnel device* (as opposed to forward a single port):
add the line "PermitTunnel Yes" to /etc/ssh/sshd_config (must install openssh-server)
root@localhost# ssh -w0:0 gatewayhost
root@localhost# ifconfig tun0 10.0.2.1 netmask 255.255.255.252
root@gatewayhost# ifconfig tun0 10.0.2.2 netmask 255.255.255.252
(each box should ping each other now respectively at these ip's)
root@localhost# route add -host gatewayhost dev eth0

• to access entire subnet of gatewayhost's locally connected hosts (add to above):
  root@localhost# route add -net gatewayhostipblock/24 dev tun0
  root@gatewayhost# echo 1 > /proc/sys/net/ipv4/ip_forward
  root@gatewayhost#/sbin/iptables-t nat -A POSTROUTING -o eth0 -j MASQUERADE

use PPP to direct connect via any serial port or chat string (example- modem on /dev/ttyS0, setup string == at&fdt, phone number == 678.445.4143 that answers CONNECT then expects a login: and password: prompt, with login sysadmin, password Test):
root@localhost# pppd connect 'chat -v "" at&fdt*70,6784454143 CONNECT "" ogin: sysadmin word: Test' /dev/ttyS0 57600 debug modem crtscts defaultroute

open up a pipe on udp socket 53 and chat to another host on it to test in a terminal:
machine1: # netcat -lu 53
machine2: # netcat -u machine1 53

setting up a VPN when there is no tun0 device and you only have root on client:

client# /usr/sbin/pppd updetach noauth silent nodeflate pty "/usr/bin/ssh root@remote-gw/usr/sbin/pppd nodetach notty noauth" ipparam vpn 10.0.0.1:10.0.0.254