Switching betwen two directories in Bash

The pushd command remembers the last pushd’ed from directory
#cd /
#pushd /home
#pwd
/home
#pushd
#pwd
/
But quicker is “~-” which will simply take you back to the last working directory
#cd /
#cd /home
#pwd
/home
#cd ~-
#pwd
/

Flushing DNS:

• Windows: ipconfig /flushdns
• Linux: service nscd restart
• Mac OS X: dscacheutil -flushcache

Is Linux 32b or 64b?
uname -m

• 32b
  i386
i686

• 64b
  x86_64

giving permission to read a specific file using sudo:
visudo
    #Support Staff
    Cmnd_Alias = LOGS = /bin/cat /var/log/squid/access.log, /bin/zcat /var/log/squid/access.log.[0-9].gz
    %logaccess ALL (all) NOPASSWD: LOGS
This gives access to the Squid Logs to anyone in the logaccess group.
(sudo zcat /var/log/squid/access.log.1.gz) | grep "microsoft.com"

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Proxy.Pac files can be tricky. The way to identify the client is:
myIPAddress()
But this fails occassionally, reporting the LocalHost address instead, especially in Linux.
Additionally, on Windows 7 multiple enabled interfaces can lead to the wrong one reporting back.
There is no fix but DISABLING the inappropriate interfaces and re-enabling them can work.

In awk you can run a command using variables from awk by saving the command to a variable first, then get line can be used to retrieve the result.
/condition/ {
    myVariable = "some value"
    CMDFILE = "echo "myVariable" | head"
    CMDFILE | getline myAnswer
    close(CMDFILE)
    print myAnswer
}

don’t forget the close statement!!,.

Squid by default on RedHat only uses a maximum number of file descriptors of a meagre 1024.
To increase this:

• Add lines in the /etc/security/limits.conf file:
  squid soft nofile 20480
squid hard nofile 20480

• Add line in the /etc/squid/squid.conf file:
  max_filedesc 20480

• Add line in the /etc/init.d/squid file:
  ulimit -HSn 20480

• Restart the squid service:
  service squid restart

This will DEFAULT to the COMPILED setting of 16k file descriptors rather than our selected value of 20K.

Using the file ~/.muttrc you can set the FROM address in outbound messages from the root user to an arbitrary but more appropriate value:
vim ~/.muttrc
Add the lines:

set from=noreply@my.domain
set envelope_from=yes
set realname=NoReply

Or as you see fit – Envelope Sender is also controlled with this setting

To share the same interface when on a Linux server you can use screen
To start a session:
screen -S {session-name}
To join a session:
screen -x {session-name}
Both administrators will then be able to type commands fully visible to each other.

When using TCPDUMP you can use -s 0 to not limit the size of the packets retrieved by the -w {Filename} option.