Examine multiple files in command less

To change to the next file after opening multiple files simultaneously in 'less' command, use this command

:n

And to change to the previous file, use

:p


The instruction should also be available from the man page.

Hostname on Mac OS X

If the dynamic hostname is provided by DHCP, and you prefer a fixed one, there are a few ways to do it.

From System Preferences/Network

• Select the network adapter which interfaces the DHCP server.
• Click on the "Advanced" button on the lower right.
• In the TCP/IP tab, specify the "DHCP client ID" text field.

Or depending on the version of Mac OS:

-- Mac OS X 10.5 and older
On Mac, the hostname shown on the terminal prompt is configured by the file,
/etc/hostconfig. If it is specified as
HOSTNAME=-AUTOMATIC-,
the name shown in the prompt is provided by DHCP or BootP server.

Change that line to
HOSTNAME=(preferred-name)

-- Mac OS X 10.6 and newer
Use the command scutil (system configuration utility)
scutil --set HostName (preferred-name)

Remember to check the name set in
/Library/Preferences/SystemConfiguration/preferences.plist,
under System/System/ComputerName

Global search in vi and list results

We use the simple command to search a pattern (or a fragment of text),

:/{pattern}

Sometimes, there would be a few dozens of matches that you want to list and browse quickly.  Use this command,

:g/{pattern}

It lists the search results in one buffer.  If you want to search the pattern case insensitive, add \c in the command, like

:g/\c{pattern}

Monitor disk IO activities on a Linux host

1. Use df first to find all the devices
2. Use iostat

iostat -dx <device> <time-interval>

For example,
iostat -dx /dev/sdb 5

to display disk IO activities every 5 seconds.

Find CPU information on a Linux

A good tip for finding out CPU and 32/64 bit kernel information on a Linux host, at http://www.cyberciti.biz/faq/linux-how-to-find-if-processor-is-64-bit-or-not/

In short, use

uname -a 

and

cat /proc/cpuinfo

Use wildcard in the command find

To use a wildcard * in the find command needs a little more care.

Without quote or backslash \, such as
find . foo*bar -print
the shell will expand * before passing it into find command.  This results is a different, and most likely a narrower, search.

To avoid that, use
find . 'foo*bar' -print
or
find . foo\*bar -print

Add one more yum repository

Read yum.conf first.  Usually the path is /etc/yum.conf.
Understand where the cache and repository configuration file directory

The following examples use default locations

## add epel to the repo list
## see what is there before change
ls /var/cache/yum/*/*
cd yum.repos.d/
## copy contents from another configured host
sudo vi epel.repo
sudo yum clean all
## make sure it is empty
ls /var/cache/yum/*/*
sudo yum makecache
## see if the new repo epel is added
ls /var/cache/yum/*/*
## add GPG key for the new repo, also do copy-paste
sudo vi /etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL

If you are not sure about the name of RPM-GPG key, run rpm to install the package.  Towards the end, rpm will complain about missing the GPG key and prompt the file name for you.

Find out it is a 32 or 64-bit Linux

Use the command
uname -m
If it shows x86_64, it is a 64-bit host.
If it shows i386 (or sometimes i686), it is a 32-bit machine

Plus, run the command
file /bin/ls
For 64-bit kernel, it should show
/bin/ls: ELF 64-bit LSB executable ....

Find installed packages on Linux hosts

To find installed packages on a Redhat/CentOS Linux, use

rpm -qa | less

Once the package name is found, use this command to find more details

rpm -qil {package-name}

To find installed packages on a Debian (also Ubuntu) Linux, use

dpkg --list

Once the package name is found, use this command to find more details

dpkg -p {package-name}

Find and delete into directories recursively

How to find a set of files or directories that are named the same?  For example, to delete all the '.svn' directories in the current working directory.

One easy way is

rm -rvf `find . -type d -name .svn`

The option [-v] prints out the files/directories found.  and rm -rf is the typical force deletion of a directory.

Another way to do it is

find . -name ".svn" -exec rm -rf {} \;

But this command prints out warning messages that sometimes confuse users.

find: ./.svn: No such file or directory
find: ./images/.svn: No such file or directory

The warning message means it found a .svn directory in the current directory and deleted as instructed ( -exec rm -rf ).  When the command find wanted to descend into .svn directory, it could not.

The trailing {} \; is required.  Pay attention to its format: there is a space between } and \.

Find socket/port status on Linux

When debugging or monitoring a client-server application or any connected application, it is necessary to find out what is going on at socket level.

To start, use
netstat -an

To start with TCP connections, use
netstat -ant

Although the option -p should show process information (PID), it usually gives a "-" for that field. Remember to use ifconfig to find interfaces and IP addresses.