Linux Tools

Debian Alternatives

Debian has a way to specify the default version of an app. For example, if more than one version of Python is present on the system, the command "update-alternatives" can be used to activate any of the available choices. Caution - it's not a good idea to switch from the version of Python which came with your distro, since there documented and undocumented dependencies in various places, on that particular version. Random things may break such as software update, applications like Dropbox etc. Caveat Emptor.

Remove an alternative version:

sudo update-alternatives --remove python /usr/bin/python3

Example above allows to fall back on the previous version, such as Python 2.7.

Version Control

Notify git of your identity:

git config --global user.email "yourname@yoursite.yourdomain"

To avoid entering git userID and password:

git config --global credential.helper 'cache --timeout 7200'

Network Tools

It is convenient to control a few machines from one screen. Typically ssh is used for this purpose, but if security is not a concern (e.g. then the network is strictly local) telnet can be also used as a quick solution. You may find it useful to utilize telnet at first in order to access all computers from one location when setting up ssh. Another advantage of ssh is X11 forwarding, which functionality telnet does not have.

nslookup

This is a very useful network information utility with diverse functionality. One simple function is to translate qualified host names to IP addresses and back.

ssh

You'll need to run the sshd service on every machine you want to connect to. On Linux, this is most frequently openssh-server and it can be trivially installed. Make sure there is a ssh entry in /etc/services, with the desired port number.

To be used productively, private and public keys will need to be generated or imported as necessary. For the private/public key pair to work, public keys should be added to the file ".ssh/authorized_keys". A matching private key must be loaded to an identity managing service (e.g. ssh-agent in case of Linux) on the machine from which you are going to connect. If it's not cached, you will likely be prompted to enter the passphrase for the key.

Typically (this depends on the flavor of your sshd) you will get a message specifying which public key is used during the login that you are attempting. This is useful to know if you have many keys and forget which was used for what connection.

Restarting the service:

sudo systemctl restart ssh

Adding a key to the agent:

eval "$(ssh-agent -s)"
ssh-add key_file

Gateways such as one operating at BNL and other Labs typically require that your public key would be uploaded and cached on their side in advance. The exact way this can be done is site-dependent. Some sites require to verify the upload by providing the public key's fingerprint. Example of how to get it:

ssh-keygen -E md5 -lf my_public_key_file

If you lost your public key (while still having your private one) you can re-create it:

ssh-keygen -yf my_private_key_file

Once it's done, a connection becomes possible, for example:

ssh username@atlasgw.usatlas.bnl.gov

Tunneling at BNL:

ssh -L 8080:130.199.23.54:3128 yourAccount@your.gateway.bnl.gov

The port 8080 is chosen as an example - by rules it must be a number larger than a certain low limit. On your local machine, you would need to specify a proxy which looks like this:

localhost:8080

Another example when going from one Linux box to another:

ssh -L 8000:localhost:8000 myRemoteHost

The above gives you access to the remote port 8000 on the local machine via localhost:8000.

telnet

While using ssh is in general preferable for many reasons, sometimes there is a chicken and an egg problem where you need to establish access fast in order to debug ssh on a remote machine. In these cases, and if security is not a concern (rare, but could happen on an entirely internal network), one may opt to use telnet.

On Ubuntu one can install the software necessary to run the telnet service in the following manner:

sudo apt-get install xinetd telnetd

Make sure there is an entry in /etc/services which looks like

telnet        23/tcp

Also, create a file /etc/xinetd.d/telnet with contents similar to this:

service telnet {    
        disable         = no
        flags           = REUSE
        socket_type     = stream
        wait            = no
        user            = root
        server          = /usr/sbin/in.telnetd
        log_on_failure  += USERID HOST
        log_on_success  += PID HOST EXIT
        log_type        = FILE /var/log/xinetd.log
}

...and start the service as follows:

sudo /etc/init.d/xinetd start

pdsh

This is an advanced parallel shell designed for cluster management. It often uses ssh as the underlying protocol although there are other options as well. Configuration is defined by files residing in /etc/pdsh. For example, the file "machines" needs to contain the list of computers to be targeted by pdsh. Optionally, this is also the place for a file that can be sourced for convenience of setup, cf

# setup pdsh for cluster users
export PDSH_RCMD_TYPE='ssh'
export WCOLL='/etc/pdsh/machines'

This of course can be done from the command line anyway, cf

export PDSH_RCMD_TYPE=ssh

Using ssh as the underlying protocol for pdsh implies that you have set up private and public keys just like you normally would for ordinary ssh login. Once this is done, you should be able to do something like this as a basic test of your setup:

pdsh -w targetHost "ls"

If the targetHost is omitted, the command will be run against all machines listed in the "machines" file as explained above. Should a command fail on a particular machine, this will be indicated (with an error code) in the output of the command, with the name of the machine listed. Redirection of stderr with something like "2>/dev/null" included with the command you run won't work with pdsh.

Databases

Postgres

Installation

sudo apt-get update
sudo apt-get install postgresql postgresql-contrib

Running

Switch over to the postgres account on your server by typing:

sudo -i -u postgres
psql

Same without switching accounts:

sudo -u postgres psql
psql

Exit out of the PostgreSQL prompt by typing: \q

Create a user/role (pretty much same in Postgres):

createuser --interactive

Example of getting help:

testdb=# \h create table

Databases and Tables

Creation of DB

From the OS prompt:

sudo -u postgres createdb foo

Also can be done from within psql.

postgres=# create database testdb;

Tables

testdb=# create table people (
testdb(# name char(50) primary key not null,
testdb(# age int not null
testdb(# );

Changing a table:

ALTER TABLE foo ADD last_maint date;

Info

List of DBs:

\l

List of schemas:

\d