Adding a command

adding your command to EOS CLI

(verified 2010 Apr. / EOS 4.3.0 / DCS-7120T-4S 03.01)

Command

Basically, EOS commands are written in Python. When user login to the system, CLI (command line interpreter) comes up and interacts with user. Then user type into a command, the associated Python program will be called. That is command.

If we understand the detail structure and the path of this process, we can add our own command to the system.

Files

The actual command files are located under /usr/lib/python2.5/site-packages/CliPlugin directory. For example, Ntp.py implements about ntp and related commands.

(You can find DonkeyCli.py hidden command and it is also a good sample code of the EOS CLI command. Must see.)

Therefore, if you write a new command imitate Ntp.py and place it to that directory, it will be available at the CLI which starts after that process.

In this document, we show the registeration path of command from Python based program to EOS CLI.

from login

When user login to the system, /usr/bin/Cli program will be kicked as a login shell, through /usr/bin/RunCli program. (/usr/bin/Cli is also included in /etc/shells )

/usr/bin/Cli is a wrapper to Cli.py. For instance, you can just start /usr/bin/Cli from bash then you see the EOS CLI prompt.

bash-3.2# /usr/bin/Cli
localhost>show vlan  
VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    vlan1                            active    Cpu, Et1
2    vlan2                            active    Cpu, Et11, Et12, Et13
3    vlan3                            active    Cpu, Et17

localhost>

Scan and Registration

After loading of /usr/lib/python2.5/site-packages/Cli.py, it calls loadPlugins() function at __init__() function. It scans /usr/lib/python2.5/site-packages/CliPlugin directory to register all commands.

For example, Ntp.py includes BasicCli.registerShowCommand() function call for adding NTP options to SHOW command. And also includes BasicCli.GlobalConfigMode.addCommand() function call for adding NTP configuration subcommands to configure mode of EOS CLI.

It means if you want to register your own command, you need to write some command registration procedure in Python and need to place it in this Plugin directory.

Preparations

Here are some preparation processes to place a file on the system directory, such as bash access, filesystem remounting and etc.

bash command is available on enable mode of EOS CLI. In almost case, it will be required the administrator permission, so just do su as follows;

mypc$ slogin -l admin arista.example.com
Password: 
localhost>enable
localhost#bash
 
Arista Networks EOS shell
 
[admin@localhost ~]$ su
Password: 
bash-3.2#

root file system is mounted as read only mode. It needs to remount as read write mode as follows;

bash-3.2# mount -o rw,remount / 
bash-3.2#

Place a program

Place a Python program which includes command registration process into /usr/lib/python2.5/site-packages/CliPlugin directory. There are several pathway to place your program to Arista directory, as follows;

Edit code in directly by vi command from bash of Arista.
Copy and paste from external system through the remote terminal. You need do "cat > sample" or something on Arista side.
Copy from USB memory. (Just insert USB storage then "cp /mnt/flash/sample ." or something.)
Place it on Web directory then get by wget command. (Arista has /usr/bin/wget in initial condition.)

Sample : Hello World

We start from the old tradition, Hello World. Make a HelloCli.py file as follows. (left number is the line number. )

1: import BasicCli, CliParser
2: tokenHello = CliParser.KeywordRule( 'hello', helpdesc='Says hello' )
3: def doHello( mode ):
4: print "World.\n"

5: BasicCli.UnprivMode.addCommand( ( tokenHello, doHello ) )

line 1 : this is the minimum import of the system API.

line 2 : CliParser.KeywordRule() register a word as a command or parameter. In Arista source code, it will be called as token. In this example, it registers 'hello' token with 'Says hello' message as a help string. ( EOS CLI shows interactive help message in every commands. )

line 3, 4 : this doHello( ) function is registered callback function. It will be called when user execute 'hello' command.

line 5 : BasicCli.UnprivMode.addCommand() function adds a token which is defined in line 2, to the non-privilege mode, with callback function name 'doHello'.

Sample operation

Here is an example to write a program by vi editor.

bash-3.2# cd /usr/lib/python2.5/site-packages/CliPlugin/
bash-3.2# vi HelloCli.py
bash-3.2# cat HelloCli.py 
import BasicCli, CliParser
tokenHello = CliParser.KeywordRule( 'hello', helpdesc='Says hello' )
def doHello( mode ):
        print "World.\n"
BasicCli.UnprivMode.addCommand( ( tokenHello, doHello ) )
bash-3.2#

Confirmation

Invoke /usr/bin/Cli from bash then try hello command.

bash-3.2# /usr/bin/Cli 
localhost>hello 
World.

localhost>exit
bash-3.2#

And type ? key on localhost> prompt. You can see the hello command and help message that you have registered also included as follows;

bash-3.2# /usr/bin/Cli 
localhost>?
....
  hello       Says hello
....
localhost>

So we have succeed to add a new command to EOS CLI.

next step

Of course it is too simple and useless command. There are some steps to climb up.

make command persistently : adding commands as Extension with SWIX format.
make a command with options
use Arista oriented services such as SysDB

We hope we have a time to try them soon.

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