Configure the interfaces File with a Bash Script

While the long term strategy for many data centers is to automate the configuration of NVIDIA switches, this is often overkill for smaller environments.

However, manually adding every switch port to the configuration file can be a dreary task. So it is a good thing NCLU supports globs so you can use one to quickly define a range or ports.

However, if you are old school and want to use the tools available through the native Bash shell, this article shows you how to quickly add the switch ports using a Bash script.

Determine the switch port identifiers on your device using the ip link show command:

cumulus@switch$ ip link show
1:  lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN mode DEFAULT
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2:  eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT qlen 1000
    link/ether 08:9e:01:f8:9b:ac brd ff:ff:ff:ff:ff:ff
3:  swp1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT qlen 500
    link/ether 08:9e:01:f8:9b:af brd ff:ff:ff:ff:ff:ff
...
53: swp51: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT qlen 500
    link/ether 08:9e:01:f8:9b:df brd ff:ff:ff:ff:ff:ff
54: swp52: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT qlen 500
    link/ether 08:9e:01:f8:9b:e0 brd ff:ff:ff:ff:ff:ff

For this device, you need to configure swp1 through swp52.

Back up the current /etc/network/interfaces file:
```
cumulus@switch$ sudo cp /etc/network/interfaces /etc/network/interfaces.orig
```
This step is an insurance policy in case you accidentally corrupt the file you are trying to update.
Create the script in the cumulus user home directory. Substitute the highest-numbered swp on your switch for the number on the line starting with while.
```
cumulus@switch$ vi populate.sh

#!/bin/bash

mycount=1

while (( $mycount <= 52 ))
do
echo  >> /etc/network/interfaces
echo auto swp$mycount >> /etc/network/interfaces
echo iface swp$mycount >> /etc/network/interfaces
((mycount++))
done
```
Transcribe the spacing and special characters exactly or the script could corrupt your /etc/network/interfaces file. This is why you made a copy in step 2.

Run the script using sudo, then apply the changes:

cumulus@switch$ chmod 744 populate.sh
cumulus@switch$ sudo bash populate.sh
cumulus@switch$ cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5), ifup(8)
#
# Please see /usr/share/doc/python-ifupdown2/examples/ for examples
#
#

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet dhcp

auto swp1
iface swp1
...

auto swp52
iface swp52

cumulus@switch$ sudo ifreload -a

Verify that the ports are active using the ip link show command:

cumulus@switch$ ip link show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN mode DEFAULT
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT qlen 1000
    link/ether 08:9e:01:f8:9b:ac brd ff:ff:ff:ff:ff:ff
3: swp1: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master bond0 state UP mode DEFAULT qlen 500
   link/ether 08:9e:01:f8:9b:af brd ff:ff:ff:ff:ff:ff
...
53: swp51: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master bond0 state UP mode DEFAULT qlen 500
   link/ether 08:9e:01:f8:9b:df brd ff:ff:ff:ff:ff:ff
54: swp52: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master bond0 state UP mode DEFAULT qlen 500
   link/ether 08:9e:01:f8:9b:e0 brd ff:ff:ff:ff:ff:ff

Bonus Section: How this Script Works

The script in step 3 works as written — just copy it onto your switch (adjusting the number of the highest numbered swp) and it just works. But if you want to start learning Bash scripting, review the following script line by line.

You can accomplish the tasks described in shell scripting using multiple methods. This example uses some preferred methods — consult other resources (for example, nixCraft) for a more complete discussion of scripting tools.

 #!/bin/bash

This statement tells the script which shell (command interpreter) you should use to run it. Typically the # character denotes a comment but here it actually passes information to the script.

 mycount=1

The script uses a counter to cycle through the swp interfaces. Here you set the variable named mycount with the initial value of 1.

while (( $mycount <= 52 ))

The while statement indicates a loop which continues to run as long as the specified condition is true. All commands contained between the do and done statements run on each pass of the loop.

The double parentheses contain a conditional check. Using the $ character substitutes in the current value of the variable it precedes — in this case mycount. This line checks whether the counter is greater than or equal to the highest port ID on your switch.

The white space in this line is critical — the notation used does not work if the spacing is not correct.

echo  >> /etc/network/interfaces
echo auto swp$mycount >> /etc/network/interfaces
echo iface swp$mycount >> /etc/network/interfaces

The echo command typically prints any arguments directly to the screen. The addition of the >> characters redirects that output and appends it to the end of the specified file. Be careful; a single > character overwrites the file rather than adding to the end of it.

This block of code adds a blank line, followed by the standard auto and iface statements while substituting in the current value of the mycount variable to generate the swp interface name.

((mycount++))

This is the most critical part of this while loop. If the counter is not incremented, the loop does not exit automatically because the conditional check never changes.

Configure the interfaces File with a Bash Script

Bonus Section: How this Script Works

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