A Bash web server
One Liners
With one line of Bash code, you can create a Bash web server for quickly viewing the output from Bash scripts and commands.
For people who do a lot of work with command-line tools or Bash code, having a Bash web server could be very handy. I was really amazed that in one line of Bash code I was able to create web servers that could:
- Send the output from a Bash command directly to a browser page
- Create diagnostic pages using standard Linux tools
- Create pages that view Raspberry PI GPIO pins
- Create a page to toggle a Rasp PI GPIO pin
One-Line Web Servers
While a number of minimal, one-line web servers exist in most programming languages [1], you can create a Bash web server using the networking utility nc
(or netcat
) as follows:
while true; do { \ echo -ne "HTTP/1.0 200 OK\r\n \ Content-Length: \ $(wc -c <index.htm)\r\n\r\n"; \ cat index.htm; } | nc -l -p 8080 ; \ done
This Bash statement echoes a string with an HTTP header, the file content length, and an HTML file to a listener connecting on port 8080 using the cat
command to show the HTML file. This one-line Bash example shows a single page (index.htm
), which isn't overly useful. There are other web server options that would work much better.
Where a Bash web server really stands out is in its ability to execute command-line utilities or scripts and send the results to a web client.
Bash Web Server Calling Bash Commands
The Bash command output can be included in the echo
string along with the HTTP header when a client listener connects. For example, the iostat
command, which is used for system monitoring, can be viewed on a web page with:
while true; do echo \ -e "HTTP/1.1 200 OK\n\n$(iostat)" \ | nc -l -k -p 8080 -q 1; done
With this Bash statement, there are two important options that need to be set on nc
: -k
, which keeps the connection open after the first connection, and -q 1
, which closes the connection after one second, so another connection can occur. Depending on the complexity of the script that is being run, the -q
timing may need to be adjusted.
Figure 1 shows a web page with the iostat
output.
Multiple Commands with Headings
Comments and multiple command-line utilities can be combined as a variable string that can be passed to the Bash web server.
The FIGlet [2] utility is useful for custom-sized ASCII headings, which can come in handy if you don't want use HTML syntax. To install FIGlet in Ubuntu enter:
sudo apt-get install figlet
To run FIGlet, simply add the text string and use the -f
option for a font presentation:
$ figlet "123 Test" -f smslant ______ ____ ______ __ < /_ ||_ / /_ __/__ ___ / /_ / / __/_/_ < / / / -_|_-</ __/ /_/____/____/ /_/ \__/___/\__/
To get the output shown in Figure 2, Listing 1 uses FIGlet headings with the sensors
and vmstat
utilities.
Listing 1
Using FIGlet Headings
title1=$(figlet Sensors -f big) cmd1=$(sensors) title2=$(figlet VMStat -f small) cmd2=$(vmstat) thebody="$title1\n$cmd1\n$title2\n$cmd2" while true; do echo \ -e "HTTP/1.1 200 OK\n\n$thebody" \ | nc -l -p 8080 -q 1; done
Bash Web Server with Raspberry Pi GPIO
For many Raspberry Pi projects, monitoring the status of the General Purpose Input/Output (GPIO) pins is quite important.
The Raspberry Pi gpio
utility is a command-line tool that can be used to read and write to GPIO pins. The readall
option can be used to show the present status of all the GPIO pins.
Rather than passing the Bash commands as a string, an alternative approach is to use a Bash script and then call (sh
) that file. An example script file (web_body.sh
) that shows the time and then calls the gpio readall
command would be:
#!/bin/bash # web_body.sh - Show the time and # PI GPIO pins date $T echo "$(gpio readall)"
To run this script file in a Bash web server, use the following command:
while true; do { \ echo -ne "HTTP/1.1 200 OK\r\n"; \ sh web_body.sh; } \ | nc -l -k -q 2 8080; \ done
Figure 3 shows the web page with the GPIO pins' time and the status.
Send GPIO Writes from the Address Bar
Client-side GET
requests can be simulated on the browser address bar. For example, entering
gpio write 7 1
in the address bar sends that string to the Bash Server as a GET
request.
In Figure 4, you can see that the HTTP request uses HTML encoding. In this example, a space is converted to %20
.
Bash code can be added to look for specific messages. In this case, you can search for the "gpio write 7 1"
or "gpio write 7 0"
messages. If found, the code then executes the extracted message.
The Bash web server code now is modified to look for the "GET gpio"
message and then decode any HTTP %20
characters to spaces. Next, the code parses out the string to get the GPIO message and finally executes the required command:
while true; do { echo -ne "HTTP/1.1 200 OK\r\n"; \ sh web_body.sh; } | \ nc -l -k -q 5 8080 | \ grep "GET /gpio" | \ sed -e 's/%20/ /g' | \ eval $( awk '{print substr($0,6,15) }') ; done
With the new code, the "gpio write" text entered in the address bar is executed, and the result can be seen in the web page (Figure 5).
Create an HTML Form
Entering commands on the command line works, but it's crude. A better way is to create an HTML Form.
The Bash web server code can remain exactly the same as in the earlier example. The original script (web_body.sh
) file can be modified to output in HTML format, and three forms can be included (Listing 2). The first and second forms will define the GET
actions to turn the GPIO pin on or off, and the third form will be used to refresh the page to check for GPIO changes. Figure 6 shows the client web page with buttons to turn on and off a GPIO pin. After toggling the GPIO pin, a refresh of the web page is required to see the new status.
Listing 2
Toggling a Rasp Pi GPIO Pin
#!/bin/bash # web_body.sh - Show the time and PI GPIO pins # - Use HTML instead of text output # - Add forms for GPIO on/off, and a refresh echo " <!DOCTYPE html><html><head> </head><body> <h1>Bash Commands in a Web Page</h1> <h2>Toggle Pin 7 On/Off</h2> <form action='gpio write 7 0'> <input type='submit' value='OFF'> </form> <form action='gpio write 7 1'> <input type='submit' value='ON'> </form> <form action=''> <input type='submit' value='Refresh Page'> </form> <pre> " date $T echo "$(gpio readall)" echo "</pre></body></html>"
The nc
utility is extremely powerful, but it can be rather dangerous in that it can create back doors into your system. In this example, the code was specifically looking for the string "GET /gpio"
. This allows only gpio
commands to be passed. However, if the code only looked for "GET /"
, then you could potentially pass any command string to your server.
Final Comments
A Bash web server is a quick and easy solution for viewing the output from Bash scripts and commands. I especially like the fact that I don't need to install any special software, nor do I need to write any HTML code.
It is important to note that the number of concurrent connections is very low (one per second if the nc -q
option is 1
).
A Bash web server supports client-side GET
and POST
requests. However, for complex requirements, the Bash code could start to get messy quickly. In that case, it would probably be best to look at another solution.
Infos
- One-line web servers: https://gist.github.com/willurd/5720255
- FIGlet documentation: http://www.figlet.org/