A Bash web server

One Liners

© Lead Image © valenty, 123RF.com

© Lead Image © valenty, 123RF.com

Article from Issue 250/2021
Author(s):

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.

Figure 1: The iostat Bash command on a web page.

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
Figure 2: Two Bash utilities with FIGlet headings.

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