Write shell scripts in OpenWrt

The default shell provided with OpenWrt is the Almquist shell, which is better known as the ash shell. The Almquist shell is the default Busybox shell. Most Linux distros, such as Ubuntu or Debian, will use the Bash, which is much bigger and more complex than ash. For example, a typical bash implementation requires approximately 1 MB of disk space, which is an extravagant waste of space memory/space in an embedded device.

By contrast, BusyBox fits in less than 512 KB of space, and, in addition to providing the ash shell, it also provides many other tools you will need to manage your OpenWrt device. Some examples of the other very useful tools provided by BusyBox include awk, grep, sed, and vi/vim. Note that the stock (i.e. factory) firmware that ships with many routers also uses BusyBox for the reasons outlined here.

For this reason, OpenWrt firmware uses BusyBox, which is extremely efficient with respect to 1) the size required to install and run in RAM, and 2) the amount of functionality it provides. Please be aware that many of the BusyBox implementations of common Linux/Unix tools might be more limited than their full desktop counterparts. However, Bash and Busybox shell are similar enough for the majority of daily use cases. If your script is using the POSIX features of the Bash shell, then it will work in the Ash shell, too.

Q: Can I install a full version of Bash or other Linux CLI tools?

A: Yes!

If your router supports USB storage and you choose to install and configure a USB drive with your OpenWrt device, you will have much more storage space for installing programs. If you choose to go this route, then you can install the bash package very easily. Also, OpenWrt contains many packages for installing the full versions of core Linux shell utilities, such as basename, cat, rm, sort, or sleep. To get an idea of the full list of available CLI packages, visit the web page for the OpenWrt packages in the Utilities category and search for all packages that start with coreutils. Also, OpenWrt contains many full sized versions of popular Linux editors such as nano and vim (see vim, vim-full, vim-fuller).

Scripts in OpenWrt should have #!/bin/sh at the first line, #!/bin/bash will not work.

Write your bash scripts with #!/bin/sh first line in https://www.shellcheck.net/ and it will check for features you cannot use in OpenWrt shell. see the https://github.com/koalaman/shellcheck/blob/master/README.md#portability

You may need to install additional command line tools in OpenWrt, as the default installation is very minimal. A Desktop PC Linux like Ubuntu has much more command line tools in default install.

Check if a tool is available by writing type tool-name and it will answer you where this tool is, or an error if there is no such tool installed. Use the package table or index (Packages link in the sidebar to the left of this article) to find the package with the tool you need.

# type time
time is /usr/bin/time

See also: Configuration in scripts