Defensive BASH Programming: The Reprint

The aim of this page📝 is to rewrite Web Archive > Defensive BASH programming > Say what? as it is no more publicly available since the domain is down at the moment.

There’s also Defensive BASH Programming which is older. There’s also a critique on hackernews Defensive BASH programming | Hacker News. Let’s just transcribe. I am also changing the formatting as per shellcheck’s recommendations.

1. Make global variables immutable

• Try to keep globals to minimum
• Globals use UPPER_CASE naming convention
• Use readonly declaration

Same as declare -r, sets a variable as read-only, or, in effect, as a constant. Attempts to change the variable fail with an error message. This is the shell analog of the C language const type qualifier.

— https://tldp.org/LDP/abs/html/internal.html

• Use globals to replace the cryptic $0, $1, etc. names passed as arguments into the script call
• Globals the author always uses in his programs are as follows

Positional parameters: $0 is the name of the script itself, $1 is the first argument, $2 the second, $3 the third, and so forth. After $9, the arguments must be enclosed in brackets, for example, ${10}, ${11}, ${12}. The special variables $* and $@ denote all the positional parameters.

• from https://tldp.org/LDP/abs/html/othertypesv.html

2. All variables should be local

• the local keyword cannot be used within global scope

pkutaj@DESKTOP-LRK1G7U:~$ local foo
-bash: local: can only be used in a function

• Self-documenting parameters

• Usually for loop use the conventional i variable ⟹ makes it essential to declare it as local

On Unix-like operating systems, shift is a builtin command of the Bash shell. When executed, it shifts the positional parameters (such as arguments passed to a bash script) to the left, putting each parameter in a lower position.

• from https://www.computerhope.com/unix/bash/shift.htm

3. Use the top-level main() function — the only global command in the code is main

• Intuitive for functional programmers
• Helps keeping all variables local

4. Everything except global immutables and calling main is in a function

• As outlined, only 2 things are running globally ..1. Global immutables ..2. main which is calling main()
• We may start with

• but improve this with

• in the second example, finding fines is the problem of get_temp_files() and not of main()
• the code also becomes testable, by unit testing of get_temp_files()

5. Instead of using -x for debugging the whole file, wrap problematic sections into set -x and set +x for precision.

• Debugging is activated via the -x flag

bash -x my_prog.sh

• Don’t do the above
• Instead, debug just a small section of code wrapping it into set -x and set +x
• In addition, you can use the $FUNCNAME + $@ variables and print them while debugging

• You’ll get something like

temp_files /var/tmp
+ ls /var/tmp
+ grep pid
+ grep -v daemon
+ set +x

6. Extract commands into names with readable functions so that code reads “like newspaper”

• can you always quickly say does the following code do?
• of course, disagreements rage around this one on HN

• Let your code speak

7. Each line does just one thing

• Break expressions with \ or with natural line continuators
• For example

Can be written much cleaner — shellcheck removes backslash and puts the pipe at the end of the line and I am keeping this as opposed to the original doc

• Good example where we see the connection between lines and the connecting rods.

Here is the conflict with shellcheck. See the diff — I suggest that the point is made and both are fine

• for explanation of && and || see the great Advanced Bash-Scripting Guide > Chapter 26. List Constructs

8. Print usage with heredoc within a dedicated function

• Do not do the following
• It should be a function

• To remove the repetition of echo in each line, use Bash Heredoc < Linuxize

9. Command line arguments

• Here is an example to complement the usage function above.

• You use it like this, using the immutable ARGS variable we defined at the top:

main() {
    cmdline $ARGS
}
main

10. Unit Testing

• Use shunit2 for unit testing

• Here is another example using df command:

• Here I have an exception, for testing, I declare DF in the global scope not readonly.
• This is because of shunit2 not allowing to change global scope functions.

LINKS

• Defensive BASH programming — Say what?
• Bash Shift Builtin Command Help and Examples