Beware of Bash – Hyoungchul Kim

In this post, I list some important precautions and best practices when working with the Bash shell. Note that this will be updated as I find more of it.

1. Word splitting

When you enter commands in Bash shell, the shell performs several operations on your commands before finally executing them. Thus, understanding how your original command will be tranformed by the shell is important to avoid any fatal errors. This is especially true for Bash as one of the problems with Bash shell is that it does not easily let you know that there is something wrong with your command. So it is very likely that you have some fatal errors without ever knowing them until the very end.

One of these operations is called “word splitting.” This is a process where shell splits the results of the expansions (brace expansion, arithmetic expansion, etc) into separate words based on the characters of the IFS variable.¹

Why can this be problematic? Word splitting works almost on every unquoted expansions. So the results of your commands can be problematic if you do not consider this.

Still wondering what would happen? Try this and you will see.

Case 1 (not quoting expansion)

    name="1 2 3 4 5"
    mkdir $name
    ls
    rmdir 1 2 3 4 5

1
2
3
4
5
index.qmd
index.rmarkdown

This creates 5 files named “1”, “2”, “3”, “4”, “5”.

Case 2 (quoting expansion)

    name="1 2 3 4 5"
    mkdir "$name"
    ls
    rmdir "1 2 3 4 5"

1 2 3 4 5
index.qmd
index.rmarkdown

This creates one file named “1 2 3 4 5”.

Hence, it is always a best practice to quote the expansion.

2. Bash is sensitive to space (white character)

In Bash, space (white character) matters a lot. Use it only when it is necessary.

Example 1: Space gives you error²

# Wrong!
car = 7; echo $?

# Correct!
car=7; echo $?

bash: line 2: car: command not found
127
0

Example 2: Sometimes though, you need space

# Wrong!
[10 -eq 10]; echo $?

# Correct!
[ 10 -eq 10 ]; echo $?

bash: line 2: [10: command not found
127
0

Footnotes

The IFS variable is a string of special characters that works as delimiters between words. The default value of IFS is space, tab, newline.↩︎
You might wonder what 127 and 0 mean in the output. In Bash, every command gives off return code. If it is 0, it means execute code was run without problem. If it has some other values, it means there is something wrong with the command.↩︎