bash Idioms

Chapter 2. Looping Lingo

It’s not just C-style for loops—bash includes other syntaxes and styles; some are more familiar to Python programmers, but each has its place. There is a for loop with no apparent arguments, useful in both scripts and inside functions. There’s also an iterator-like for loop with explicit values and values that can come from other commands.

Looping Constructs

Looping constructs are common in programming languages. Since the invention of the C language, many programming languages have adopted the C-style for loop. It’s such a powerful, readable construct because it groups the initialization code, the termination condition, and the iteration code all into one place. For example, in C (or Java, or…):

/* NOT bash */
for (i=0; i<10; i++) {
    printf("%d\n", i);
}

With just a few minor syntax differences, bash follows much the same approach:

for ((i=0; i<10; i++)); do
    printf '%d\n' "$i"
done

Note, especially, the use of double parentheses. Rather than braces, bash uses do and done to enclose the statements of the loop. As with C/C++, an idiomatic use of the for loop is the empty for loop, giving a deliberate infinite loop (you’ll also see while true; do):

for ((;;)); do
    printf 'forever'
done

But that’s not the only kind of for loop in bash. Here’s a common idiom in shell scripts:

for value; do
    echo "$value"
    # Do more stuff with $value...
done

This looks like something is missing, doesn’t it? Where does value get its values? This won’t do anything ...