Bash: Inline Iterate over multiple values

Question

Morning,

I'm currently in a situation where I have to repeat a command for several datasets and am wondering, if there is a more elegant solution than nesting loops or building a dedicated program.

So basically something like:

> echo "Hello from [NYC,Chicago] to [Berlin,Paris]"
Hello from NYC to Berlin
Hello from NYC to Paris
Hello from Chicago to Berlin
Hello from Chicago to Paris

Sure, some "for i in x,y,z"-magic would work, but that gets hard to read as soon as multiple datasets are involved.

Is there a way to easily iterate over multiple datasets in Bash? How would you solve this to keep it somewhat readable?

Answer 1

Use brace expansion.

printf '%s\n' 'Hello from '{NYC,Chicago}' to '{Berlin,Paris}

Note that it doesn't work for variables, only for literal values (as brace expansion happens before variable expansion).

Answer 2

If it's a quick one-off then a brace expansion would be quick and easy. If you're looking for a programmatic variation then a couple of loops might be more suitable.

Example 1, with trivially short datasets

# Iterate until done
for colour in red green blue
do
    for shape in square triangle circle
    do
        printf "> %s - %s <\n" "$colour" "$shape"
    done
done

Example 2, with larger datasets

# Prepare the scenario with two files, "colourfile" and "shapefile"
cat >colourfile <<'X'
red
green
blue
X

cat >shapefile <<'X'
square
triangle
circle
X

# Loop until done
while IFS= read -r colour
do
    while IFS= read -r shape
    do
        printf "> %s - %s <\n" "$colour" "$shape"
    done <shapefile
done <colourfile

This will read shapefile once through for each entry in colourfile. Fortunately disk caching will help mitigate the repeated I/O. If you are really concerned then reading the inner file into an array would trade disk accesses for memory usage.

Output for both examples

> red - square <
> red - triangle <
> red - circle <
> green - square <
> green - triangle <
> green - circle <
> blue - square <
> blue - triangle <
> blue - circle <