Fish Shell:
Interpolating a string

How to:

In Fish, you use double quotes and place the variable or command you want to interpolate with a dollar sign $ straight in the string.

set name "world"
echo "Hello, $name!"

Output:

Hello, world!

To include the output of a command within a string:

echo "I have (count (ls)) files in this directory."

Output might be:

I have 9 files in this directory.

Variables and commands get evaluated and neatly tucked into the place you put them.

Deep Dive

Before Fish and other modern shells, you’d often use a more clunky combo of quotes and concatenation—or rely on external tools—to get variables into strings.

In bash, for example, it’d look like this:

name="world"
echo "Hello, "$name"!"

Not as slick, right?

Fish not only streamlines this process but also handles errors more gracefully. If a variable doesn’t exist, Fish will insert an empty string, lessening the chance of a crash from mishandled interpolations.

Alternatives to direct interpolation include using the printf command:

set animal "narwhal"
printf "The %s is an awesome creature!" $animal

Output:

The narwhal is an awesome creature!

In this case, %s is a placeholder for the string variable $animal that gets replaced by printf.

In terms of implementation, when Fish processes the command line, it parses the double-quoted strings and swaps the variables with their values on the fly. It’s elegant and mimics the variable interpolation found in higher-level languages like Ruby or PHP.

See Also

For more on Fish string manipulation and scripting, check these out:

• Fish Shell Documentation: Quotes
• Fish Shell Tutorial
• Stack Overflow: How to use variables in a command in Fish