Bash variable interpolation: a guide for shell programmers

If you're a shell programmer working with Bash, you'll know that variable interpolation is a crucial part of working with shell scripts. In short, variable interpolation is the process of inserting the value of a variable into a string or command, allowing you to dynamically build commands and output strings.

Syntax

The syntax for variable interpolation is fairly simple. To interpolate a variable, simply prefix its name with a dollar sign ($), like so:

echo "Hello, $USER"

This will output "Hello, " followed by the value of the $USER variable. Note that you don't need to use any special syntax to interpolate variables in strings - simply including the variable name in the string is enough.

Escaping

Sometimes, you'll want to include a literal dollar sign in your output string, without actually interpolating a variable. In these cases, you'll need to escape the dollar sign with a backslash (), like so:

echo "You owe me $10"

This will output "You owe me $10".

Brace expansion

Bash also supports a feature called brace expansion, which allows you to generate multiple values from a single expression. For example:

echo {1..5}

This will output "1 2 3 4 5". You can also use brace expansion to generate filenames, like so:

touch file{1..5}.txt

This will create five files: file1.txt, file2.txt, file3.txt, file4.txt, and file5.txt.

Conclusion

Variable interpolation is a powerful feature of Bash that allows you to build dynamic commands and output strings. By mastering the syntax and techniques involved, you can become a more efficient and effective shell programmer.