3 Expansion

Expansion in a command shell is somewhat like macro expansion in macro parsers (such as cpp and m4), but in a command shell, they are less often used for constants, and usually for using variables and string manipulation.7 For example, $var on a line expands to the value of the variable var when the line is executed. Expansions are usually passed as arguments, but may also be used as commands.8

You can concatenate expansions with regular string arguments or even other expansions. In the simplest case, when the expansion returns a string value, this is equivalent to ordinary string concatenation; for example, ‘${echo "foo"}bar’ returns ‘foobar’. The exact behavior depends on the types of each value being concatenated:

both strings

Concatenate both values together.

one or both numbers

Concatenate the string representation of each value, converting back to a number if possible.

one or both (non-nil) lists

Concatenate “adjacent” elements of each value (possibly converting back to a number as above). For example, ‘$(list "a" "b")c’ returns ‘("a" "bc")’.

anything else

Concatenate the string representation of each value.



Eshell has no string-manipulation expansions because the Elisp library already provides many functions for this.


E.g., entering just ‘$var’ at the prompt is equivalent to entering the value of var at the prompt.