Dynamic command evaluation

Two built-in utilities support dynamic command evaluation.

Evaluating command strings

The eval built-in evaluates a command string. This is useful for constructing and executing commands dynamically.

For example, you can use eval to assign a value to a variable whose name is chosen at runtime:

echo "Type a variable name:"
read -r varname
eval "$varname='Hello, world!'"
eval "echo 'The value of $varname is:' \$$varname"

Reading and executing files

The . (dot) built-in reads and executes commands from a file. This is useful for organizing scripts and reusing code.

For example, you can use . to source a file containing variable definitions:

# contents of vars.sh
greeting="Hello, world!"
farewell="Goodbye, world!"

# main script
. ./vars.sh
echo "$greeting"
echo "$farewell"

source is a non-POSIX synonym for the . built-in.