How Do I Assign an Expression in Bash without Evaluating It?

In Bash, You might want to store an expression as a string in a variable without having Bash immediately evaluate it. This is useful when You want to pass the expression around, manipulate it, or evaluate it later.

Here's How You can achieve this:

1. Assign the expression to a variable as a string:

- To prevent Bash from evaluating the expression immediately, enclose it in single quotes ('). Single quotes prevent variable expansion and command substitution.

- Example:

expression='2 + 2'

- In this case, expression will contain the literal string 2 + 2, not the result of the addition.

2. Evaluate the expression later:

- To evaluate the stored expression, You can use the eval command. Be cautious when using eval, as it can execute arbitrary code if the expression contains user-supplied input. It is best practice to avoid eval unless absolutely necessary.

- Example:

expression='2 + 2'
eval "result=$expression"
echo "$result" # Output: 4

3. Using printf for safe evaluation (Bash 4.2+):

- A safer alternative to eval, especially when the expression may contain untrusted input, is to use printf with the %q format specifier (available in Bash 4.2 and later):

- Example:

expression='2 + 2'
printf -v result '%((%s))' "$expression"
echo "$result" # Output: 4

4. Using declare -p to avoid evaluation:

- You can also declare a variable using `declare -p` to view how the variable is stored, which can be useful for debugging and ensuring the expression is stored as a string.

- Example:

expression='2 + 2'
declare -p expression

This will output something like: `declare -- expression="2 + 2"` showing the variable is stored as a string.

In summary, using single quotes ensures the expression is stored as a string, and You can later evaluate it using eval or, preferably, printf. Always exercise caution when evaluating expressions, especially if they contain user-supplied input.

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