CODEWITHSUNDEEP
bash
Mark Galeck
Mark Galeck

Reputation: 6405

why do I have to eval to > in bash?

In bash, why do I have to eval a variable whose value is a redirection:

bash> foo=echo
bash> $foo true
true
bash> foo='>/dev/null'
bash> $foo true
bash: >/dev/null: No such file or directory
bash> eval $foo true
bash>

How to do this without eval?

Upvotes: 0

Views: 180

Answers (2)

konsolebox
konsolebox

Reputation: 75618

Redirections are simply not parsed after word splitting with a variable. Only pathname expansion and similar stuffs are recognized.

With your example, >/dev/null is treated literally there as if you ran:

">/dev/null" "false"

In which case, > doesn't have a special meaning.

That is the reason why eval is needed to re-evaluate the resulting arguments as a new command. Whatever you see with echo is what you get with eval.

# foo='>/dev/null'
# echo $foo true
>/dev/null true

So sometimes you also need to quote to prevent word splitting and other expansions before evaulation:

echo "$foo true"

Upvotes: 3

roelofs
roelofs

Reputation: 2170

eval means evaluate, telling the shell to evaluate (or perform) the expression inside the quotes. Otherwise it's regarded as just a literal string, likely only having meaning to the user.

If you provide more detail in your question, I might be able to help with alternative approaches.

Upvotes: 0

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