why do I have to eval to > in bash?

Question

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?

similar question here http://stackoverflow.com/questions/11065077/eval-command-in-bash-and-its-typical-uses — Jayesh Bhoi, Aug 16 '14 at 08:38
yes I am aware of that one, I read that carefully, somehow it did not yield an answer here for me — Mark Galeck, Aug 16 '14 at 08:41

konsolebox · Accepted Answer · 2014-08-16T08:42:29.703

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"

score 0 · Answer 2 · answered Aug 16 '14 at 08:19

0

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.

answered Aug 16 '14 at 08:19

roelofs

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OK, then why does `$foo true` work when foo is echo ?? – Mark Galeck Aug 16 '14 at 08:27

why do I have to eval to > in bash?

2 Answers2