In bash, I can evaluate a string as a statement, e.g.
a="echo hello"
eval $a
I recently found that you can also just expand a using
$a
and it seems to work just as well.
Multi-line input does not work, but other than that, is $a generally a legal substitution for eval $a?
No. First of all, you want to do eval "$a", or what you evaluate could run away from you. But more than that, $a will not do any expansions:
a='echo {a,b}{c,d}'
$a
# => {a,b}{c,d}
eval `$a`
# => ac ad bc bd
a='echo ~'
$a
# => ~
eval "$a"
# => /home/amadan
foo=bar
a='echo $foo'
$a
# => $foo
eval "$a"
# => bar
a='echo $(which echo)'
$a
# => $(which echo)
eval "$a"
# => /usr/bin/echo
a='echo $((1+2))'
$a
# => $((1+2))
eval "$a"
# => 3
a='sort <(echo foo; echo bar)'
$a
# => sort: cannot read: <(echo: No such file or directory
eval "$a"
# => bar
# => foo
a='echo *'
$a
# => echo *.txt
eval "$a"
# => first.txt second.txt
You can use eval to assign variables:
a='baz=quux'
$a
# => -bash: baz=quux: command not found
eval "$a"
# no output
echo "$baz"
# => quux
There may be more that I did not think of.