Execute command within variable

Question

I'm trying to execute a command, stored within a variable:

cmd="grep -i \"word1\" filename | grep -i \"word2\""
eval $cmd

But when I execute the script I get the errors:

grep: |: No such file or directory
grep: grep: No such file or directory

How can I execute commands like the one in my example without getting this errors?

Answer 1

You need to quote "$cmd" - and maybe avoid the "double-quotes. Anyway, to run this you do need to eval it - and this is due to the |pipe. A shell variable does not expand beyond the limits of a single simple command - and you're trying to run a compound command.

So:

cmd='grep -i "word1" filename | grep -i "word2"'

eval "$cmd"

Probably when you were expanding $cmd without quotes you ran into filename generation and/or $IFS issues. Just make sure it's reliably quoted and you'll be fine. Also verify that whatever is in "word[12]" doesn't contain double-quotes or backslashes or whatever - else you'll need to reevaluate your quoting style there.

Looking closer now at your error and I can tell you exactly what it was:

grep: |: No such file or directory
grep: grep: No such file or directory

So if I do:

echo | echo

The first echo prints a \newline to the second echo's stdin. If I do:

set \| echo
echo "$@"

The first echo prints each of its arguments, which are |pipe and echo respectively.

The difference is that the |pipe in the first case is interpreted by the shell's parser to be a token and is interpreted as such. In the second case, at the same the shell is scanning for the |pipe token it is also scanning for the $expand token - and so the |pipe is not yet there to be found because "$@" has not yet been replaced with its value.

So if you do:

grep -i "word" filename \| grep -i "word2"

...you're likely to get the same results because the | there does not delimit the commands, and is instead an argument to grep in its infiles position.

Here's a look at how many fields you get when you split $cmd with a default $IFS:

printf '<%s> ' $cmd
<grep> <-i> <"word1"> <filename> <|> <grep> <-i> <"word2">

And here's what filename generation might do:

touch 'echo 1simple &&' 'echo 2simple'
eval echo*

Answer 2

As for me it wrong way to use variable to save and to execute any command. There is better choice -- function:

my_cmd() {grep -i "word1" filename | grep -i "word2"}

That is all.

Or you can use different arguments like:

my_cmd() {grep -i "$1" "$3" | grep -i "$2"}

Then call it by

my_cmd word1 word2 filename

Answer 3

For the example you posted, you shouldn't need the eval. Just execute the variable like so:

cmd="grep -i \"word1\" filename | grep -i \"word2\""; # added missing " at the end    
$cmd

Watch your quotes. Maybe something like this is less prone to mistakes:

cmd='grep -i "word1" filename | grep -i "word2"';