Run-time code generation in Linux shell scripts

Question

Let's say I have the following snippet of code, which at runtime generates a string which I want executed in the shell (just like if I'd just pasted the string into a terminal and hit enter":

cmd=""
for i in first second third
do
    cmd="$cmd | grep $i"
done
cmd="cat /usr/share/dict/words $cmd"
$cmd

The problem here is that the pipe characters are not interpreted as such, however; they're treated as parameters to cat, as though I had escaped them:

cat: |: No such file or directory
cat: grep: No such file or directory
cat: first: No such file or directory
cat: |: No such file or directory
cat: grep: No such file or directory
cat: second: No such file or directory
cat: |: No such file or directory
cat: grep: No such file or directory
cat: third: No such file or directory

Is there a way to "execute" the string with the pipe characters unescaped? Is there a better way to generate code like this at runtime?

I'd prefer a portable solution but bashisms are acceptable.

If you can keep from getting too trusting, look at eval, i.e. eval $cmd. Constructing command lines programmatically is begging to get pwnt. Ask around about Little Bobby Tables. — jthill
– jthill, Commented Apr 16, 2014 at 0:16

austinmarton · Accepted Answer · 2014-04-16 00:16:52Z

4

To execute the string you can use eval $cmd

answered Apr 16, 2014 at 0:16

austinmarton

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