Question

I have a command that I need to use repeatedly within a shell script. This is command contains a pipe and the output of the whole command will be piped to other commands.

e.g. Let’s say for simplicity sake the command is ls | tee. Then I might pipe it to other commands, says ls | tee | someprogram or ls | tee | anotherprogram.

So naturally I’ll want to keep ls | tee is a variable. The problem is that I can’t seem to execute a variable with a pipe in it.

#!/bin/sh
TEST="ls | tee"
$TEST

Gives the following output

ls: cannot access |: No such file or directory
ls: cannot access tee: No such file or directory

How do I execute a variable like $TEST above, whist being able to pipe the output to other commands?

Answer 1

The short answer is eval.

eval $TEST somefile

eval $TEST otherfile | more

However, you need to be aware that eval means problems with quoting special characters and blanks and the like. If everything is simple (TEST="ls -l | tee"), then it is easy. If you have spaces in arguments or shell metacharacters, then it is hard — very hard — to do it right. At that point, you’d be better off creating a function or separate shell script.

You might well be better off with a function or shell script even so.

If the string you eval comes from a user, you have to worry even more!