Piping to the command substitution of a string containing pipes

Question

This works—

$ x='grep a'
$ echo ab | $x
ab

This doesn't—

$ x='grep a | grep b'
$ echo ab | $x
grep: |: No such file or directory
grep: grep: No such file or directory
grep: b: No such file or directory

It appears in the latter case, grep a | grep b is seen as a single command, i.e. grep a \| grep b.

How do I get the second example to work without modifying x?

Solution 1

When you want to run variables that contain code you'll often want to use the command eval. This will expand the contents of the variable so that they can be executed.

Example

$ x='grep a | grep b'
$  echo ab | eval "$x"
ab

Using eval is often discourage though, so use caution, see this BashFAQ titled: Eval command and security issues for more examples!

References

• Execution of a command in a variable with eval

Solution 2

Ah, I figured it out after some experimentation—use alias—

$ x='grep a | grep b'
$ alias y=$x
$ echo ab | y
ab

Please do post any other ways to do this—I'd be interested in alternatives.

Comments

[valid]

When such an alias is run non-interactively, i.e. in a script, you need shopt -s expand_aliases before the first use of the alias - interestingly not necessarily before the alias definition.

[Barmar]

You should double-quote $x.

[slm]

@barmar good catch will do