Question

Im doing this that works fine

utnso@utnso-vm:~$ read command
ps -fea
utnso@utnso-vm:~$ $command
UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 23:34 ?        00:00:02 /sbin/init
root         2     0  0 23:34 ?        00:00:00 [kthreadd]
root         3     2  0 23:34 ?        00:00:00 [ksoftirqd/0]
...

But when trying to run this command it fails…

utnso@utnso-vm:~$ read command
ps -fea | grep bash
utnso@utnso-vm:~$ $command
ERROR: Garbage option.
********* simple selection *********  ********* selection by list *********
-A all processes                      -C by command name
-N negate selection                   -G by real group ID (supports names)
-a all w/ tty except session leaders  -U by real user ID (supports names)
-d all except session leaders         -g by session OR by effective group name
-e all processes                      -p by process ID
T  all processes on this terminal     -s processes in the sessions given


   ...
    ...
    ...
    ...
                        ********* misc options *********
-V,V  show version      L  list format codes  f  ASCII art forest
-m,m,-L,-T,H  threads   S  children in sum    -y change -l format
-M,Z  security data     c  true command name  -c scheduling class
-w,w  wide output       n  numeric WCHAN,UID  -H process hierarchy

Does anybody know whats going on??

Thanx in advanced!

Answer 1

When your command line consists of a single parameter to be expanded, such as

$ $command

bash does the following:

1. Expand the parameter, and perform word splitting on the result.
2. Execute the first word as a simple command, passing the remaining words as arguments.

That’s it. No further parsing of the expansion is done to process the result as any type of compound command or variable assignment or redirection.

Anything more complicated needs to be passed to the the eval command, but only if you fully understand how eval works and know the security implications of evaluating an arbitrary string. In my opinion, eval should be considered a tool of last resort, not a convenience.