如何在bash shell中重定向命令输出？

Question

I'm having a problem writing a small bash command. Basically I want to echo the wrapper command and redirect the output of the real command to a log file.

Something like this in my .bashrc doesn't work -- the output still gets to the console.

cmd="some_command >& output.log";
echo $cmd;
$cmd;

But the following works -- the output is directed into the log file.

cmd = "some_command";
echo $cmd" >& output.log";
$cmd >& output.log;

What is wrong with the first method? How to fix it?

Thanks!

Answer 1

Using eval works, but is bad practice for security reasons. The Right Thing, when you need to perform redirections inside code stored for reuse, is to define a function:

cmd() { some_command &> output.log; } # define it
declare -p cmd                        # print it
cmd                                   # run it

If you don't need redirections, then the right thing is an array:

cmd=( something 'with spaces' 'in args' ) # define it
printf '%q ' "${cmd[@]}"; echo            # print it
"${cmd[@]}"                               # run it

This is safer, inasmuch as array contents won't go through a full eval pass. Think about if you did cmd="something-with $filename" , and filename contained $(rm -rf /) . If you used eval , this would run the rm command!

---

To provide a more specific example, this would hose your system if run as root:

# !!! I AM DANGEROUS DO NOT RUN ME !!!
evil_filename='/tmp/foo $(rm -rf /)'
cmd="echo $evil_filename"             # define it (BROKEN!)
eval "$cmd"                           # run it    (DANGEROUS!)

On the other hand, this would be safe:

evil_filename='/tmp/foo $(rm -rf /)'
cmd=( echo "$evil_filename" )         # define it (OK!)
printf '%q ' "${cmd[@]}"; echo        # print it  (OK!)
"${cmd[@]}"                           # run it    (OK!)

...and it would still be safe even if you left out some of the quotes -- it would work wrong, but still not break your system:

# I'm broken, but not in a way that damages system security
evil_filename='/tmp/foo $(rm -rf /)'
cmd=( echo $evil_filename )           # define it (BROKEN!)
${cmd[@]}                             # run it    (BROKEN!)

And this would be safe too:

evil_filename='/tmp/foo $(rm -rf /)'
cmd() { echo "$1"; }                  # define it (OK!)
cmd "$evil_filename"                  # run it    (OK!)

---

For a more in-depth discussion, see BashFAQ #50 (on properly storing command sequences for reuse), and BashFAQ #48 (on why eval is dangerous).

Answer 2

What is wrong with the first method?

When you include the redirection operators within a variable, the shell doesn't treat those as special. Instead those are considered as arguments to the program in question.

One solution is to make use of eval :

cmd="some command >& output.log";
eval $cmd;

As an aside, the following is wrong:

cmd = "some command";

You cannot have spaces around = in a variable assignment.