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";
$cmd >& output.log;
What is wrong with the first method? How to fix it?
Thanks!
Using eval works, but is bad practice for security reasons. The Right Thing, when you need to perform redirections inside code stored for reuse, is to define a function:
cmd() { some_command &> output.log; } # define it
declare -p cmd # print it
cmd # run it
If you don't need redirections, then the right thing is an array:
cmd=( something 'with spaces' 'in args' ) # define it
printf '%q ' "${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).
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.
& in the command above isn't to send it in the background. >& output.log is equivalent to saying > output.log 2>&1.
.in your second code example is not treated as a concatenator, which seems to be what you expect (I guess you're probably coming from Perl or something?); instead, it's printed literally. Just remove it. (Also, as devnull pointed out, you need to remove the spaces around=.)echo one two threeis the same asecho "one two three"orecho one two three, butecho "one two three"is different because the quotation marks will cause the spaces to be preserved.