Redirecting stdout only if command failed?

Question

I'm writing a bash script that is supposed to be "transparent" to the user. It reads commands from the user and intercepts them, allowing only some of them to be executed by bash, depending on some criteria. It (basically) works like this:

while true; do
   read COMMAND
   can_be_done $COMMAND
   if [ $? == 0 ]; then
      eval $COMMAND
      if [ $? != 0 ]; then
         echo "Error: command not found"
      fi
   fi
done

The problem is, when the command fails, you also get stuff printed to the console. BUT, if I keep the result in a variable and only print it when it doesn't fail, like so:

RESULT=$(eval $COMMAND)

Then there's another problem: The special formatting gets lost (for example, "ls --color" doesn't show colors anymore)

My question is: Is there a way to have the command print to STDOUT if successful, but to /dev/null if it fails?

Answer 1

Do you really need the second part, replacing the output of the command with an error message? Linux commands print their own error messages, which aren't necessarily "command not found". You'd be hiding the true error (permission denied, file not found, out of memory, segfault, etc.) with an oftentimes incorrect error message (command not found).

If you remove that check, you could simplify the loop to something like this:

while true; do
   read -e COMMAND
   if can_be_done "$COMMAND"; then
      eval "$COMMAND"
   fi
done

• read -e uses readline to obtain the command, making the prompt a lot more shell-like (↑ and ↓ for history, for instance).
• command; if [ $? == 0 ]; then is more idiomatically written as if <command>; then.
• Quoting makes sure special characters and whitespace are handled properly.

Answer 2

I would argue strongly that you should not do this. If you do not want to see output, redirect it to /dev/null. If you do want to see errors, do not redirect stderr. If you are using a program that prints its error messages on stdout instead of stderr, FIX THE PROGRAM! Error messages belong on stderr. Note that this means your program is broken, as it ought to read:

 echo "Error: command not found" >&2

I'm not sure if it is rule number 1, but it certainly belongs in the top 10, and it may be the most often violated rule: Error messages belong on stderr. A program which prints error messages on stdout is broken.

Answer 3

if false > /dev/null;then echo 1; else echo 2; fi 2> /dev/null

Will output 2

if true > /dev/null;then echo 1; else echo 2; fi 2> /dev/null

Will output 1

remove the > /dev/null to print the command also to stdout

for example if echo 123;then echo 1; else echo 2; fi 2> /dev/null

Will output 123 & 1

Answer 4

Assuming that the command is not very expensive to run you can do this:

   test `ls /mooo 2>/dev/null` || echo moo not found

test will return true only if the command exits with 0, in this case ls is the command. You could have put this in an if statement too like so:

if [ `ls /moo 2>/dev/null` ];then 
  echo moo is a folder
fi