Bash redirections in broader contexts

Everybody uses redirections. Whether it is to or from /dev/null, or a file, or even a command. But every so often I see sequential commands redirecting their output to the same place. A proof that very few grasp that you can use bash redirections in broader contexts, like compound commands, groups or blocks of code.

Brace list

Take the following example:

command1 2>/dev/null
command2
command3 2>/dev/null

There is, indeed, the case where we need the stderr information from command2, but in real life, I’ve found that quite rare. For all the other cases, when we don’t need the stderr of command2, we can group all the three commands under the same umbrella with the brace compound command:

{ command1
  command2
  command3
} 2>/dev/null

And this is probably the most common grouped redirection I’ve seen out there. I am biased here, but that the second form looks more logical and organised.

But wait, there’s more 🙂

Loops

You might have already noticed my grep replacement filtering article where I’m passing the contents of a file via the input redirection, directly to the while conditional. Well, it goes the other way as well.

Let’s say you have to execute an lslocks command once every second for one minute, prepend that information with the current date and time, add a delimiting line, and redirect everything in the same file. The whole while loop can redirect its output to the same file, and you don’t even need to use the append mode:

while (( SLEEP++ &lt 60 )) && sleep 1; do
  echo ====
  date
  lslocks
  echo
done >/tmp/output

Not only all four commands from within the while block will redirect their output to the /tmp/output file, but also the conditional command will do so. So the above could easily be replaced with:

while (( SLEEP++ < 60 )) && sleep 1 && echo ====; do
  date
  lslocks
  echo
done >/tmp/output

and you’ll get the same result.

We can use the for loop in the same way. Then the case, select and until blocks, they are all similar in this. The two conditional compound commands (( )) and [[ ]] also fall into this category.