Published On

Mar 9, 2022

Bash parallels

I spent 2+ hours working on a function to sync my private notes Repos. Actually getting the initial concept to work was quick, but then I wanted to make it work in parallel, easy. But then I wanted to capture the output and make it print out in serial. Not so easy.

Here is the my final solution:

notessync() {
  set +m
  declare -A outputs=()
  local msg="===-<output>-===\n"
  local dirs=(
    $HOME/docs/corpus
    $HOME/docs/notes/corpus
    $HOME/docs/notes/captainslog
  )

  echo "===-<starting>-==="
  for dir in ${dirs[@]}; do
    outputs[$dir]=$(mktemp /tmp/notessync.XXX)
    { run-job $dir 2 >&$outputs[$dir] &>1 & } 2>/dev/null
  done

  wait

  for dir in ${dirs[@]}; do
    msg+="$(cat $outputs[$dir])\n"
  done

  echo $msg
  set -m
}

What did I learn?

  • I need to store output to a file
    • This was done with a temp file (huray mktemp!)
      • outputs[$dir]=$(mktemp /tmp/notessync.XXX)
  • I learned about associative arrays in bash
    • declare -A output=()
    • i.e. output["foo"]="bar"
  • How to capture the output with >&
    • i.e. echo "foo" >&/tmp/some/file 2>&1
    • the end bit pipes stderr to stdout which is piped by >& to the tmp file
  • I need to set +m (disables job control?)
    • with that disabled, it removes some output about currently running background tasks
  • use wait to wait for background tasks to complete before ending function
  • then fold outputs back into msg and print out

Remaining Questions

I use a bash formatter. One weird thing I noticed is that the formatter would re-arrange the order of things

# from
run-job $dir >&$outputs[$dir] 2&>1
# to
run-job $dir 2 >&$outputs[$dir] &>1

Not sure what that's about but it doesn't seem to change the outcome. 🤷

Edit - 03-26-2022

Ok, sometimes I'm an idiot and sometimes my dyslexia sneaks one by.

The above re-arranging was do to a syntax error. It should be

run-job $dir >&$outputs[$dir] 2>&1

Also, >& some-file is equivalent to > some-file 2>&1. So the last redirect is not required.

On top of that, the preferred syntax is &>some-file. The result:

run-job $dir &>$outputs[$dir]

I read it as apply run-job to $dir, store stdout to $outputs[$dir] and redirect stderr to stdout

I've left the original script above with the mentioned errors, but below is the script with the issues resolved.

notessync() {
  set +m
  declare -A outputs=()
  local msg="===-<output>-===\n"
  local dirs=(
    $HOME/docs/corpus
    $HOME/docs/notes/corpus
    $HOME/docs/notes/captainslog
  )

  echo "===-<starting>-==="
  for dir in ${dirs[@]}; do
    outputs[$dir]=$(mktemp /tmp/notessync.XXX)
    { run-job $dir &>$outputs[$dir] & } 2>/dev/null
  done

  wait

  for dir in ${dirs[@]}; do
    msg+="$(cat $outputs[$dir])\n"
  done

  echo $msg
  set -m
}

Happy Coding,

=<<Berkeley>>=

Tags

bash