BASH 4 - capture error messsage from subcommand

Question

I'm trying to capture the stderr of a subcommand, my idea is something like this:

subErr(){
  echo "${1}"
  exit 1
}

BLOB=$(some long command || subErr $2)
aws configure set x_security_token_expires $(echo $BLOB|jq -r '.Credentials.Expiration') --profile DEFAULT

So basically if the command fails, feed the stderr to a function which prints the error and exits.

If the command succeeds assign stdout to BLOB and continue processing

How can I achieve this?

Ed Morton · Accepted Answer · 2023-08-23T18:52:37.137

How about saving stderr to a file for later consumption by your function:

errFile=$(mktemp) || exit
trap 'rm -f "$errFile"; exit' 0

subErr(){
  cat "$errFile"
  exit 1
}

BLOB=$(some long command 2>"$errFile") || subErr

You could redirect all stderr to the same file with exec 2>"$errFile" so you don't need to add that to each command call but idk if you want to do that for all stderr output.

Alternatively, see redirect-stderr-and-stdout-to-different-variables-without-temporary-files where I particularly like:

{
  IFS= read -rd '' err
  IFS= read -rd '' out
  IFS= read -rd '' status
} < <({ out=$(ls /dev/null /x); } 2>&1; printf '\0%s' "$out" "$?")

which you'd have to massage to suit your needs.

BASH 4 - capture error messsage from subcommand

1 Answers1