Get rid of "warning: command substitution: ignored null byte in input"

Question

I'm getting -bash: warning: command substitution: ignored null byte in input when I run model=$(cat /proc/device-tree/model)

bash --version
GNU bash, version 4.4.12(1)-release (arm-unknown-linux-gnueabihf)

With bash version 4.3.30 it's all OK

I understand the problem is the terminating \0 character in the file, but how can I suppress this stupid message? My whole script is messed up since I'm on bash 4.4

Answer 1

If you just want to delete the null byte:

model=$(tr -d '\0' < /proc/device-tree/model)

Answer 2

There are two possible behaviors you might want here:

• Read until first NUL. This is the more performant approach, as it requires no external processes to the shell. Checking whether the destination variable is non-empty after a failure ensures a successful exit status in the case where content is read but no NUL exists in input (which would otherwise result in a nonzero exit status).

  IFS= read -r -d '' model </proc/device-tree/model || [[ $model ]]
  

• Read ignoring all NULs. This gets you equivalent behavior to the newer (4.4) release of bash.

  model=$(tr -d '\0' </proc/device-tree/model)
  

  You could also implement it using only builtins as follows:

  model=""
  while IFS= read -r -d '' substring || [[ $substring ]]; do
    model+="$substring"
  done </proc/device-tree/model