As Ciro Santilli notes using command substitutions will drop trailing newlines. Their workaround adding trailing characters is great, but after using it for quite some time I decided I needed a solution that didn't use command substitution at all.

My approach now uses read along with the printf builtin's -v flag in order to read the contents of stdin directly into a variable.

# Reads stdin into a variable, accounting for trailing newlines. Avoids needing a subshell or
# command substitution.
read_input() {
  # Use unusual variable names to avoid colliding with a variable name
  # the user might pass in (notably "contents")
  : "${1:?Must provide a variable to read into}"
  if [[ "$1" == '_line' || "$1" == '_contents' ]]; then
    echo "Cannot store contents to $1, use a different name." >&2
    return 1
  fi

  local _line _contents
   while read -r _line; do
     _contents="${_contents}${_line}"$'\n'
   done
   _contents="${_contents}${_line}" # capture any content after the last newline
   printf -v "$1" '%s' "$_contents"
}

This supports inputs with or without trailing newlines.

Example usage:

$ read_input file_contents < /tmp/file
# $file_contents now contains the contents of /tmp/file

As Ciro Santilli notes using command substitutions will drop trailing newlines. Their workaround adding trailing characters is great, but after using it for quite some time I decided I needed a solution that didn't use command substitution at all.

My approach now uses read along with the printf builtin's -v flag in order to read the contents of stdin directly into a variable.

# Reads stdin into a variable, accounting for trailing newlines. Avoids
# needing a subshell or command substitution.
# Note that NUL bytes are still unsupported, as Bash variables don't allow NULs.
# See https://stackoverflow.com/a/22607352/113632
read_input() {
  # Use unusual variable names to avoid colliding with a variable name
  # the user might pass in (notably "contents")
  : "${1:?Must provide a variable to read into}"
  if [[ "$1" == '_line' || "$1" == '_contents' ]]; then
    echo "Cannot store contents to $1, use a different name." >&2
    return 1
  fi

  local _line _contents=()
   while IFS='' read -r _line; do
     _contents+=("$_line"$'\n')
   done
   # include $_line once more to capture any content after the last newline
   printf -v "$1" '%s' "${_contents[@]}" "$_line"
}

This supports inputs with or without trailing newlines.

Example usage:

$ read_input file_contents < /tmp/file
# $file_contents now contains the contents of /tmp/file

As Ciro Santilli notes using command substitutions will drop trailing newlines. Their workaround adding trailing characters is great, but after using it for quite some time I decided I needed a solution that didn't use command substitution at all.

My approach now uses read along with the printf builtin's -v flag in order to read the contents of stdin directly into a variable.

# Reads stdin into a variable, accounting for trailing newlines. Avoids needing a
# subshell or command substitution.
read_input() {
  # Use unusual variable names to avoid colliding with a variable name
  # the user might pass in (notably "contents")
  local __line __contents __varname
  __varname=${1:?Must provide a variable to read into}
   while read -r __line; do
     __contents="${__contents}${__line}"$'\n'
   done
   __contents="${__contents}${__line}" # capture any content after the last newline
   printf -v "$__varname" '%s' "__contents"
}

This supports inputs with or without trailing newlines.

As Ciro Santilli notes using command substitutions will drop trailing newlines. Their workaround adding trailing characters is great, but after using it for quite some time I decided I needed a solution that didn't use command substitution at all.

My approach now uses read along with the printf builtin's -v flag in order to read the contents of stdin directly into a variable.

# Reads stdin into a variable, accounting for trailing newlines. Avoids needing a subshell or
# command substitution.
read_input() {
  # Use unusual variable names to avoid colliding with a variable name
  # the user might pass in (notably "contents")
  : "${1:?Must provide a variable to read into}"
  if [[ "$1" == '_line' || "$1" == '_contents' ]]; then
    echo "Cannot store contents to $1, use a different name." >&2
    return 1
  fi

  local _line _contents
   while read -r _line; do
     _contents="${_contents}${_line}"$'\n'
   done
   _contents="${_contents}${_line}" # capture any content after the last newline
   printf -v "$1" '%s' "$_contents"
}

This supports inputs with or without trailing newlines.

Example usage:

$ read_input file_contents < /tmp/file
# $file_contents now contains the contents of /tmp/file