shell shell-script dash stdin – Check if Standard Input Comes from a File or Pipe in a Script

dashshellshell-scriptstdin

I know this has been poorly covered previously, but those answers either lack explanation or don't apply.

Basically at some point my scripts needs check if a file is specified, if it was it will be used later as input.

[ -f "$1" ] && TINPUT="$1"

simple enough… Now if a file was not found, or not specified, I would have TINPUT="-" which would tell the later command to read stdin.

Here is my question… How do i get the script to die with error, if it was run without a pipe or without a file specified?

I'm using dash, the Debian POSIX complient shell, so I can't use Bashisms. I also prefer to use lists, over ifs but most ifs could be written in lists anyway.

Best Answer

You can test whether standard input is a terminal:

if [ -n "$1" ]; then
  exec <"$1"
elif tty >/dev/null; then
  echo 1>&2 'Cowardly refusing to read data from a terminal.'
  exit 2
# else we're reading from a file or pipe
fi

Related Solutions

Shell – Can the shell warn me if a program is waiting for standard input

If you know you're never going to use grep to read from the terminal, you could redefine grep as:

grep() {
  if [ -t 0 ]; then
    < /dev/null command grep "$@"
  else
    command grep "$@"
  fi
}

That will not give you any warning about you typo. But at least it will return without a match immediately. It will also affect behaviour when - or /dev/stdin is passed as an argument to grep.

Edit:

Actually, a way to get a warning would be to close stdin instead of redirecting it from /dev/null:

grep() {
  if [ -t 0 ]; then
    <&- command grep "$@"
  else
    command grep "$@"
  fi
}

$ grep foobar.txt
grep: (standard input): Bad file descriptor

Background Process – Pipe Input in Unix/Linux

Yes, as required by POSIX, commands started in background with & have their standard input redirected from /dev/null.

And indeed

{ cmd <&3 3<&- & } 3<&0

is the most obvious way to work around it.

It's not clear why you'd want to run part of pipeline in background though.

Best Answer

Related Solutions

Shell – Can the shell warn me if a program is waiting for standard input

Background Process – Pipe Input in Unix/Linux

Related Question