Way to ‘tee’ input to a program or script

I'd make heavier use of I/O redirection:

#!/bin/bash
[[ $1 ]] && [[ ! -f $1 ]] && echo "file $1 dne" && exit 1
[[ $1 ]] && exec 3<$1 || exec 3<&0
[[ $2 ]] && exec 4>$2 || exec 4>&1
fgrep -v "stuff" <&3 >&4

Explanation

• [[ $1 ]] && [[ ! -f $1 ]] && echo "file $1 dne" && exit 1

  Test if an input file has been specified as a command line argument and if the file exists.

• [[ $1 ]] && exec 3<$1 || exec 3<&0

  If $1 is set, i.e. an input file has been specified, the specified file is opened at file descriptor 3, otherwise stdin is duplicated at file descriptor 3.

• [[ $2 ]] && exec 4>$2 || exec 4>&1

  Similarly if the $2 is set, i.e. an output file has been specified, the specified file is opened at file descriptor 4, otherwise stdout is duplicated at file descriptor 4.

• fgrep -v "stuff" <&3 >&4

  Lastly fgrep is invoked, redirecting its stdin and stdout to the previously set file descriptors 3 and 4 respectively.

Reopening standard input and output

If you'd prefer not to open intermediate file descriptors, an alternative is to replace the file descriptors corresponding to stdin and stdout directly with the specified input and output files:

#!/bin/bash
[[ $1 ]] && [[ ! -f $1 ]] && echo "file $1 dne" && exit 1
[[ $1 ]] && exec 0<$1
[[ $2 ]] && exec 1>$2
fgrep -v "stuff"

A drawback with this approach is that you loose the ability to differentiate output from the script itself from the output of the command which is the target for the redirection. In the original approach, you can direct script output to the unmodified stdin and stdout, which in turn might have been redirected by the caller of the script. The specified input and output files could still be accessed via the corresponding file descriptors, which are distinct from the script stdin and stdout.

When this happens, run bash from the command line, then exit (to return the first bash). Should work again. Somewhat interesting details here: https://stackoverflow.com/questions/19895185/bash-shell-read-error-0-resource-temporarily-unavailable.