Run A Command Line Process On All Files In A Directory And Collect The Output Into A New File

The Script

This was a test script for setting up a larger process. It reads all the files in a directory, passes them through a secondary process, then puts all the outputs in a single file:

Code

#!/bin/bash

OUTPATH="results file.txt"
: > "$OUTPATH"

for file in input_dir/*.txt
do
  cat "$file" \
  | tr '[:upper:]' '[:lower:]' \
  >> "$OUTPATH" 
done

Details

Using quotes around `$OUTPATH`` allows spaces in the path
The first `: < "$OUTPATH"`bash` line clears out the file. From the bash manual, the `:`bash` "Do nothing beyond expanding arguments and performing redirections. The return status is zero." When you point that to a file with `<`bash` it clears it out
This version uses a `for`bash` loop. I've seen other stuff that uses `file`bash` but haven't explored the differences yet
The meat of the program runs `cat`bash` on each file to output it then pipes that output to `tr`bash` with `PIPE_HERE`bash` before appending it to the output file via `<\<`bash`
The `tr`bash` command changes all upper case letters to lowercase
Any command that works in the pipeline can be used in place of `tr`bash`. It's simply what's being used for the illustration

Example

Given these three input files:

input_dir/alfa.txt

Pitch The Straw

input_data/bravo.txt

TacK iT dOWn

input_data/charlie.txt

RAISE THE SAIL

The script will produce:

Results

pitch the straw
tack it down
raise the sail