how does a while read loop work in bash? - bash

Question

This is a crawler from GitHub that I am to implement on myself but am unable to read the bash since am a novice. Can this be explained in answer
#!/bin/bash
# Create an array files that contains list of filenames
files=($(< url.txt))
cities=($(< city.txt))
url="http://www.grotal.com/"
citycodes=($(<citycode.txt))
# Read through the url.txt file and execute wget command for every filename
while IFS='=| ' read -r param uri; do
for file in "${files[#]}"; do
for city in "${cities[#]}"; do
mkdir "${city}"
mkdir "${city}/${file}"
wget -O "${city}/${file}/${file}${citycodes[#]}" "${uri}${url}${city}/${file}-${citycodes[#]}/"
done
done
done < url.txt
specifically these (even if u choose to downvote...)
while IFS='=| ' read -r param uri;
and then this:
done < url.txt

Answer 1

Let's break this down into pieces:
read, unless given a non-default -d argument to specify a terminator to use in place of the newline, reads a single line from stdin (that is, reads up to the next newline); splits that line on IFS characters, and writes each field into a different variable. If it stops being able to read more data before reaching a newline, then it emits a nonzero exit status, even if it successfully populated the variables given. (The -r argument prevents read from treating backslashes as continuation characters rather than literals; unless you have a specific reason to have continuation characters available in the context at hand, you should make a habit of passing -r to read by default).
< url.txt redirects a read handle on url.txt into stdin for the command (including a compound command such as a while loop) to which it's appended.
A while loop runs the conditional command it's given, checks whether that conditional reports success or failure, and then proceeds to run the body and restart on success, or exit on failure.
Thus, if you have IFS='=| ' read -r param uri, it will read a single line from stdin; assign everything up to the first =, | or space to the variable named param, and assign what's left to the variable uri.
If you put that in the conditional part of a while loop, then the loop will operate until that read fails -- as it will if there isn't more content (up to and including a newline character) available to be read.
For more in-depth discussion of the idiom and its uses, see BashFAQ #1.
Some asides:
Using mkdir -p -- "${city}/${file}" will let you have only a single mkdir command that creates both directories (and avoids generating error messages if they already exist).
Using readarray -t files < url.txt is a more robust way to read the contents of url.txt into an array named files, though it requires bash 4.0 or newer. For older versions of the shell, consider IFS=$'\n' read -r -d '' -a files <url.txt || (( ${#files[#]} )). These will behave far better than the original idiom if you have wildcards, whitespace, or other unexpected content in your input files.