Increment variable in bash for loop

Question

Trying to create a script using curl to probe multiple URLs for updated information. Currently, I'm using individual commands for each URL, which I am trying to shorten lines of codes by using increment variables in a for loop. What I have at the moment: (dummy urls as examples)

#!/bin/bash

url01="https://askubuntu1.com"
url02="https://askubuntu2.com"
url03="https://askubuntu3.com"
url04="https://askubuntu4.com"
url05="https://askubuntu5.com"

for i in {01..05}
    do
        url=$((url$i))
        curl $url 2>&1| grep 'askubuntu' >> output.txt
done &

Error Output:

line 10: https://askubuntu1.com: syntax error in expression (error token is "://askubuntu1.com")

Expected result: command input to bash shell for each of curl queries pipes to grep then outputs to "output.txt".

curl "https://askubuntu1.com"
curl "https://askubuntu2.com"
.
.
curl "https://askubuntu3.com"

Thank You

Answer 1

The assignment url=$((url$i)) attempts to evaluate url$i as an arithmetic expression - this is not appropriate here. What you could do instead is make use of variable indirection:

for url in url{01..05}
do
  curl "${!url}" 2>&1| grep 'askubuntu' >> output.txt
done

However it would probably cleaner to use an array:

#!/bin/bash

urls=(
  "https://askubuntu1.com"
  "https://askubuntu2.com"
  "https://askubuntu3.com"
  "https://askubuntu4.com"
  "https://askubuntu5.com"
)

for url in "${urls[@]}"
do
  curl "$url" 2>&1| grep 'askubuntu' >> output.txt
done

---

You could also loop over the indices of the array ${!urls[@]} - which would give you the desired incrementing integer variable from which to construct a sequence of numbered output files ex.:

for i in ${!urls[@]}
do
  printf -v output 'file%02d.txt' $((i+1))
  curl "${urls[i]}" 2>&1 | grep 'askubuntu' > "$output"
done

The $((i+1)) is because bash array indexing starts from 0 - if you don't mind the outputs being file00 through file04 then you can use plain $i.

For the version based on variable indirection, you could do either

for i in {01..05}
do
  url="url$i"
  curl "${!url}" 2>&1| grep 'askubuntu' > "file${i}.txt"
done

or

for url in url{01..05}
do
  curl "${!url}" 2>&1| grep 'askubuntu' > "${url/#url/file}.txt"
done