Add shellcheck CI check + address its concerns in few bash scripts found

[yarikoptic]

I (newbie to slackdump) was looking for incremental backup setup. Ran into bash script -- decided to contribute but also to robustify before I "mass try it" since shellcheck is right in warning (generally ;-) ) about such constructs as ( $(im_channels) ) since they might not handle correctly cases where names have spaces. I do not have proof yet though here that it would apply. Neither I have yet tested in this script that the helper append_to_array works.

but here is a script which demonstrates problem and checks different solutions for it

```shell #!/bin/bash # Disabling shellcheck check here since adding escaped symbols for demo # shellcheck disable=SC2028 function func { echo word echo "phrase with space" echo 'wordwith\n' } function func2 { echo 'last-word\r' } function append_to_array { local array_name="$1" shift #local cmd="$@" # Read output of the command into a temporary array, splitting on newline # shellcheck disable=SC2034 if [ -n "$*" ]; then IFS=$'\n' read -r -d '' -a temp_array < <("$@") else # no command provided -- read from stdin IFS=$'\n' read -r -d '' -a temp_array fi # Use eval to safely append temp_array to the named array eval "$array_name+=(\"\${temp_array[@]}\")" } function show { for i in "${LIST[@]}"; do echo "<$i>" done } echo "Can't use mapfile since not portable -- not available on OSX" echo "Simple array: not good -- would not be good for spaces" LIST=( $(func) ) show echo "Tuned read: not good for appends to array" IFS=$'\n' read -r -d '' -a LIST < <(func) IFS=$'\n' read -r -d '' -a LIST < <(func2) show echo "Custom helper: good -- we will append to last listed but tested to work if there no array yet" append_to_array LIST func show echo "Trying more complicated case with a pipe" LIST=() append_to_array LIST < <(func | tr o X) show ```

[rusq]

Hey @yarikoptic, thanks for your contribution!

[rusq]

I have left some comments in the other PR that you have submitted, if you could have a look at it too: #271