Delete .shellcheckrc (#1947)

* CI: disable Ubuntu 16.04 as it's EOL

https://github.blog/changelog/2021-04-29-github-actions-ubuntu-16-04-lts-virtual-environment-will-be-removed-on-september-20-2021/

* main: lint false positive

* install: lint

* plugins/cmd-returned-notify: don't `export`

* plugins/xterm: lint

* plugins/git: lint

* plugins/goenv: lint

* plugins/alias-completion: lint false positives

* plugins/alias-completion: fix SC2155, SC2154

Declare `locals` at the top of the function

* completion: lint completions using `bash_completion` functions

Match the style of the existing code

* completion/knife: lint false positives

* completion/knife: lint

* completion/sdkman: lint

* completion/composer: lint

* Move `.shellcheckrc` under `themes/`

* lib/theme: fix SC2155, SC2154, SC2034

* lib/colors: don't warn on unused variables

We assign a large number of variables here and they may or may not be used anywhere else, so disable SC2034 for this file (only).

Alsö disable SC2005 as the functions in this file were written before `printf` was invented and have to do some fancy metascripting to get escape sequences interpreted reliably. I’m not smart enough to fix this to use `printf`, so leave it for now.

* themes/agnoster: lint

* themes: disable SC2154 for colors

Each one of these themes will need it’s own fix for SC2154, possibly upstream.

Due to the way themes are, it's entirely normal to have a *lot* of false positives for SC2034. So much so, that I have to admit that it is probably just not worth linting for SC2034 despite my dislike of blanket ignore rules.

* themes: disable SC2154, fix SC2155

Each one of these themes will need it’s own fix for SC2154, possibly upstream.

Due to the way themes are, it's entirely normal to have a *lot* of false positives for SC2034. So much so, that I have to admit that it is probably just not worth linting for SC2034 despite my dislike of blanket ignore rules.

* Delete `.shellcheckrc`

* remove executable bit

themes/base.theme.bash

@@ -1,4 +1,5 @@
 # shellcheck shell=bash
+# shellcheck disable=SC2034 # Expected behavior for themes.
 
 CLOCK_CHAR_THEME_PROMPT_PREFIX=''
 CLOCK_CHAR_THEME_PROMPT_SUFFIX=''
@@ -130,12 +131,11 @@ function scm {
 }
 
 scm_prompt() {
-	local CHAR=$(scm_char)
+	local CHAR
+	CHAR="$(scm_char)"
 	local format=${SCM_PROMPT_FORMAT:-'[%s%s]'}
 
-	if [[ $CHAR = "$SCM_NONE_CHAR" ]]; then
-		return
-	else
+	if [[ "${CHAR}" != "$SCM_NONE_CHAR" ]]; then
 		# shellcheck disable=2059
 		printf "$format\n" "$CHAR" "$(scm_prompt_info)"
 	fi
@@ -352,15 +352,15 @@ function svn_prompt_vars {
 # - .hg is located in ~/Projects/Foo/.hg
 # - get_hg_root starts at ~/Projects/Foo/Bar and sees that there is no .hg directory, so then it goes into ~/Projects/Foo
 function get_hg_root {
-	local CURRENT_DIR=$(pwd)
+	local CURRENT_DIR="${PWD}"
 
-	while [ "$CURRENT_DIR" != "/" ]; do
-		if [ -d "$CURRENT_DIR/.hg" ]; then
+	while [[ "${CURRENT_DIR:-/}" != "/" ]]; do
+		if [[ -d "$CURRENT_DIR/.hg" ]]; then
 			echo "$CURRENT_DIR/.hg"
 			return
 		fi
 
-		CURRENT_DIR=$(dirname "$CURRENT_DIR")
+		CURRENT_DIR="${CURRENT_DIR%/*}"
 	done
 }
 
@@ -552,7 +552,7 @@ function prompt_char {
 
 function battery_char {
 	if [[ "${THEME_BATTERY_PERCENTAGE_CHECK}" = true ]]; then
-		echo -e "${bold_red}$(battery_percentage)%"
+		echo -e "${bold_red:-}$(battery_percentage)%"
 	fi
 }
 
@@ -594,7 +594,7 @@ function __check_precmd_conflict() {
 function safe_append_prompt_command {
 	local prompt_re
 
-	if [ "${__bp_imported}" == "defined" ]; then
+	if [ "${__bp_imported:-missing}" == "defined" ]; then
 		# We are using bash-preexec
 		if ! __check_precmd_conflict "${1}"; then
 			precmd_functions+=("${1}")
@@ -609,7 +609,7 @@ function safe_append_prompt_command {
 			prompt_re="\<${1}\>"
 		fi
 
-		if [[ ${PROMPT_COMMAND} =~ ${prompt_re} ]]; then
+		if [[ ${PROMPT_COMMAND[*]:-} =~ ${prompt_re} ]]; then
 			return
 		elif [[ -z ${PROMPT_COMMAND} ]]; then
 			PROMPT_COMMAND="${1}"