Further work on the content of the "Rationale" section.

SC1014.md

@@ -1,16 +1,24 @@
 ## Use `if cmd; then ..` to check exit code, or `if [ "$(cmd)" = .. ]` to check output.
 
 ### Problematic code:
-
 ```sh
+# WRONG
 if [ grep -q pattern file ]
 then
   echo "Found a match"
 fi
 ```
 
-### Correct code:
+```sh
+# WRONG
+if ! [[ logname == $LOGNAME ]]
+then
+    echo "Possible `su` shell"
+fi
+```
 
+
+### Correct code:
 ```sh
 if grep -q pattern file
 then
@@ -18,20 +26,40 @@ then
 fi
 ```
 
+```sh
+if ! [[ $(logname) == $LOGNAME ]]
+then
+    echo "Possible `su` shell"
+fi
+```
+
 
 ### Rationale:
-
 `[ ... ]` as shell syntax is a simple command that tests for whether certain conditions are true or false, such as whether the value assigned to a variable has a non-zero length (`[ -n "${foo}" ]`) or whether a file system object is a directory (`[ -d "${dir}" ]`). `If-then-(elif-then)-else-fi` statements are logical constructs which themselves contain lists of commands which can include simple commands. 
 
 `[` is just regular command, like `whoami` or `grep`, but with a funny name (see `ls -l /bin/[`). It's a shorthand for `test`. `[[` is similar to both `[` and `test`, but `[[` offers some additional unary operators, such as '=~' the regular expression comparison operator.  It allows one to use extglobs such as `@(foo|bar)` (a "bashism"), among some other less commonly used features. 
 
 `[[`, `[` and `test` are often used within `if...fi` constructs in the conditional commands position: which is between the 'if' and the 'then.' 
 
-There are certain shell syntaxes which can be wrapped directly around simple commands, in particular: (1) `{ ...;}` compound command groupings, (2) `$( ... )` command substitutions and (3) `<( ... )` and `>( ... )` process substitutions.  Some examples include `{ [ -d "${HOME}/file" ]; echo $?;}`, `var=$(logname)` and `readarray -d "" -t files < <( find ~ -iname '*bash*' -print0 2> /dev/null )`, respectively.  However, the various testing commands' syntaxes are not to be included among this list.
+There are certain shell syntaxes which can be wrapped **directly** around simple commands, in particular:
+- (1)  `{ ...;}`, group commands, 
+- (2)  `$( ... )`, command substitutions, 
+- (3)  `<( ... )` and `>( ... )`, process substitutions, 
+- (4)  `( ... )`, subshells, and
+- (5)  `$(( ... ))` and `(( ... ))`, arithmetic evaluations.
+
+Some examples include: 
+- (1)  `{ echo {a..z}; echo {0..9};} > ~/f`, 
+- (2)  `[[ $(logname) == $LOGNAME ]]`, 
+- (3)  `readarray -t files < <( find ...)`,
+- (4)  `(cd /foo || exit 1; tar ...)`, and
+- (5)  `dd bs=$((2**12)) count=1 if=/dev/zero of=/tmp/zeroed-block`, respectively.  
+
+Note how in example (2) `logname` is enclosed **directly** within a command substitution, which is itself enclosed within a `[[` reserved word / conditional expression / compound command. 
 
 If you want to check the exit status of a certain command, use that command directly as demonstrated in the correct code, above.
 
-If you want to check the output of a command, use `"$(..)"` to get its output, and then use `test` or  `[`/`[[` to do a string comparison:
+If you want to check the output of a command, use `"$(..)"` to get its output, and then use `test`/`[` or  `[[` to do a string comparison:
 
 ```sh
 # Check output of `whoami` against the string `root`
@@ -41,13 +69,15 @@ then
 fi
 ```
 
-For more information, see [this problem in the Bash Pitfall](http://mywiki.wooledge.org/BashPitfalls#if_.5Bgrep_foo_myfile.5D) list, or generally [Tests and Conditionals](http://mywiki.wooledge.org/BashGuide/TestsAndConditionals) in the WoolEdge BashGuide 
 
 ### Exceptions:
 
 None.
 
-### Related resources:
+
+### Related Resources:
+
+For more information, see [this problem in the Bash Pitfall](http://mywiki.wooledge.org/BashPitfalls#if_.5Bgrep_foo_myfile.5D) list, or generally [Tests and Conditionals](http://mywiki.wooledge.org/BashGuide/TestsAndConditionals) in the wooledge.org BashGuide 
 
 * [How do I use a file grep comparison inside a bash if/else statement?](https://stackoverflow.com/questions/2480584/how-do-i-use-a-file-grep-comparison-inside-a-bash-if-else-statement)
 * [Bash Pitfalls: `if [grep foo myfile]`](https://mywiki.wooledge.org/BashPitfalls#if_.5Bgrep_foo_myfile.5D)