Update 'bash_programming.md'

bash_programming.md

@@ -108,13 +108,28 @@ EOT
 }
 ```
 
+Bash test. Returns True if test expression passes
+```
+    if [ -z "${MAKE_TARGETS}" ]; then
+        # do stuff
+    fi
+```
+
+
+Bash negative test. Returns True if test expression does not passes
+```
+    if [ ! -z "${MAKE_TARGETS}" ]; then
+        # do stuff
+    fi
+```
+
 Common bash test expressions
 https://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html and https://stackoverflow.com/a/21164441
 ```
 The following test operators return "True" if the file exists
 -a file :: Any file type
--e file :: Any file type
+-e file :: Check for file existence, regardless of type (node, directory, socket, etc.)
--f file :: Any regular file (ie not a block, socket, pipe, symlink, etc.)
+-f file :: Any regular file that is not a directory
 -b file :: Block special file
 -c file :: Character file
 -d file :: Directory type
@@ -135,6 +150,7 @@ Other common test operators
 -v varname :: True if 'varname' is set and has been assigned a value
 -R varname :: True if varname is set and is a name reference
 -n string :: True if length of string is not zero
+-z string :: True if length of string is zero
 string1 == string2 :: True if the strings are equal
 string1 = string2 :: True if the strings are equal. POSIX conformant
 string1 != string2 :: True if the strings are not equal
@@ -149,3 +165,28 @@ arg1 -le arg2 :: True if arg1 is less than or equal to arg2
 arg1 -gt arg2 :: True if arg1 is greater than arg2
 arg1 -ge arg2 :: True if arg1 is greater than or equal to arg2
 ```
+
+Command substitution
+https://www.gnu.org/software/bash/manual/html_node/Command-Substitution.html
+```
+$(command)
+`command`
+```
+
+Shell Arthimetic - https://www.gnu.org/software/bash/manual/html_node/Shell-Arithmetic.html
+
+Brace expansions
+https://www.gnu.org/software/bash/manual/html_node/Brace-Expansion.html
+```
+mkdir /usr/local/src/bash/{old,new,dist,bugs}
+```
+or
+```
+chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
+```
+
+Parameter (variable) expansion - https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html and https://wiki.bash-hackers.org/syntax/pe
+
+GNU BASH reference - https://www.gnu.org/software/bash/manual/html_node/index.html
+
+BASH Hackers Wiki - https://wiki.bash-hackers.org/