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--- ops102:bash_scripting_1 [2024/11/18 20:21] – [Conditional Logic: if / then / elif / else / fi] chris
+++ ops102:bash_scripting_1 [2025/03/11 15:11] (current) – chris
@@ Line 38: / Line 38: @@
   The current date and time is:
   Sat Mar  6 12:03:32 EST 2038
+===== Comments =====
+A comment in a bash script starts with a sharp symbol (#) and is ignored by the shell interpreter:
+<code># test script #1
+# written by Jason Bourne</code>
+A comment may also be just one portion of a line:
+<code>cd - # change to the user's previous working directory</code>
+Note that a shbang line is a comment from the point of view of the shell interpreter -- it's there for the kernel to use, not the shell!
 ===== Variables =====
@@ Line 47: / Line 60: @@
   A=5
   B=World
+  TheNameOfTheUser=Jason
 If the variable does not exist, it will be created. If it does exist, the previous value will be discarded.
@@ Line 54: / Line 68: @@
 Unlike some computer languages such as C, variables do not need to be declared. Variables are not typed -- they may be used as strings, integers, or decimal values.
+Note that there must not be a space on either side of the equal sign.
 ==== Accessing a Variable ====
@@ Line 102: / Line 117: @@
   Hello $B
-You should always double-quote variables that may contain a space in their value when using them as command arguments.
+You should always double-quote variables that may contain a space in their value when using them as command arguments. This is especially true for filenames -- you never know when a user is going to put a space in a filename! Many scripts work find with opaque filenames (those containing no whitespace) but fail with non-opaque names.
-This is especially true for filenames -- you never know when a user is going to put a space in a filename! Many scripts work find with opaque filenames (those containing no whitespace) but fail with non-opaque names.
+Here is an example -- the difference between the first ''ls'' statement (which does not double-quote the variable) and the second ''ls'' statement (where the variable expansion is double-quoted):
+  $ touch "red maple"
+  $ FILE="red maple"
+  $ ls $FILE
+  ls: cannot access 'red': No such file or directory
+  ls: cannot access 'maple': No such file or directory
+  $ ls "$FILE"
+  'red maple'
 === Backslashes ===
@@ Line 188: / Line 214: @@
   $ A=`ls`
-This is an archaic syntax with is depricated -- avoid doing this. Some fonts make it hard to distiguish between backticks and single-quotes, and nesting backticks is difficult.
+This is an archaic syntax which is depricated -- avoid doing this. Some fonts make it hard to distiguish between backticks and single-quotes, and nesting backticks is difficult.
 ===== Arithmetic =====
-Bash can perform __integer__ arithmetic.
+Bash can perform integer arithmetic.
 To evaluate an arithmetic expression and return a value, use ''<nowiki>$(( ))</nowiki>'':
@@ Line 203: / Line 229: @@
   $ echo $B
 Note that inside the double-parenthesis, spaces don't matter, and it is not necessary to use a dollar-sign [$] in front of variables being accessed.
@@ Line 214: / Line 241: @@
   $ ((C=A*B*2))
-  $ echo "The answer is $C'
+  $ echo "The answer is $C"
   The answer is 2626