shekkbuilder · November 22, 2019 03:35 · Sep 18, 2017 · Sep 18, 2017 · Sep 18, 2017
diff --git a/bash_cheatsheet.md b/bash_cheatsheet.md
@@ -131,8 +131,6 @@ do
 done
 ```
 
-```
-
 * ```select``` looping works just like ```for```
 * with select, use p3 prompt. User options get a number prefix and waits for number response.
 

diff --git a/bash_cheatsheet.md b/bash_cheatsheet.md
@@ -7,6 +7,10 @@
 
 * ```#!/usr/bin/env bash``` for portability, or less good, ```#!/bin/bash```
 
+# Commenting
+
+```# this symbol makes everything after it on a line a comment```
+
 # Variables
 
 * No data types. Number, char, string, ok!
@@ -35,10 +39,35 @@ $$             #Process id of script (really id of the shell running the script)
 * Piping allows for robust mini programs to be built together like lego kits of separate parts
 * Example ```cat *.txt | sort | uniq > result-file``` Sorts the output of all the .txt files and deletes duplicate lines, saving outcome to *result-file*
 
+# Input
+
+* ```read``` command e.g. ```read num``` asks for input and puts it in $num
+
+# Arithmetic
+
+* uses the ```expr``` command
+
+```
+result=`expr $1 + 2`
+result2=`expr $2 + $1 / 2`
+result=`expr $2 \* 5`               #note the \ on the * symbol
+```
+
 # Conditionals
 
+* Usually surround comparisons and conditions in brackets not parens e.g. ```if [ $# -gt 1 ]```
+* ```&&``` is logical *and*, ```||``` is logical *or*
+
 ## Comparisons
 
+### Booleans
+
+```
+!     #not
+-a    #and
+-o    #or
+```
+
 ### Numerical comparisons
 
 ```
@@ -123,8 +152,11 @@ select opt in $OPTIONS; do
 		echo bad option
 	fi
 done
+```
 
 ### WHILE loop
+
+```
 while [ condition ]
 do
 	command
@@ -156,6 +188,18 @@ e Hello #will echo Hello when called
 
 ```#!/bin/bash -x``` Adding ```-x``` to the shebang produces output information
 
+# Additional shell features
+
+```
+$var           #Value of shell variable var.
+${var}abc      #Example: value of shell variable var with string abc appended.
+#              #At start of line, indicates a comment.
+var=value      #Assign the string value to shell variable var.
+cmd1 && cmd2   #Run cmd1, then if cmd1 successful run cmd2, otherwise skip.
+cmd1 || cmd2   #Run cmd1, then if cmd1 not successful run cmd2, otherwise skip.
+cmd1; cmd2     #Do cmd1 and then cmd2.
+cmd1 & cmd2    #Do cmd1, start cmd2 without waiting for cmd1 to finish.
+(cmds)         #Run cmds (commands) in a subshell.
+```
 
-# Resources
-* Other better(!) [Bash cheatsheet](http://johnstowers.co.nz/pages/bash-cheat-sheet.html)
+* Additional shell features from this [Bash cheatsheet](http://johnstowers.co.nz/pages/bash-cheat-sheet.html)
diff --git a/bash_cheatsheet.md b/bash_cheatsheet.md
@@ -0,0 +1,161 @@
+# Bash scripting cheatsheet
+
+* by Lee2sman
+* Updated 2017-09-18
+
+# Bash shebang
+
+* ```#!/usr/bin/env bash``` for portability, or less good, ```#!/bin/bash```
+
+# Variables
+
+* No data types. Number, char, string, ok!
+* No need to declare. Just assign a value to create it.
+* Get the value of a variable (whatever is stored in it) by calling it with ```$``` in front.
+* declare a local variable in a function by saying ```local``` before creating it.
+
+### Builtin shell variables
+
+```
+$0             #Name of this shell script itself.
+$1             #Value of first command line parameter (similarly $2, $3, etc)
+$#             #In a shell script, the number of command line parameters.
+$*             #All of the command line parameters.
+$-             #Options given to the shell.
+$?             #Return the exit status of the last command.
+$$             #Process id of script (really id of the shell running the script)
+```
+
+# File redirection and Piping
+
+* Three default files: 0. standard input (stdin), 1. standard output (stdout) and 2. standard error (stderr)
+* ```> filename``` redirects stdout to a file (and creates the file if it didn't already exist)
+* ```>> filename``` redirects stdout to a file but it *appends* to the end of the file if it already exists
+* ```|``` is the pipe. **Piping** is used to chain commands, scripts, files and programs together.
+* Piping allows for robust mini programs to be built together like lego kits of separate parts
+* Example ```cat *.txt | sort | uniq > result-file``` Sorts the output of all the .txt files and deletes duplicate lines, saving outcome to *result-file*
+
+# Conditionals
+
+## Comparisons
+
+### Numerical comparisons
+
+```
+-eq == equal to 
+-ne ≠ (not equal to)
+-gt > greater than
+-ge >= greather than or equal to
+-lt < less than
+-le <= less than or equal to
+< less than
+<= less than or equal to ((within double parenthesis))
+> greater than ((within double parenthesis))
+>= greater than or equal to ((within double parenthesis))
+= equal to, including for strings, e.g. ```if [ "$a" = "$b" ]```
+```
+
+### STRING COMPARISONS
+
+```
+== is equal to
+!= is NOT equal to
+< less than (ascii-betically)
+> greater than (ascii-betically)
+-z string is null
+-n string is not null
+```
+
+### Example comparisons
+
+```
+if [ "$VAR1" = "$VAR2" ]; then
+	echo "expression evaluated as true"
+else
+	echo "expression evaluated as false"
+fi
+```
+
+```
+case "$C" in
+"1")
+	do_this()
+	;;
+"2" | "3")
+	do_what_you_are_supposed_to_do()
+	;;
+*)    #fallback default case
+	do_nothing()
+	;;
+esac
+```
+
+# Looping
+
+* Iterates over a string of values
+
+```
+### FOR loop
+for i in 1 2 3 4 5    # can also be written for i in {1..5} or {start..end..increment}
+do
+	echo "Welcome $i times"
+done
+```
+
+```
+
+* ```select``` looping works just like ```for```
+* with select, use p3 prompt. User options get a number prefix and waits for number response.
+
+## Example - Basic text menu
+
+```
+#!/bin/bash
+OPTIONS="Hello Quit"
+select opt in $OPTIONS; do
+		if [ "$opt" = "Quit" ]; then
+		echo done
+		exit
+	elif [ "$opt" = "Hello" ]; then
+		echo Hello World
+	else
+		clear
+		echo bad option
+	fi
+done
+
+### WHILE loop
+while [ condition ]
+do
+	command
+done
+```
+
+```
+### UNTIL loop
+until [ condition ] # executes until condition = true
+do
+	command
+done
+```
+
+# Functions
+
+* works same as other languages
+* can take parameters
+
+```
+function e {
+	echo $1
+}
+
+e Hello #will echo Hello when called
+```
+
+# Debugging
+
+```#!/bin/bash -x``` Adding ```-x``` to the shebang produces output information
+
+
+# Resources
+* Other better(!) [Bash cheatsheet](http://johnstowers.co.nz/pages/bash-cheat-sheet.html)