If and Decision making is done

Bash-Scripting.txt

@@ -336,3 +336,128 @@
 * [[ $STRING == pattern_with_wildcards ]]
 * [[ $STRING == file[0-9].txt ]]
 * [[ $STRING == rich* ]]
+
+
+-> Regular Expressions
+
+|-----------------------------------------------------------|
+|   Symbol  |   Description |   Example |   Example matches |
+|-----------------------------------------------------------|
+|   .       |   any single  |   bo.k    |   book,bo1k, ...  |
+|           |   character   |           |                   |
+|-----------------------------------------------------------|
+|   *       |   preceeding  |   co*l    |   cl,col,cool, ...|
+|           |   item must   |           |                   |
+|           |   match zero  |           |                   |
+|           |   or more     |           |                   |
+|           |   times       |           |                   |
+|-----------------------------------------------------------|
+|   ^       |   Start of    |   ^hello  |   line starting   |
+|           |   the line    |           |   with hello      |
+|           |   marker      |           |                   |
+|-----------------------------------------------------------|
+|   $       |   End of the  |   hello$  |   line ending     |
+|           |   line marker |           |   with hello      |
+|-----------------------------------------------------------|
+|   []      |   Any one of  |   coo[kl] |   cook or cool    |
+|           |   of character|           |                   |
+|           |  enclosed in[]|           |                   |
+|-----------------------------------------------------------|
+|   [-]     |  Any character|  file[1-3]|  file1,file2,file3|
+|           |   within the  |           |                   |
+|           |   range       |           |                   |
+|-----------------------------------------------------------|
+|   [^]     |  Any character|   file[^0 |   file8,file9     |
+|           |   except those|   1234567]|                   |
+|           |   enclosed in |           |                   |
+|           |   brackets    |           |                   |
+|-----------------------------------------------------------|
+|   ?       |   preceeding  |   colou?r |   color,colour    |
+|           | item must     |           |   NOT colouur     |
+|           | one or zero   |           |                   |
+|           |   times       |           |                   |
+|-----------------------------------------------------------|
+|   +       | preceeding    |   file1+  |   file1,file11,   |
+|           | item must     |           |   file111         |
+|           | match one or  |           |   NOT file        |
+|           |   more times  |           |                   |
+|-----------------------------------------------------------|
+|   {n}     | preceeding    |   [0-9]{3}|   Any three digit:|
+|           | item must     |           |   097,256,787,... |
+|           | match n times |           |                   |
+|-----------------------------------------------------------|
+|   {n,}    | Minimum number|  [0-9]{3,}|   Any three digit |
+|           | of times the  |           |   111,097,256,787.|
+|           | preceeding    |           |                   |
+|           | item should   |           |                   |
+|           | match         |           |                   |
+|-----------------------------------------------------------|
+|   {n,m}   | Minimum and   | [0-9]{1,3}|   1,158,26, ...   |
+|           | maximum number|           |   NOT 1258,1111,..|
+|           | of times the  |           |                   |
+|           | preceeding    |           |                   |
+|           | item should   |           |                   |
+|           | match         |           |                   |
+|-----------------------------------------------------------|
+|   \       | Escape        |   hell\.o |   hell.o          |
+|           | character -   |           |   NOT:helllo,     |
+|           | escape any of |           |   hell1o...       |
+|           | the special   |           |                   |
+|           | character,    |           |                   |
+|           | ignore the    |           |                   |
+|           | meaning of    |           |                   |
+|           | them          |           |                   |
+|-----------------------------------------------------------|
+
+-> Regex in Bash
+
+    * [[ $STRING =~ $REGEX ]]
+    * REGEX="http://.*\.jpg"
+    * ${BASH_REMATCH[0]} -> part of the STRING which match
+                            REGEX (e.g. http://images.jpg)
+    * ${BASH_REMATCH[1]} -> part of the REGEX which is
+                            enclosed in first parentheses,
+                            e.g.
+                            REGEX="http://(.*)\.jpg" -> which
+                            would represents as "images"
+
+
+-> Filesystem Related Tests
+
+|-----------------------------------------------------------------------|
+|   File Test operators |   Description, what is returned               |
+|-----------------------------------------------------------------------|
+|   [ -e $VAR ]         |   True if variable hold existing file (file   |
+|                       |   or directory)                               |
+|-----------------------------------------------------------------------|
+|   [ -f $VAR ]         |   True if variable holds an existing regular  |
+|                       |   file                                        |
+|-----------------------------------------------------------------------|
+|   [ -d $VAR ]         |   True if variable holds an existing directory|
+|-----------------------------------------------------------------------|
+|   [ -x $VAR ]         |   True if variable is an executable file      |
+|-----------------------------------------------------------------------|
+|   [ -L $VAR ]         |   True if variable holds th path of a symlink |
+|-----------------------------------------------------------------------|
+|   [ -r $VAR ]         |   True if variable holds file that is readable|
+|-----------------------------------------------------------------------|
+|   [ -w $VAR ]         |   True if variable holds file that is writable|
+|-----------------------------------------------------------------------|
+
+        * Example:
+            if [ -f $FILE ]; then
+                echo File exists
+            else
+                echo File does not exists
+            fi
+
+-> && and ||
+
+    *examples
+        [ $? -eq 0 ] && echo command OK || echo something went wrong
+            |-----------------|                      |
+        if true             execute this             |
+                            if NOT true              |
+                                |--------------------|
+
+                        

filetest.sh

@@ -0,0 +1,49 @@
+#!/bin/bash
+
+#for provided file print the summary of what permissions users has granted
+#example:   ./filetest.sh hello.txt
+#READ:      YES
+#WRITE:     NO
+#EXECUTABLE:NO
+
+#argument check
+
+if [ $# -ne 1 ]; then
+    echo "Provide exactly one argument"
+    exit 1
+fi
+
+#variable assignment
+FILE=$1
+
+#check if the file exists 
+if [ -f $FILE ]; then
+
+    #default variables
+    VAR_READ="NO"
+    VAR_WRITE="NO"
+    VAR_EXEC="NO"
+
+    #check if file is readable
+    if [ -r $FILE ]; then
+        VAR_READ="YES"
+    fi
+
+    #check if file is writable
+    if [ -w $FILE ]; then
+        VAR_WRITE="YES"
+    fi
+
+    #check if file is executable
+    if [ -x $FILE ]; then
+        VAR_EXEC="YES"
+    fi
+
+    #write permissions summary to user
+    echo "==FILE:   $FILE=="
+    echo "READ:         $VAR_READ"
+    echo "WRITE:        $VAR_WRITE"
+    echo "EXECUTABLE:   $VAR_EXEC"
+else
+    echo $FILE does not exists
+fi

regex.sh

@@ -0,0 +1,38 @@
+#!/bin/bash
+
+#provide one word/sentence as an argument to the script.
+#If in that sentence will be ip addresss, find out, if that
+#ip address is reachable or not.
+
+#argument check
+if [ $# -ne 1 ]; then
+    echo "Provide exactly one argument e.g. $0 argument"
+    exit 1
+fi
+
+VAR1=$1
+#ip address regex
+REGEX="[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[[:digit:]]{1,3}"
+
+#regex check
+if ! [[ $VAR1 =~ $REGEX ]]; then
+    echo "No IP address provided"
+    exit 2
+fi
+
+IP=${BASH_REMATCH[0]}
+
+#find if ip address is reachable or not
+ping -c4 $IP 1>/dev/null
+
+#1>/dev/null
+#It will redirect standard output and prevent
+#any output to print on screen.
+
+if [ $? -eq 0 ]; then
+    STATUS="ALIVE"
+else
+    STATUS="DEAD"
+fi
+
+echo "IP found: $IP ($STATUS)"

secret.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+
+echo -n Your age:
+read AGE
+
+#Problem with output as () cause to execute command in subshell.
+#[ $AGE -lt 20 ] && (echo You are not allowed to see secret; exit 1) || echo Welcome
+
+#command execute it same shell
+[ $AGE -lt 20 ] && { echo You are not allowed to see secret; exit 1;} || echo Welcome
+
+echo Secret is that there is no secret