Bash variables scope

Bash variables are defaults to global

What makes a variable local?

• A variable declared as local is one that is visible only within the block of code in which it appears. It has local "scope." In a function, a local variable has meaning only within that function block.
  Ex:
  local variable name

func ()
{
local loc_var=23
# Declared as local variable.
# Uses the 'local' builtin.
echo "\"loc_var\" in function = $loc_var"
global_var=999

# Not declared as local.
# Defaults to global.
echo "\"global_var\" in function = $global_var"
}

func
# Now, to see if local variable "loc_var" exists outside function.

echo "\"loc_var\" outside function = $loc_var"

# $loc_var outside function =
# No, $loc_var not visible globally.

echo "\"global_var\" outside function = $global_var"

# $global_var outside function = 999
# $global_var is visible globally.
exit 0

• Before a function is called, all variables declared within the function are invisible outside the body of the function, not just those explicitly declared as local
  

func ()
{
global_var=37

# Visible only within the function block
# before the function has been called.
} # END OF FUNCTION

echo "global_var = $global_var"
# global_var =
# Function "func" has not yet been called,
# so $global_var is not visible here.

func

echo "global_var = $global_var"
# global_var = 37 Has been set by function call.

• Using the declare builtin restricts the scope of a variable
  

foo ()
{
FOO="bar"
}

bar ()
{
foo
echo $FOO
}

bar
# Prints bar

However . . .

foo (){
declare FOO="bar"
}
bar ()
{
foo
echo $FOO
}
bar
# Prints nothing.

Bash variables type

Unlike many other programming languages, Bash does not separate its variables by "type". Essentially, Bash variables are character strings, but, depending on context, Bash permits integer operations and comparisons on variables. The determining factor is whether the value of a variable contains only digits.
Integer or string?
a=2334
# Integer.
let "a += 1"
echo "a = $a "
# a = 2335
# Integer, still.

b=${a/23/BB}
# Substitute "BB" for "23".
# This transforms $b into a string.
echo "b = $b"
# b = BB35
declare -i b
# Declaring it an integer doesn't help.
echo "b = $b"
# b = BB35

let "b += 1"
# BB35 + 1 =
echo "b = $b"
# b = 1
c=BB34
echo "c = $c"
# c = BB34
d=${c/BB/23}
# Substitute "23" for "BB".
# This makes $d an integer.
echo "d = $d"
# d = 2334
let "d += 1"
# 2334 + 1 =
echo "d = $d"
# d = 2335
# What about null variables?
e=""
echo "e = $e"
# e =
let "e += 1"
# Arithmetic operations allowed on a null variable?
echo "e = $e"
# e = 1
# Null variable transformed into an integer.

# What about undeclared variables?
echo "f = $f"
# f =
let "f += 1"
# Arithmetic operations allowed?
echo "f = $f"
# f = 1
# Undeclared variable transformed into an integer.

The burden is on the programmer to keep track of what type the script variables are.
Bash will not do it for you.
But “declare” or “typeset “built-ins permit restring the properties of variables.
This is very weak form of the typing available in certain programming languages.

Ex:1
declare –i number
# The script will treat subsequent occurrences of “number” as an integer
number=3
echo “Number = $number”
# Number = 3
number=three
echo “Number = $number”
#Number = 0
#Tries to evaluate the string “three” as an integer

Backup files to tarred and zip file

#!bin/bash

# backs up all files in current directory modified within last 24 hours
# in a tarred and zipped file
# Replace 1 with how many day's you want to back up files

BACKUPFILE=backup-`date +"%m-%d-%Y"`
# Embeds date in backup filename

archive=${1:-$BACKUPFILE}
#If no filename specified default to backup-MM-DD-YYYY

find . -mtime -1 -type f -print0 xargs -0 tar rvf "$archive.tar"

#check bellow command
# tar cvf - $(find . -mtime -1 -type f -print) > $archive.tar
# It works But will fail to backup file names contain space
# if there is no files containing spaces it is good

# ALSO USE bellow code but it is slow and not portable
# find . -mtime -1 -type f -exec tar rvf "$archive.tar" {} \;
# use rvf option instead of cvf otherwise only one file will be archived

gzip $archive.tar && echo "Directory $pwd backed up in \"$archive.tar.gz\" File"

Traversing directory using depth first search

#!/bin/bash
# Traverse a directory using depth first traversal technique
# Usage $0 directorypath
# otherwise it takes current working directory as directory path

depth()
{
#Do a small depth checking how deep into the tree we are
k=0
while [ $k -lt $1 ]
do
echo -n " "
let k++
#or use k=`expr $k + 1`
done
}

traverse()
{
# Traverse a directory

ls "$1"while read i
do
depth $2
if [ -d "$1/$i" ]
then
echo Directory: $1/$i

traverse "$1/$i" `expr $2 + 1`
# Calling this as a subshell means that when the called
# function changes directory, it will not affect our
# current working directory
# If you call this in current shell it gives the error
# bash: traverse: No such file or directory after changing
# the current directory
else

echo File: $1/$i
fi
done
}

# $1 is directory path

if [ -z "$1" ]
then
# Here we are giving '0' is the current depth of direcory
traverse . 0
else
traverse $1 0
fi