Day-05 shell scripting
1.RegEx: We can use RegEx to search for specific patterns in a text. For example, finding dates, emails, phone numbers, or specific keywords.
2. Arrays: Array is nothing but list of components.
3. Break: The break command in shell scripting is important for controlling the flow of loops based on specific conditions.
4. Continue: In shell scripting, the continue command can be used to skip the current iteration of a loop and move on to the next iteration. which can effectively control the flow according to specific conditions.
5. Custom Exit Codes : Custom exit codes in shell scripts are used to indicate the success or failure of a script or command.
1.RegEx:
if [[ $USER =~ ^[a-zA-Z]+$ ]];
=~: Allows you to check if a string matches a given pattern.
^[a-zA-Z]+$ : is valid (only letters).
\d : The expression \d matches any one of the digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9.
\D : Identify names from a string that contains a mix of text and numbers.
\s : Is used to match any whitespace character. This includes spaces, tabs, and newlines.
\w : Is a helpful tool in RegEx that matches any letter, number, or underscore, making it great for working with text where you want to focus on valid “word-like” characters.
-> nano regex.sh
#!/bin/bash
if [ $# -gt 0 ]; then
for USER in $@; do
echo $USER
if [[ $USER =~ ^[a-zA-Z]+$ ]]; then
EXISTING_USER=$(cat /etc/passwd | grep -i -w $USER | cut -d ‘:’ -f1)
if [ “${USER}” = “${EXISTING_USER}” ]; then
echo “$USER is already exisitin, Please create a New user”
else
echo “Lets create the New $USER”
sudo useradd -m $USER — shell /bin/bash
SPEC=$(echo ‘!@#$%^&*()_’ | fold -w1 | shuf | head -1)
PASSWORD=”IndianArmy@${RANDOM}${SPEC}”
echo “$USER:$PASSWORD” | sudo chpasswd
echo “The termporary password for the user is ${PASSWORD}”
passwd -e $USER
fi
else
echo “The User Must Contain Alphabets”
fi
done
else
echo “Please pass the Argument”
fi
-> In regular expression we have mentioned only lower and uppercase letters that’s the reason. The RegEx taking only lower and uppercase letters.
-> Now i can restrict the user along with the 3 letters and 3 numbers.
if [[ $USER =~ ^[a-z]{3}[0–9]{3}$ ]];
#!/bin/bash
if [ $# -gt 0 ]; then
for USER in $@; do
echo $USER
if [[ $USER =~ ^[a-z]{3}[0–9]{3}$ ]]; then
EXISTING_USER=$(cat /etc/passwd | grep -i -w $USER | cut -d ‘:’ -f1)
if [ “${USER}” = “${EXISTING_USER}” ]; then
echo “$USER is already exisitin, Please create a New user”
else
echo “Lets create the New $USER”
sudo useradd -m $USER — shell /bin/bash
SPEC=$(echo ‘!@#$%^&*()_’ | fold -w1 | shuf | head -1)
PASSWORD=”IndianArmy@${RANDOM}${SPEC}”
echo “$USER:$PASSWORD” | sudo chpasswd
echo “The termporary password for the user is ${PASSWORD}”
passwd -e $USER
fi
else
echo “The User Must Contain Alphabets”
fi
done
else
echo “Please pass the Argument”
fi
2. Array:
Example : We can take one variable and assign a value into the variable.
Name : ‘surya prasad’
echo $Name
### Break, Exit Codes and Continue.
-> Now we can check for break
aws_regions=(us-east-1 us-east-2 hyd-india-1 eu-north-1 ap-south-1 eu-west-3 eu-west-2 eu-west-1 ap-northeast-2)
echo “Running the function to list VPCs using the regions list”
for region in “${aws_regions[@]}”; do
echo “Getting VPCs in $region .. “
vpc_list=$(aws ec2 describe-vpcs — region “$region” | jq -r .Vpcs[].VpcId)
vpc_arr=(${vpc_list[@]})
if [ ${#vpc_arr[@]} -gt 0 ]; then
for vpc in “${vpc_list[@]}”; do
echo “The VPC-ID is: $vpc”
done
echo “##########”
else
echo “Invalid Region..!!”
echo “#######”
echo “# Breaking at $region #”
echo “################”
break
fi
done
-> Now we can check for exit 99
######EXIT CODE############
#!/bin/bash
aws_regions=(us-east-1 us-east-2 hyd-india-1 eu-north-1 ap-south-1 eu-west-3 eu-west-2 eu-west-1 ap-northeast-2)
echo “Running the function to list VPCs using the regions list”
for region in “${aws_regions[@]}”; do
echo “Getting VPCs in $region .. “
vpc_list=$(aws ec2 describe-vpcs — region “$region” | jq -r .Vpcs[].VpcId)
vpc_arr=(${vpc_list[@]})
if [ ${#vpc_arr[@]} -gt 0 ]; then
for vpc in “${vpc_list[@]}”; do
echo “The VPC-ID is: $vpc”
done
echo “##########”
else
echo “Invalid Region..!!”
echo “#######”
echo “# Breaking at $region #”
echo “################”
#break
exit 99
fi
done
-> Now we can check for Continue.
# CONTINUE
#!/bin/bash
aws_regions=(us-east-1 us-east-2 hyd-india-1 eu-north-1 ap-south-1 eu-west-3 eu-west-2 eu-west-1 ap-northeast-2)
echo “Running the function to list VPCs using the regions list”
for region in “${aws_regions[@]}”; do
echo “Getting VPCs in $region .. “
vpc_list=$(aws ec2 describe-vpcs — region “$region” | jq -r .Vpcs[].VpcId)
vpc_arr=(${vpc_list[@]})
if [ ${#vpc_arr[@]} -gt 0 ]; then
for vpc in “${vpc_list[@]}”; do
echo “The VPC-ID is: $vpc”
done
echo “##########”
else
echo “Invalid Region..!!”
echo “#######”
echo “# Breaking at $region #”
echo “################”
#break
#exit 99
continue
fi
done
