link ---> https://www.freecodecamp.org/news/author/zaira/

How to define variables??

We can define a variable by using the syntax variable_name=value.

To get the value of the variable, add $ before the variable.

#!/bin/bash

# A simple variable example

greeting=Hello

name=Mahesh

echo $greeting $name

output: Hello Mahesh

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Arithmetic Expressions

Below are the operators supported by bash for mathematical calculations:

OPERATOR USAGE

+ addition

- subtraction

* multiplication

/ division

** exponentiation

% modulus

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How to read user input

Sometimes you'll need to gather user input and perform relevant operations.

In bash, we can take user input using the read command.

read variable_name

To prompt the user with a custom message, use the -p flag.

read -p "Enter your age" variable_name

Example:

#!/bin/bash

echo "Enter a numner"

read a

echo "Enter a numner"

read b

var=$((a+b))

echo $var

read -p "Enter first number: " a

read -p "Enter second number: " b

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Numeric Comparison logical operators

Comparison is used to check if statements evaluate to true or false. We can use the below shown operators to compare two statements:

OPERATION SYNTAX EXPLANATION

Equality num1 -eq num2 is num1 equal to num2

Greater than equal to num1 -ge num2 is num1 greater than equal to num2

Greater than num1 -gt num2 is num1 greater than num2

Less than equal to num1 -le num2 is num1 less than equal to num2

Less than num1 -lt num2 is num1 less than num2

Not Equal to num1 -ne num2 is num1 not equal to num2

Syntax:

if [ conditions ]

then

commands

fi

Example:

Let's compare two numbers and find their relationship:

read x

read y

if [ $x -gt $y ]

then

echo X is greater than Y

elif [ $x -lt $y ]

then

echo X is less than Y

elif [ $x -eq $y ]

then

echo X is equal to Y

fi

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Conditional Statements (Decision Making)

Conditions are expressions that evaluate to a boolean expression (true or false). To check conditions, we can use if, if-else, if-elif-else and nested conditionals.

The structure of conditional statements is as follows:

if...then...fi statements

if...then...else...fi statements

if..elif..else..fi

if..then..else..if..then..fi..fi.. (Nested Conditionals)

Syntax:

if [[ condition ]]

then

statement

elif [[ condition ]]; then

statement

else

do this by default

fi

To create meaningful comparisons, we can use AND -a and OR -o as well.

The below statement translates to: If a is greater than 40 and b is less than 6.

if [ $a -gt 40 -a $b -lt 6 ]

Example: Let's find the triangle type by reading the lengths of its sides.

read a

read b

read c

if [ $a == $b -a $b == $c -a $a == $c ]

then

echo EQUILATERAL

elif [ $a == $b -o $b == $c -o $a == $c ]

then

echo ISOSCELES

else

echo SCALENE

fi

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Looping and skipping

For loops allow you to execute statements a specific number of times.

Looping with numbers:

In the example below, the loop will iterate 5 times.

#!/bin/bash

for i in {1..5}

do

echo $i

done

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Looping with strings:

We can loop through strings as well.

#!/bin/bash

for X in cyan magenta yellow

do

echo $X

done

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While loop

While loops check for a condition and loop until the condition remains true. We need to provide a counter statement that increments the counter to control loop execution.

In the example below, (( i += 1 )) is the counter statement that increments the value of i.

Example:

#!/bin/bash

i=1

while [[ $i -le 10 ]] ; do

echo "$i"

(( i += 1 ))

done

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How to execute commands with back ticks

If you need to include the output of a complex command in your script, you can write the statement inside back ticks.

Syntax:

var= ` commands `

Example: Suppose we want to get the output of a list of mountpoints with tmpfs in their name. We can craft a statement like this: df -h | grep tmpfs.

To include it in the bash script, we can enclose it in back ticks.

#!/bin/bash

var=`df -h | grep tmpfs`

echo $var

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How to get arguments for scripts from the command line

It is possible to give arguments to the script on execution.

$@ represents the position of the parameters, starting from one.

#!/bin/bash

for x in $@

do

echo "Entered arg is $x"

done

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How to Automate Scripts by Scheduling via cron Jobs

Cron is a job scheduling utility present in Unix like systems. You can schedule jobs to execute daily, weekly, monthly or in a specific time of the day. Automation in Linux heavily relies on cron jobs.

Below is the syntax to schedule crons:

# Cron job example

* * * * * sh /path/to/script.sh

Here, * represent represents minute(s) hour(s) day(s) month(s) weekday(s), respectively.

Below are some examples of scheduling cron jobs.

SCHEDULE SCHEDULED VALUE

5 0 * 8 * At 00:05 in August.

5 4 * * 6 At 04:05 on Sunday.

0 22 * * 1-5 At 22:00 on every day-of-week from Monday through Friday.

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How to Check Existing Scripts in a System

Using crontab

crontab -l lists the already scheduled scripts for a particular user.

My scheduled scripts

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Using the find command

The find command helps to locate files based on certain patterns. As most of the scripts end with .sh, we can use the find script like this:

find . -type f -name "*.sh"

`

Where,

. represents the current directory. You can change the path accordingly.

-type f indicates that the file type we are looking for is a text based file.

*.sh tells to match all files ending with .sh.

Practical examples of find with bash scripts

We can combine find with rm or mv to create meaningful bash scripts that can be automated.

Let's say we want to create a script that moves log files older than 7 days to a backup path.

From there, it deletes log files older that older than 30 days.

We can create a script and schedule it with cron. You can learn more about cron jobs here.

Let's view the script:

#!/bin/bash

# Script to move from logs older than 7 days to backup logs path: /app/backup_logs/ESB0*

# move ESB01 logs to backup

find /logs/esb01/audit -name "*.tar.gz" -mtime +7 -exec mv {} app/backup_logs/ESB01/ \;

# Remove logs from backup path after 30 days

find /app/backup_logs/ESB01 -name "*.tar.gz" -mtime +30 -exec rm {} \;

Note that we are using exec with find. Basically, exec executes the command provided ( mv and rm in our case).

{} is the placeholder which holds the results of the command.

Lastly, we provide the delimiter ;.

As we do not want the shell to interpret the semicolon, we escape it with \.

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Grep command Tutorial

Grep Command in Linux – Usage, Options, and Syntax Examples

If you are a system admin who needs to scrape through log files or a developer trying to find certain occurrences in the code file,

then grep is a powerful command to use.

In this article, we will discuss the grep command's syntax and its usage with some examples.

Syntax of the grep Command

The syntax of the grep command is as follows:

grep [OPTION...] PATTERNS [FILE...]

Grep syntax

In the above syntax, grep searches for PATTERNS in each FILE. Grep finds each line that matched the provided PATTERN.

It is a good practice to close the PATTERN in quotes when grep is used in a shell command.

In this article, we will discuss the following options that can be used with grep:

-i, --ignore-case: Ignores case distinctions in patterns and input data.

-v, --invert-match: Selects the non-matching lines of the provided input pattern.

-n, --line-number: Prefix each line of the matching output with the line number in the input file.

-w: Find the exact matching word from the input file or string.

-c: Count the number of occurrences of the provided pattern.

In the coming examples, we will use the file fruits.txt with the following content:

apples and pears

citrus – oranges, grapefruits, mandarins and limes

stone fruit – nectarines, apricots, peaches and plums

tropical and exotic – bananas and mangoes

berries – strawBERRIES, raspberries, blueberries, kiwifruit and passionfruit

melons – watermelons, rockmelons and honeydew melons

tomatoes and avocados.

How to Find a Matching String with grep

If we want to find the string "fruit" in the file fruits.txt, we can do so like this:

grep "fruit" fruits.txt

How to Ignore Case Distinctions using -i

We can command grep to return results while ignoring the case of the matching string.

Let's find the word "berries" from our sample file. It should match all occurrences of "berries" regardless of their case.

grep -i "berries" fruits.txt

How to Select the Non-Matching Lines using -v

We can reverse the results of the grep command to include non-matching results. Let's say,

if we want to get all the lines that do not contain the word "berries", the command would look like this:

grep -v "berries" fruits.txt

The output returned all the lines that do not contain "berries".

How to Find the Line Numbers Against Matching Input using -n

There are times when we want to get the line numbers against the matching string. For that, we can supply the -n flag to grep like this:

grep -n "berries" fruits.txt

How to Find the Exact Matching Word from the Input File or String using -w

If you were to match an exact word rather than just the occurrence of a pattern, you can do so by using the -w flag.

If we grep "fruit" without any flags, it would return any occurrence of the word "fruit". This would include occurrences like "grapefruit", "dragonfruit" and so on like this:

image-77

But, if we only want "fruit" in our results, we can use the -w flag like this:

grep -w "fruit" fruits.txt

How to Count the Number of Occurrences of the Provided Pattern using -c

We can count the number of times the matched string appears in the file. This is how the -c flag works:

grep -c "fruit" fruits.txt

The word "fruit" appears 3 times in the file.

Tip: The -c flag can be very useful if you have to count the number of times an error message appeared in a log file.

How to Scan Files for Matching Input

Until now we had discussed how to search for matching patterns in a file or input string. We can also use grep to narrow down the files that contain a matching pattern. We can use the following flags to separate files that contain the matching pattern:

-l, --files-with-matches: Prints the file name that contains the provided matching pattern.

-L, --files-without-match: Prints the file name that does not contain the provided matching pattern.

Let's say we have the following files in our folder:

grep -l "echo" *.sh

grep -L "even" *.sh