Bash Scripting Basics, Control Structures, Loops

📌 Bash Shell Scripting - Wikibooks

Basics

BASH shell scripts are essentially a series of bash commands that are stored in a file that can be executed by running the script.

hello-world.sh

#!/bin/bash

# This is a comment
echo "Hello world"

#!/bin/bash - the first line says the script should be run using bash
- #! = shebang
# = pound/hash - used for comments
There are a few ways to execute the script:

bash hello-world.sh
	Hello world
# bash is the executable program

# To start the script without "bash" command, the script must be executable
chmod +x hello-world.sh
./hello-world.sh
	Hello world

executing-commands-example.sh

#!/bin/bash

# ************************************
# Executing commands example

# Execute whoami command
user=$(whoami)

# Execute hostname command
hostname=$(hostname)

# Execute print working directory (pwd) command
directory=$(pwd)

# Display information
echo "User=[$user] Host=[$hostname] Working dir=[$directory]"

# Display contents of directory
echo "Contents:"
ls

bash executing-commands-example.sh 
    User=[user] Host=[UbuntuVM] Working dir=[/home/user/scripts/linux101]
    Contents:
    case-example.sh			   comparison-examples.sh	  generate-passord.sh	hello-world.sh	loop-examples.sh
    command-line-arguments-example.sh  executing-commands-example.sh  generate-password.sh	if-examples.sh	process-information.sh

Control Structures

Conditionals allow the script to take different actions depending on some sort of state, referred to as if-then rules.

if-examples.sh

if statements.
- The expression inside the brackets [[ ... ]] is evaluated and used to determine if the conditional code is executed.
- If the expression evaluates to TRUE, the block (the code inside the IF statement) is executed.
- If the expression evaluates to FALSE, BASH skips over all of the conditional code and starts execute after the "fi" keyword.
else statements
- Code inside the else block is executed if the test returns FALSE.
elif statements (else-if)
- If the first test in the if statement fails, the elif statement will be evaluated.
- elif statements will be tested from top down.
- the code block associated with the first TRUE evaluated test will be executed and the rest of the conditional will be skipped.

#!/bin/bash

# ************************************
# If examples

# Test to see if /etc is a directory
if [[ -d /etc/ ]]; then
        echo /etc/ is indeed a directory
fi

# Check to see if a file exists
if [[ -e sample.txt ]]; then
        echo The file sample.txt exists
else
        echo The file sample.txt does NOT exist
fi

# Check a variable's value
TEST_VAR="test"
if [[ $TEST_VAR == "test" ]]; then
        echo TEST_VAR has a value of "test"
elif [[ $TEST_VAR == "again" ]]; then
        echo TEST_VAR has a value of "again"
else
        echo TEST_VAR has an unknown value
fi

bash if-examples.sh 
    /etc/ is indeed a directory
    The file sample.txt does NOT exist
    TEST_VAR has a value of test

comparison-examples.sh

Examples of performing numerical and string comparisons.

#!/bin/bash

# ************************************
# Numerical comparison examples

# Create some variables
x=1
echo x=["$x"]
y=2
echo y=["$y"]
z=2
echo z=["$z"]


# Perform some comparisons
# Numeric: Not equals
if [[ "$x" -ne "$y" ]]; then
	echo ["$x"] ne ["$y"]
fi

# Numeric: Equals
if [[ "$y" -eq "$z" ]]; then
	echo ["$y"] eq ["$z"]
fi

# Numeric: Greater than
if [[ "$y" -gt "$x" ]]; then
	echo ["$y"] gt ["$x"]
fi

# Numeric: Greater than or equal to
if [[ "$y" -ge "$z" ]]; then
	echo ["$y"] ge ["$z"]
fi

# Numeric: Less than
if [[ "$x" -lt "$y" ]]; then
	echo ["$x"] lt ["$y"]
fi

# Numeric: Less than or equal to
if [[ "$y" -le "$z" ]]; then
	echo ["$y"] le ["$z"]
fi

# ************************************
# String comparison examples

# Create some variables
a="A"
echo a=["$a"]
b="B"
echo b=["$b"]
anotherA="A"
echo anotherA=["$anotherA"]

# Perform some comparisons
# String: Equals
if [[ "$a" == "$anotherA" ]]; then
	echo ["$a"] "==" ["$anotherA"]
fi

# String: Not equals
if [[ "$a" != "$b" ]]; then
	echo ["$a"] "!=" ["$b"]
fi

# String: Less than
if [[ "$a" < "$b" ]]; then
	echo ["$a"] "<" ["$b"]
fi

# String: Greater than
if [[ "$b" > "$a" ]]; then
	echo ["$b"] ">" ["$a"]
fi

bash comparison-examples.sh 
    x=[1]
    y=[2]
    z=[2]
    [1] ne [2]
    [2] eq [2]
    [2] gt [1]
    [2] ge [2]
    [1] lt [2]
    [2] le [2]
    a=[A]
    b=[B]
    anotherA=[A]
    [A] == [A]
    [A] != [B]
    [A] < [B]
    [B] > [A]

case-example.sh

A case statement is essentially an if statement with multiple elif statements.
- it uses a command line argument - $1

#!/bin/bash

# ************************************
# Case example

# Switch off of the first command line argument
case $1 in
[1-3])
	message="Argument is between 1 and 3 inclusive"
	;;
[4-6])
	message="Argument is between 4 and 6 inclusive"
	;;
[7-9])
	message="Argument is between 7 and 9 inclusive"
	;;
1[0-9])
	message="Argument is between 10 and 19 inclusive"
	;;
*)
	message="I don't understand the argument or it is missing"
	;;
esac

# Print out a message describing the result
echo $message

bash case-example.sh 6
    Argument is between 4 and 6 inclusive
bash case-example.sh 22
	I don't understand the argument or it is missing

Loops

Loops allow to write code once and execute it multiple times.

Definite loops

know the number of loops (times it is executed) before the loop ever starts.
most common: for loop

Indefinite loops

unknown number of loops until the end (it depends on the user input for example).
most common: while loop

Infinite loops

loops that never end (whether by design or accident), != indefinite loops

loop-examples.sh

#!/bin/bash

# ************************************
# For loop examples

echo -----------------------------------
echo For loops

# Iterate through the numbers 1 through 5 and print them out
echo Print out a hard-coded sequence
for i in 1 2 3 4 5; do
    echo Index=[$i]
done

# Same as above, but generate the sequence
echo Print out a generated sequence
for i in {1..5}; do
    echo Index=[$i]
done

# Same as above, but use a more conventional format
#   NOTE: Double parenthesis are used since we are doing arithmetic
echo Print out a generated sequence using the 3-expression format
for(( i=1; i<=5; i++ ))
do
    echo Index=[$i]
done

# Print out the last line of each shell script in the current directory
echo Print out the last line of each shell script
for FILE in *.sh
do
    echo =====================================
    echo File=[$FILE]
    tail -n 1 $FILE
done

echo ''

# ************************************
# While loop example

echo -----------------------------------
echo While loop

# Countdown to blastoff
echo Executing a while loop to countdown to blastoff
counter=5
while [[ $counter -gt 0 ]]; do
    echo Countdown [$counter]
    counter=$(($counter - 1))
done
echo Blastoff

bash loop-examples.sh 
    -----------------------------------
    For loops
    Print out a hard-coded sequence
    Index=[1]
    Index=[2]
    Index=[3]
    Index=[4]
    Index=[5]
    Print out a generated sequence
    Index=[1]
    Index=[2]
    Index=[3]
    Index=[4]
    Index=[5]
    Print out a generated sequence using the 3-expression format
    Index=[1]
    Index=[2]
    Index=[3]
    Index=[4]
    Index=[5]
    Print out the last line of each shell script
    =====================================
    File=[case-example.sh]
    echo $message
    =====================================
    File=[command-line-arguments-example.sh]
    fi
    =====================================
    File=[comparison-examples.sh]
    fi
    =====================================
    File=[executing-commands-example.sh]
    ls
    =====================================
    File=[generate-passord.sh]
    echo $password
    =====================================
    File=[generate-password.sh]

    =====================================
    File=[hello-world.sh]
    echo "Hello world"
    =====================================
    File=[if-examples.sh]
    fi
    =====================================
    File=[loop-examples.sh]
    echo Blastoff
    =====================================
    File=[process-information.sh]
    echo Child process exit status $?

    -----------------------------------
    While loop
    Executing a while loop to countdown to blastoff
    Countdown [5]
    Countdown [4]
    Countdown [3]
    Countdown [2]
    Countdown [1]
    Blastoff

Examples

command-line-arguments-example.sh

Variables usage - $#, $*

#!/bin/bash

# ******************************************************************************
# Processing command line arguments

# What is the name of the executed script?
echo Name of script [$0]

# How many were provided?
echo Command line argument count [$#]

# Iterate through each argument
for arg in $@; do
	echo Argument [$arg]
done

# Display all the arguments as a string
echo All arguments [$*]

# Use parenthesis for arguments with numbers 10 or larger
if [ "${12}" != "" ]; then
	echo Argument 12 is [${12}]
	echo Argument 12 is NOT [$12]
fi

bash command-line-arguments-example.sh one Two 3 4th 999
    Name of script [command-line-arguments-example.sh]
    Command line argument count [5]
    Argument [one]
    Argument [Two]
    Argument [3]
    Argument [4th]
    Argument [999]
    All arguments [one Two 3 4th 999]

generate-password.sh

This script generates a password by combining a specified number of words.
Each word is capitalized and separated by a separator character provided as a command line argument.

#!/bin/bash

# Grab the command line arguments
passwd_word_count=$1
separator=$2

# Start with a blank password
password=''

# Get the total number of words in the word list
total_word_count=`wc -l wordlist.txt | awk '{print $1;}'`


# Build the password using the specified number of words
for (( i=1; i<=$passwd_word_count; i++ ))
do
    # Generate a random number using OpenSSL to be cryptographically secure
    rand_num_hex=`openssl rand -hex 4`
    rand_num_dec=$((16#$rand_num_hex))

    # Use the random number as an index into the word list
    word_index=$(($rand_num_dec % $total_word_count))
    random_word=`awk -v idx="$word_index" '{if (NR==idx) print $1}' wordlist.txt`

    # Capitalize the word
    random_word_upper=`echo ${random_word^}`

    # Insert a separator if this isn't the first word in the password
    if [[ ${#password} -gt 0 ]]; then
        password=$password$separator$random_word_upper
    else
        password=$random_word_upper
    fi

done

# Display the password
echo $password

bash generate-password.sh 6 '-'
	Rinse-Triage-Seizing-Vocation-Atlas-Narrow
bash generate-password.sh 4 '**'
	Probing**Cricket**Autopilot**Immodest
bash generate-password.sh 8 '!-!'
	Movable!-!Chimp!-!Abreast!-!Why!-!Shell!-!Dipped!-!Unlimited!-!Calamity

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