2 - Advanced Bash Script

Linux Directory Structure

Directories	Description
/bin	binary or executable programs.
/etc	system configuration files.
/home	home directory. It is the default current directory.
/opt	optional or third-party software.
/tmp	temporary space, typically cleared on reboot.
/usr	User related programs.
/var	log files.

File descriptors

File descriptors (FDs) are numeric identifiers that represent open files or streams in Unix/Linux systems. In bash, every process automatically gets three standard file descriptors:

Standard File Descriptors

0 - stdin (Standard Input)

Purpose: Where a program reads input from
Default: Connected to keyboard/terminal
Example: When you type commands or data

1 - stdout (Standard Output)

Purpose: Where a program sends normal output
Default: Connected to terminal screen
Example: Regular command output, echo results

2 - stderr (Standard Error)

Purpose: Where a program sends error messages
Default: Connected to terminal screen (same as stdout)
Example: Error messages, warnings

#!/bin/bash

echo "=== File Descriptor Redirection Demo ==="

# Setup: Create some test files
echo -e "banana\napple\ncherry" > fruits.txt
echo "Setting up demo files..."

echo -e "\n1. REDIRECT STDOUT TO FILE"
echo "This goes to a file" > message.txt
echo "✓ Check message.txt - you won't see the output on screen"

echo -e "\n2. REDIRECT STDERR TO FILE"
ls thisfiledoesnotexist 2> error_log.txt
echo "✓ Error message went to error_log.txt instead of screen"

echo -e "\n3. REDIRECT BOTH STDOUT AND STDERR"
# This command will produce both output and an error
{ echo "Success message"; ls fakefile; } > all_output.txt 2>&1
echo "✓ Both success and error messages went to all_output.txt"

echo -e "\n4. SHORTHAND FOR BOTH"
{ echo "Another success"; ls anotherfakefile; } &> shorthand_output.txt
echo "✓ Same result using &> shorthand"

echo -e "\n5. REDIRECT STDIN FROM FILE"
echo "Sorting fruits from fruits.txt:"
sort < fruits.txt

echo -e "\n6. DISCARD OUTPUT (send to /dev/null)"
echo "Running ls on fake file - you won't see any error:"
ls nonexistent 2> /dev/null
echo "✓ Error was discarded"

echo -e "\n7. DISCARD EVERYTHING"
{ echo "This output disappears"; ls fakefile; } &> /dev/null
echo "✓ Both output and errors were discarded"

echo -e "\n=== Demo complete! Check these files: ==="
echo "- message.txt"
echo "- error_log.txt"
echo "- all_output.txt"
echo "- shorthand_output.txt"

# Cleanup option
echo -e "\nTo clean up demo files, run:"
echo "rm -f message.txt error_log.txt all_output.txt shorthand_output.txt fruits.txt"

local vs global variable

Global variable

#!/bin/bash
# Global variable
username="Alice"

function display_user() {
    echo "User: $username"  # Can access global variable
}

function change_user() {
    username="Bob"          # Modifies the global variable!
}

echo "Initial: $username"  # Alice
display_user               # User: Alice
change_user
echo "After change: $username"  # Bob (global was modified)

OUTPUT

c@c:~/cos3105$ ./test.sh
Initial: Alice
User: Alice
After change: Bob

local variable

#!/bin/bash
username="Alice"  # Global

function test_local() {
    local username="Bob"    # Local variable, shadows global
    echo "Inside function: $username"  # Bob
}

echo "Before: $username"   # Alice
test_local                 # Inside function: Bob
echo "After: $username"    # Alice (global unchanged)

OUTPUT

c@c:~/cos3105$ ./test.sh
Before: Alice
Inside function: Bob
After: Alice

echo vs printf

echo for simple output
printf for formatted output

#!/bin/bash
name="John"
age=25
score=87.5

# echo - basic concatenation
echo "Name: $name, Age: $age, Score: $score"

# printf - formatted output
printf "Name: %-10s Age: %3d Score: %6.2f\n" "$name" "$age" "$score"
# Output: Name: John       Age:  25 Score:  87.50

# Without formatting
printf "%s\n" "hello"
# Output: hello

# With %-10s (left-aligned, 10 characters wide)
printf "%-10s|\n" "hello"
# Output: hello     |
#         ^^^^^     ^ (5 spaces added to make it 10 chars total)

# Compare with right-aligned (no minus sign)
printf "%10s|\n" "hello"
# Output:      hello|
#         ^^^^^     ^ (5 spaces before the text)

OUTPUT

c@c:~/cos3105$ ./test.sh
Name: John, Age: 25, Score: 87.5
Name: John       Age:  25 Score:  87.50
hello
hello     |
     hello|

Function with parameters and return value

Example 1

#!/bin/bash
var1=$1
var2=$2
var3=$3
var4=$4

add(){
  #Note the $1 and $2 variables here are not the same of the
  #main script...
  echo "The first argument to this function is $1"
  echo "The second argument to this function is $2"
  result=$(($1+$2))
  echo $result

}

add $var1 $var2
add $var3 $var4

OUTPUT

c@c:~/cos3105$ ./test.sh 1 2 3 4
The first argument to this function is 1
The second argument to this function is 2
3
The first argument to this function is 3
The second argument to this function is 4
7

Example 2

#!/bin/bash
var1=$1
var2=$2
var3=$3
var4=$4

add(){
  result=$(($1+$2))
  echo $result
}
minus(){
    result=$(($1-$2))
    echo $result
}
result1=$(add $var1 $var2)
result2=$(add $var3 $var4)
result3=$(minus $result2 $result1)
echo "$result2 - $result1 = $result3"

OUTPUT

c@c:~/cos3105$ ./test.sh 1 2 3 4
7 - 3 = 4

Example 3

read string into array

#!/bin/bash

read first second third <<< "a b c"
echo $first   # a
echo $second  # b
echo $third   # c

# With -a (reads into array)
read -a array <<< "a b c"
echo ${array[0]}  # a
echo ${array[1]}  # b
echo ${array[2]}  # c

OUTPUT

c@c:~/cos3105$ ./test.sh
a
b
c
a
b
c

Example 4

return string of array and read array

#!/bin/bash

get_user_info() {
    echo "john" "doe" "30"
}

read -a user_info <<< "$(get_user_info)"
printf "Name = %s, Last name = %s, Age = %s\n" "${user_info[0]}" "${user_info[1]}" "${user_info[2]}"

OUTPUT

c@c:~/cos3105$ ./test.sh
Name = john, Last name = doe, Age = 30

Example 5

return string of array with comma separate and read array
IFS=", " for tells read to split string with ", "

#!/bin/bash

get_user_info() {
    echo "john, doe, 30"
}

IFS=", " read -a user_info <<< "$(get_user_info)"
printf "Name = %s, Last name = %s, Age = %s\n" "${user_info[0]}" "${user_info[1]}" "${user_info[2]}"

OUTPUT

c@c:~/cos3105$ ./test.sh
Name = john, Last name = doe, Age = 30

Terminal User Interface (TUI)

dialog: Original tool, developed first (1990s)
whiptail: Fork/reimplementation of dialog, designed to be more lightweight
dialog vs whiptail
- dialog more widgets and features
- whiptail less widgets

Example 1: Message Box

message="Whiptail: Tool Basics"
whiptail --msgbox --title "Intro to Whiptail" "$message" 0 0

The number 0 0 is for auto size

Another example

message="Whiptail: Tool Basics"
whiptail --msgbox --title "Intro to Whiptail" "$message" 25 80

The number 25 80 is height and width.

25 is the height (number of rows/lines)
80 is the width (number of columns/characters)

OUTPUT

Whiptail Message Box

Example 2: Yes/No dialog

#!/bin/bash

if whiptail --title "Example Dialog" --yesno "This is an example of a yes/no box." 0 0; then
    echo "User selected Yes, exit status was $?."
else
    echo "User selected No, exit status was $?."
fi

Whiptail Yes/No Dialog

Example 3: Input dialog

#!/bin/bash
username="$(whiptail --inputbox "What is your username?" 10 30 "$USER" 3>&1 1>&2 2>&3)"
printf "Your username is : %s\n" "${username}"

OUTPUT

Whiptail Input dialog

This is a file descriptor swap - it switches stdout and stderr!

3>&1 1>&2 2>&3

Step by step:

3>&1 - Save stdout (1) into temp descriptor (3)
1>&2 - Point stdout (1) to stderr (2)
2>&3 - Point stderr (2) to temp descriptor (3)

Before:  1 → screen    2 → screen
         3 → (unused)

Step 1:  1 → screen    2 → screen    3 → screen (copy of 1)
Step 2:  1 → screen    2 → screen    3 → screen
         ↳─────────────────↗
Step 3:  1 → screen    2 → screen    3 → screen
         ↳─────────────────↗   ↳--────────↗

Result:

stdout now goes where stderr used to go
stderr now goes where stdout used to go
3 is like temporary variable

Why use 3?

File descriptors 0, 1, 2 are already taken (stdin, stdout, stderr) Descriptor 3 is the first available “free” descriptor It’s temporary - we only need it for the swap operation

Example 4: Password dialog

#!/bin/bash
password="$(whiptail --passwordbox "Please enter your password:" 0 0 3>&1 1>&2 2>&3)"
printf "Your password is : %s\n" "$password"

OUTPUT

Whiptail Password dialog

#!/bin/bash
result=$(whiptail \
  --menu "What is your favorite shell?" 0 0 2 \
  "Bash" "(Bourne Again shell)" \
  "Zsh"  "(Z-Shell)" \
  3>&1 1>&2 2>&3)

echo "You selected: $result"

OUTPUT

Whiptail Menu dialog

Example 6: Multiple selected dialog

#!/bin/bash
result=$(whiptail \
  --checklist "What is your favorite shell?" 0 0 3 \
  "Bash" "(Bourne Again shell)" 1 \
  "Zsh"  "(Z-Shell)" 0 \
  "Dash" "(Dash shell)" 0 \
  3>&1 1>&2 2>&3)

echo "You selected: $result"

OUTPUT

Whiptail multiple selected dialog

Example 7: Textbox with scroll text

#!/bin/bash
cat > temp.txt << 'EOF'
This is a very long text that will demonstrate
the scrollable text box feature in whiptail.
You can scroll up and down through this content
using the arrow keys.

This is a very long text that will demonstrate
the scrollable text box feature in whiptail.
You can scroll up and down through this content
using the arrow keys.


This is a very long text that will demonstrate
the scrollable text box feature in whiptail.
You can scroll up and down through this content
using the arrow keys.
EOF

whiptail --textbox temp.txt 0 0 --scrolltext
rm temp.txt

OUTPUT

Whiptail Textbox dialog

Example 8: Radio lists

#!/bin/bash
result=$(whiptail \
  --radiolist "What is your favorite shell?" 0 0 3 \
  "Bash" "(Bourne Again shell)" 1 \
  "Zsh"  "(Z-Shell)" 0 \
  "Dash" "(Dash shell)" 0 \
  3>&1 1>&2 2>&3)

echo "You selected: $result"

OUTPUT

Whiptail radio lists dialog

Example 9: Progress bar

#!/bin/bash

# Simple file processing progress bar
{
    for i in {0..100..20}; do
        echo "XXX"
        echo $i
        echo "Processing file $((i/20 + 1)) of 6..."
        echo "XXX"
        sleep 1
    done
} | whiptail --gauge "File Processing" 6 50 0

echo "Done!"

OUTPUT

Whiptail progress bar

Handle errors from C program

Example of C program

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[]) {
    if (argc < 2) {
        fprintf(stderr, "Usage: %s <command>\n", argv[0]);
        return 1;  // Error code
    }

    // Your logic here
    if (argc == 2) {
        fprintf(stderr, "Error: Something went wrong\n");
        return 2;  // Specific error code
    }

    if (argc > 2) {
        fprintf(stderr, "Error: File not found\n");
        return 3;  // Different error code
    }

    // Success case
    printf("Operation successful\n");
    return 0;  // Success
}

Example of bash script

#!/bin/bash

# Capture stdout and stderr separately
result=$(./contacts2 mode email email_body 2>&1)
exit_code=$?

if [ $exit_code -eq 0 ]; then
    echo "Success: $result"
else
    echo "Error (exit code $exit_code): $result"
fi

OUTPUT

c@c:~/cos3105$ ./contacts2.sh
Error (exit code 3): Error: File not found

2 - Advanced Bash Script

Linux Directory Structure

File descriptors

local vs global variable

echo vs printf

Function with parameters and return value

Example 1

Example 2

Example 3

Example 4

Example 5

Terminal User Interface (TUI)

Example 1: Message Box

Example 2: Yes/No dialog

Example 3: Input dialog

Example 4: Password dialog

Example 5: Menu dialog

Example 6: Multiple selected dialog

Example 7: Textbox with scroll text

Example 8: Radio lists

Example 9: Progress bar

Handle errors from C program