Bash Notes

This page is dedicated to info regarding bash scripting

Bash Conditionals Examples

You can find a link to the GNU manual below.

Reference:

1. If-Else Conditional

Syntax:

if [ condition ]; then
  # Code to execute if condition is true
elif [ another_condition ]; then
  # Code to execute if another_condition is true
else
  # Code to execute if none of the above conditions are true
fi

Example:

#!/bin/bash
num=10

if [ $num -gt 5 ]; then
  echo "$num is greater than 5"
elif [ $num -eq 5 ]; then
  echo "$num is equal to 5"
else
  echo "$num is less than 5"
fi

2. Comparison Operators

Numeric Comparisons:

String Comparisons:

| Operator | Description | Example | |———-|——————————-|————————| | = | Strings are equal | [ "$a" = "$b" ] | | != | Strings are not equal | [ "$a" != "$b" ] | | < | String is less than (lexicographically)| [ "$a" \< "$b" ] | | > | String is greater than | [ "$a" \> "$b" ] | | -z | String is empty | [ -z "$a" ] | | -n | String is not empty | [ -n "$a" ] |

3. Logical Operators

Operator	Description	Example
`!`	Logical NOT	`[ ! condition ]`
`&&`	Logical AND	`[ condition1 ] && [ condition2 ]`
`\|\|`	Logical OR	`[ condition1 ] \|\| [ condition2 ]`

Example:

is_admin=true
is_logged_in=true

if [ "$is_admin" = "true" ] && [ "$is_logged_in" = "true" ]; then
  echo "Access granted"
else
  echo "Access denied"
fi

4. Using `[[` for Advanced Conditions

[[ is more flexible than [. It supports pattern matching and avoids certain pitfalls.

Example:

str="Hello"
if [[ $str == H* ]]; then
  echo "String starts with H"
fi

5. Case Statement

Syntax:

case $variable in
  pattern1)
    # Code to execute if pattern1 matches
    ;;
  pattern2)
    # Code to execute if pattern2 matches
    ;;
  *)
    # Default case
    ;;
esac

Example:

#!/bin/bash
fruit="apple"

case $fruit in
  apple)
    echo "It's an apple!"
    ;;
  banana)
    echo "It's a banana!"
    ;;
  *)
    echo "Unknown fruit."
    ;;
esac

6. File Tests

Operator	Description	Example
`-e`	File exists	`[ -e file ]`
`-f`	File exists and is a regular file	`[ -f file ]`
`-d`	Directory exists	`[ -d directory ]`
`-r`	File is readable	`[ -r file ]`
`-w`	File is writable	`[ -w file ]`
`-x`	File is executable	`[ -x file ]`
`-s`	File is not empty	`[ -s file ]`

Example:

#!/bin/bash
file="example.txt"

if [ -f "$file" ]; then
  echo "$file exists and is a regular file."
else
  echo "$file does not exist."
fi

Summary

Use if, case, and logical operators for branching.
Use [ ] or [[ ]] for conditions.
Combine numeric, string, and file tests as needed.

These tools are the foundation of Bash scripting logic.

Bash Error Handling

1. Using `$?` (Exit Status)

Every command in Bash returns an exit status:

0: Indicates success.
Non-zero: Indicates failure.

Example:

#!/bin/bash

cp file.txt /nonexistent-directory
if [ $? -ne 0 ]; then
  echo "Error: Failed to copy file."
fi

2. `set` Built-In Command

`set -e`: Exit on Error

Automatically exits the script if a command returns a non-zero exit status.

#!/bin/bash
set -e

echo "This runs."
cp file.txt /nonexistent-directory  # Causes the script to exit
echo "This will not run."

`set -u`: Treat Undefined Variables as Errors

Triggers an error if an undefined variable is used.

#!/bin/bash
set -u

echo "This is defined: $DEFINED_VAR"

Common Combination:

set -euo pipefail

-e: Exit on error.
-u: Treat undefined variables as errors.
pipefail: Ensures the script fails if any command in a pipeline fails.

3. `trap` Command

The trap command lets you handle signals and cleanup tasks.

Example: Cleanup on Exit

#!/bin/bash
trap "echo 'An error occurred. Exiting...'; exit 1" ERR

cp file.txt /nonexistent-directory  # Error triggers trap
echo "This will not run."

Example: Cleanup Temporary Files

#!/bin/bash
trap "rm -f /tmp/tempfile; echo 'Cleaned up.'" EXIT

touch /tmp/tempfile
echo "Doing work..."

4. Using `||` (OR) for Fallback

Run an alternate command if the first one fails.

Example:

#!/bin/bash

mkdir /example-directory || echo "Failed to create directory."

5. Using `if` Statements

Manually check the success of commands.

Example:

#!/bin/bash

if cp file.txt /nonexistent-directory; then
  echo "File copied successfully."
else
  echo "Failed to copy file."
fi

6. Using Functions for Error Handling

Encapsulate commands and error handling logic into functions.

Example:

#!/bin/bash

error_exit() {
  echo "Error: $1"
  exit 1
}

cp file.txt /nonexistent-directory || error_exit "Failed to copy file."

7. Combining `pipefail` with Pipelines

By default, only the last command in a pipeline affects the exit status. Use set -o pipefail to fail the pipeline if any command fails.

Example:

#!/bin/bash
set -o pipefail

cat file.txt | grep "text" | sort || echo "Pipeline failed."

8. Redirecting Errors to a Log File

Redirect error messages to a file for later inspection.

Example:

#!/bin/bash

command 2> error.log

2>: Redirects stderr (standard error).

9. Retry Logic

Retry a command if it fails.

Example:

#!/bin/bash

for i in {1..3}; do
  command && break || echo "Attempt $i failed."
  sleep 1
done

10. Error Messages with `>&2`

Send error messages to stderr.

Example:

#!/bin/bash

echo "This is an error message." >&2

Summary of Error Handling Techniques:

Use $? to check the exit status of commands.
Use set -euo pipefail for more robust scripts.
Use trap to handle unexpected errors and perform cleanup.
Redirect stderr to log files for debugging.

These techniques can help make your Bash scripts more robust and capable of handling unexpected conditions.

11. Parameter expansion in bash

Parameter expansion allows you to modify and manipulate the values of variables.

Bash Shortcuts Cheat Sheet

Navigating the Command Line

Ctrl+A: Move to the beginning of the line.
Ctrl+E: Move to the end of the line.
Alt+F: Move forward one word.
Alt+B: Move backward one word.
Ctrl+L: Clear the screen (same as clear command).

Editing Commands

Ctrl+U: Cut (delete) from the cursor to the beginning of the line.
Ctrl+K: Cut (delete) from the cursor to the end of the line.
Ctrl+W: Cut (delete) the word before the cursor.
Alt+D: Cut (delete) the word after the cursor.
Ctrl+Y: Paste the last cut text (yank).
Ctrl+T: Swap the last two characters before the cursor.
Alt+T: Swap the last two words before the cursor.
Ctrl+_: Undo the last editing action.

Searching and Reusing Commands

Ctrl+R: Reverse search through command history.
Ctrl+S: Forward search through command history (if enabled).
Ctrl+G: Cancel search in history.
Up Arrow: Scroll backward through command history.
Down Arrow: Scroll forward through command history.
!!: Repeat the last command.
!<n>: Execute command number <n> from history.
!<prefix>: Execute the most recent command starting with <prefix>.

Controlling Processes

Ctrl+C: Kill the current process.
Ctrl+Z: Suspend the current process.
jobs: List suspended or background processes.
fg: Resume the most recent suspended process in the foreground.
bg: Resume the most recent suspended process in the background.

Moving and Manipulating Text

Ctrl+Left Arrow: Move to the beginning of the previous word (if enabled).
Ctrl+Right Arrow: Move to the beginning of the next word (if enabled).
Alt+.: Insert the last argument of the previous command.

Shell Features

Ctrl+D: Log out of the current shell session.
Ctrl+P: Scroll backward through history (same as up arrow).
Ctrl+N: Scroll forward through history (same as down arrow).
Ctrl+X Ctrl+E: Open the current command in your default editor.
Tab: Autocomplete files, directories, or commands.

Terminal Control

Ctrl+S: Pause terminal output.
Ctrl+Q: Resume terminal output.
Ctrl+Z: Suspend the current foreground process.
Ctrl+D: Send EOF (end of file) to the terminal, often used to log out or close the terminal.

Special Characters

Ctrl+V: Insert the next character literally (e.g., type Ctrl+V Tab to insert a literal Tab character).
Ctrl+O: Execute the command and bring the next one from history.

Debugging

Ctrl+X Ctrl+V: Print the version of Bash.

Miscellaneous

Ctrl+G: Exit the current editing mode or cancel a command.

Bash Notes

Bash Conditionals Examples

1. If-Else Conditional

Syntax:

Example:

2. Comparison Operators

Numeric Comparisons:

String Comparisons:

3. Logical Operators

Example:

4. Using [[ for Advanced Conditions

Example:

5. Case Statement

Syntax:

Example:

6. File Tests

Example:

Summary

Bash Error Handling

1. Using $? (Exit Status)

Example:

2. set Built-In Command

set -e: Exit on Error

set -u: Treat Undefined Variables as Errors

Common Combination:

3. trap Command

Example: Cleanup on Exit

Example: Cleanup Temporary Files

4. Using || (OR) for Fallback

Example:

5. Using if Statements

Example:

6. Using Functions for Error Handling

Example:

7. Combining pipefail with Pipelines

Example:

8. Redirecting Errors to a Log File

Example:

9. Retry Logic

Example:

10. Error Messages with >&2

Example:

Summary of Error Handling Techniques:

11. Parameter expansion in bash

Bash Shortcuts Cheat Sheet

Navigating the Command Line

Editing Commands

Searching and Reusing Commands

Controlling Processes

Moving and Manipulating Text

Shell Features

Terminal Control

Special Characters

Debugging

Miscellaneous

4. Using `[[` for Advanced Conditions

1. Using `$?` (Exit Status)

2. `set` Built-In Command

`set -e`: Exit on Error

`set -u`: Treat Undefined Variables as Errors

3. `trap` Command

4. Using `||` (OR) for Fallback

5. Using `if` Statements

7. Combining `pipefail` with Pipelines

10. Error Messages with `>&2`