The Complete Guide to Bash Commands

Basic Syntax and Variables

Variables

name="John"
echo $name    # Basic output
echo "$name"  # Recommended way (with quotes)
echo "${name}"  # Most explicit way

String Quotes

Double quotes allow variable expansion: "Hello $name"
Single quotes preserve literal text: 'Hello $name'

Shell Execution

echo "Current directory: $(pwd)"   # Modern syntax
echo "Current directory: `pwd`"    # Legacy syntax

Control Flow

Conditional Execution

git commit && git push             # Run second command only if first succeeds
git commit || echo "Commit failed" # Run second command only if first fails

Functions

get_name() {
    echo "John"
}
echo "You are $(get_name)"

Conditionals

if [[ -z "$string" ]]; then
    echo "String is empty"
elif [[ -n "$string" ]]; then
    echo "String is not empty"
fi

Parameter Expansions

Basic Operations

name="John"
echo "${name}"           # Standard output
echo "${name/J/j}"      # Substitution (john)
echo "${name:0:2}"      # Slicing (Jo)
echo "${name::2}"       # Slicing from start (Jo)
echo "${name::-1}"      # Slice except last char (Joh)
echo "${name:(-1)}"     # Last character (n)
echo "${food:-Cake}"    # Default value if unset

Advanced Expansions

str="/path/to/foo.cpp"
echo "${str%.cpp}"      # Remove .cpp
echo "${str%.cpp}.o"    # Replace with .o
echo "${str%/*}"        # Remove last segment
echo "${str##*.}"       # Get extension
echo "${str##*/}"       # Get filename
echo "${str#*/}"        # Remove first segment
echo "${str/foo/bar}"   # Replace foo with bar

Arrays

Array Operations

# Definition
Fruits=('Apple' 'Banana' 'Orange')

# Access
echo "${Fruits[0]}"     # First element
echo "${Fruits[-1]}"    # Last element
echo "${Fruits[@]}"     # All elements
echo "${#Fruits[@]}"    # Number of elements
echo "${#Fruits}"       # Length of first element

# Modification
Fruits+=('Watermelon')  # Add element
unset Fruits[2]         # Remove element
Fruits=("${Fruits[@]}") # Recreate array

Dictionaries (Associative Arrays)

# Declaration
declare -A sounds
sounds[dog]="bark"
sounds[cow]="moo"

# Access
echo "${sounds[dog]}"   # Single value
echo "${sounds[@]}"     # All values
echo "${!sounds[@]}"    # All keys
echo "${#sounds[@]}"    # Number of elements

File Conditions

[[ -e FILE ]]    # Exists
[[ -r FILE ]]    # Readable
[[ -h FILE ]]    # Symlink
[[ -d FILE ]]    # Directory
[[ -w FILE ]]    # Writable
[[ -s FILE ]]    # Size > 0 bytes
[[ -f FILE ]]    # Regular file
[[ -x FILE ]]    # Executable

String Conditions

[[ -z STRING ]]          # Empty string
[[ -n STRING ]]          # Not empty string
[[ STRING == STRING ]]   # Equal
[[ STRING != STRING ]]   # Not equal
[[ STRING =~ REGEX ]]    # Regex match

Numeric Conditions

[[ NUM -eq NUM ]]   # Equal
[[ NUM -ne NUM ]]   # Not equal
[[ NUM -lt NUM ]]   # Less than
[[ NUM -le NUM ]]   # Less than or equal
[[ NUM -gt NUM ]]   # Greater than
[[ NUM -ge NUM ]]   # Greater than or equal

Loops

For Loops

# Basic for loop
for i in /etc/rc.*; do
    echo "$i"
done

# C-style for loop
for ((i = 0; i < 100; i++)); do
    echo "$i"
done

# Range loop
for i in {1..5}; do
    echo "Welcome $i"
done

# With step size
for i in {5..50..5}; do
    echo "Welcome $i"
done

While Loops

# Reading lines
while read -r line; do
    echo "$line"
done <file.txt

# Infinite loop
while true; do
    echo "Forever"
done

Input/Output

Redirection

command > output.txt      # stdout to file
command >> output.txt     # stdout to file (append)
command 2> error.log      # stderr to file
command 2>&1             # stderr to stdout
command &>/dev/null      # stdout and stderr to null

Reading Input

echo -n "Proceed? [y/n]: "
read -r ans
echo "$ans"

read -n 1 ans    # Read single character

Special Variables

$?      # Exit status of last command
$!      # PID of last background process
$$      # Current shell PID
$0      # Script name
$_      # Last argument of previous command
$#      # Number of arguments
$@      # All arguments as separate words
$*      # All arguments as a single word

Debugging and Error Handling

Strict Mode

set -euo pipefail
IFS=$'nt'

Trap Errors

trap 'echo Error at about $LINENO' ERR

# Or with function
traperr() {
    echo "ERROR: ${BASH_SOURCE[1]} at about ${BASH_LINENO[0]}"
}
set -o errtrace
trap traperr ERR

Advanced Features

Case Statements

case "$1" in
    start|up)
        vagrant up
        ;;
    stop|down)
        vagrant halt
        ;;
    *)
        echo "Usage: $0 {start|stop}"
        ;;
esac

Here Documents

cat <<END
hello world
END

Command Substitution

result=$(command)
lines=(`cat "logfile"`)

Best Practices

Always quote variables unless you specifically need word splitting
Use set -e to make scripts exit on error
Use meaningful variable names
Comment your code
Use functions for repeated operations
Check for required commands at script start
Provide usage information for scripts
Use shellcheck to validate your scripts

Remember that Bash is both powerful and complex. These commands and constructs can be combined in numerous ways to create sophisticated scripts. Always test your scripts thoroughly, especially when dealing with file operations or system commands.

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