Bash scripting notes

if [[ ... ]] vs if [ ... ]

# At the beginning of the script
set -x
trap read debug

# This is a single-line comment

: << COMMENT
This is bash
multi-line
comment
COMMENT

$?          # Exit code of last command
$#          # Number of arguments supplied
$0          # Current filename (first argument)
$1-$9       # Command line arguments (one by one)
$@          # Array of arguments
$$          # Current PID

#!/usr/bin/env bash                     # Shebang: run with Bash shell
 
# TL;DR
set -ueo pipefail
 
# Exit if undefined variable has been found
set -u                                      
 
# Exit if any exit code of executed functions is not equal zero
set -e
 
# Break pipeline if our script failed
set -o pipefail                               

var1=100                                # Number
var2="Number: $var1"                    # Double quotes == interpolation
var3='Number is stored in $var1'        # Single quotes == no interpolation
var4=$(cat test.txt)                    # Result of command
var5=("str1" "str2" "str3")             # Array of strings
var6=(1 5 9 43 23 43)                   # Array of numbers
read var7                               # Read variable from stdin

# If statements (spaces around expression matter)
if [ "$var" = "Test value" ]; then
    command
elif [ "var" = "abcd" ]; then
    command
else
    command
fi
 
# Negation
if ! [[ -f $var1 ]]                     # If file doesn't exist
 
# Number comparision
if [[ $var1 -lt $var2 ]]                # If less than
if [[ $var1 -gt $var2 ]]                # If greater than
if [[ $var1 -eq $var2 ]]                # If equal
if [[ $var1 -ne $var2 ]]                # If not equal
 
# String comparision
if [[ $var1 = $var2 ]]                  # If string equal
if [[ $var1 != $var2 ]]                 # If string not equal
 
# Others
if [[ -z $var1 ]]                       # If null or zero length
if [[ -n $var1 ]]                       # If not null and not zero length
if [[ -d $var1 ]]                       # If directory exist
if [[ -f $var1 ]]                       # If file exist
 
# Compund expressions
if [[ $v1 = $v2  || $v1 != $v3 ]]       # Or
if [[ $v1 = $v2  && $v1 != $v3 ]]       # And

# For loops
for number in {1..12}; do
    echo "Current number: $number"
    break
    continue
done
 
for file in /bin/*                      # Iterate over files
for num in {1..12..2}                   # {start..end..step}
for num in 1 9 4 3 3                    # Interate over list of items
for item in "${arr[@]}"                 # Spread array to list of items (above)
for (( i=0; i<5; i++ ))                 # C-like for loop
for name in $(cat names.txt)            # Line by line output of a command
for arg in "$@"                         # Iterate over arguments
 
# Iterate over lines of variable
while read -r line; do
    echo $line
done <<< "$content"
 
# Iterate over lines of file
while read -r line; do
    echo $line
done < file.txt

function func1() {
    echo $1                             # Echo 1st argument
    echo $2                             # Echo 2nd argument
}
 
func1 "test-argument" 234               # Call a function

var='super'
echo ${var:0:1} OR ${var::1}  => 's'    # Get substring of a string
echo ${var:2:2}               => 'pe'   # ${var:start:n}        
echo ${var:2}                 => 'per'  # From :2 to end
echo ${var:(-2)}              => 'e'    # Index from end
echo ${#var}                  => 5      # Get length
 
# Case manipulation
echo ${str^}            => 'Super'      # Upper first letter
echo ${str^^}           => 'SUPER'      # Uppercase
var='SUPER'
echo ${str,}            => 'sUPER'      # Lower first letter
echo ${str,,}           => 'super'      # Lowercase
 
# Split into variables
str='1:2'
IFS=: read -r var1 var2 <<< "$str"  =>  var1 == 1, var2 == 2 

$((1 + 1))                              # Math expression        
$((x + y))                              # Variables math

arr=("1" "2" "3")                       # Define array
arr[3]="4"                              # Define item
arr+=("5")                              # Add item
echo ${arr[3]}                          # Get item
echo ${arr[@]}                          # Get all items

# Check matching and group extraction
exp='123(.*)456'
if [[ $var1 =~ $exp ]]; then
    echo $BASH_REMATCH[0]               # Entire regex match
    echo $BASH_REMATCH[1]               # First group
fi

echo $RANDOM                            # Get random number
date +%s%N                              # Get UNIX timestamp (nanoseconds)