☑️ 8. Scripting with bash

Writing scripts

Writing a simple script is as easy as putting some commands in a file:

echo 'echo "Hello world!"' > my_first_script

You can use any editor to author your scripts, but I would recommend an editor that supports useful plugins:

• syntax highlighting
• linters
• LSPs

Executing scripts

There are 2 main ways to execute a script:

# execute explicitly using bash
bash ./my-script

# execute as a command
# implicitly uses the default shell
# requires execute permissions on the file
./my-script

Shebang

You can instruct the system to use a specific execution environment by adding a special instruction as the first line of your script.

This is often referred to as a shebang:

#!/bin/bash

# your actual script starts here

• The shebang makes sure that your script is properly setup and reduces the knowledge and setup required from its user.

• Shebangs are NOT limited to bash, you can use them with any language such as JavaScript or Python

#!/bin/env bash

# your script starts here

Test

The test command allow us to assert many types of conditions:

• string comparisons
• number comparisons
• file existence
• file properties
• variable properties
• and more

Running tests

There are 2 ways to run the test command:

# execute explicitly
test <condition>

# using square brackets
[ <condition> ]

NOTE: the spaces around the condition when using brackets are required

To read the results of the test examine the its exit status:

test 'a' = 'a'
echo "$?" # prints 0 i.e. success

[ 'a' = 'b' ]
echo "$?" # prints 1 i.e. failure

Testing strings

# str1 matches str2
str1 = str2

# str1 does not match str2
str1 != str2

# str1 is less than str2 (alphabetically)
str1 < str2

# str1 is greater than str2 (alphabetically)
str1 > str2

Test: numbers

1 -lt 2   # less than

3 -le 4   # less than or equal

5 -eq 6   # equal

7 -ge 8   # greater than or equal

9 -gt 10   # greater than

11 -ne 12   # not equal

Test: file properties

-e file   # file exists
-d file   # directory exists
-f file   # file exists and is a regular file
# (not a directory or other special type of file)

-s file   # file exists and is not empty
-N file   # file was modified since it was last read

file1 -nt file2 # file1 is newer than file2
file1 -ot file2 # file1 is older than file2

Test: variables

-v var    # variable is set

-n $var   # variable value is NOT empty

-z $var   # variable is unset or value is empty

Test: logical operators

# cond1 AND cond2
[ condition1 -a condition2 ]

# cond1 OR cond2
[ condition1 -o condition2 ]

# NOT condition
[ ! condition ]

Bash specific testing

Bash extends the capabilities of the POSIX testing utility.

To use bash-specific features use double square brackets:

[[ <condition> ]]

Some useful extensions only available in bash are:

# using globs in tests
[[ "hello.txt" == *.txt ]]

# use character classes in tests
[[ "$input" == [0-9]* ]]

# use regular expressions in tests
[[ "$version" =~ ^[0-9]+\.[0-9]+\.[0-9]+$ ]]

Logical operators outside test

You can use short-circuiting logical AND and logical OR when chaining commands

# only execute cmd2 if cmd1 succeeds
cmd1 && cmd2

# only execute cmd2 if cmd1  fails
cmd1 || cmd2

Conditionals

You do NOT need to check the result of a test through the exit status.

if

if condition; then
  statements
elif condition; then
  statements
else
  statements
fi

case

Case patterns can contain globs like *, ?, [a-z]:

case expression in
  pattern1 )
    statements ;;
  pattern2 )
    statements ;;
esac

Loops

Loops can use globs as well:

for f in ~/mydir/*.txt; do
  echo $f
done

for i in {1..5}; do
    echo "Number $i"
done

cat file.txt | while read line; do
  echo $line
done

Arrays

# declare an array
myarray=(1 "A" 2 "B")

# assign to an index in an array
myarray[0]="hi"

# print array element at index i
echo ${myarray[i]}

# print length of array element at index i
echo ${#myarray[i]}

# print length of array itself
echo ${#myarray[@]}

# append items to an array
myarray+=(7 "Z")

Looping over each element of an array:

colors=("red" "green" "yellow")
for color in "${colors[@]}"; do
  echo "The traffic light is $color"
done

Associative arrays

These act as dictionaries or hashmaps in other languages allowing you to store key - value pairs.

# define an associative array
declare -A PETS

PETS["Norwegian"]="cat"
PETS["Dane"]="horse"
PETS["Brit"]="bird"
PETS["German"]="?"

# Read the value for a given key
echo ${PETS['Brit']}

# Adding new values
PETS["Swede"]="dog"

# Alternative syntax for adding elements
PETS+=(["Bulgarian"]="goat")

Handling inputs

There are special variable that allow you to handle script arguments.

# print number of arguments passed to the script
echo "$#"

# an array containing all arguments
echo "$@"

# arguments by order
echo "$1 $2 $3"

# calling a script with arguments
my-script hello world

Prompting for user input

Use the read command to block execution and allow the user to provide input through stdin:

# program stops and waits for the user to input a line of text
read line
echo "$line"

If more than one variable is provided, the line will be split into words.

The -p option allows for adding a prompt:

read -p "Full Name: " first last
echo "My name is $last, $first $last"

Handling signals

Use the trap command to execute code as a response to a system signal:

# Delete temp file when receiving an EXIT signal
tempfile=/tmp/tmpdata
trap "rm -f $tempfile" EXIT

Functions

Functions can be used to group logical units and repetitive actions.

# declare a function
my_func() { echo "Hello $1"; }

# alternative syntax
function myfunc() {
  echo "same as above"
}

# calling a function is like calling a command
my_func "friend"

In bash functions do NOT return values.

Use the declare built-in command to examine functions defined in your environment:

# Show function body
declare -f my_func

# List all function names
declare -F

The meaning of certain special variable changes inside a function:

# number of arguments to the function
myfunc() { echo "$#"; }`

# all arguments to the function
function myfunc() {
  echo "$@"
}

# function arguments by order
myfunc() { echo "$1 $2 $3"; }

Doing math in bash

By default everything in the console is treated as a string. Use the $((<expression>)) syntax to perform math.

echo "1 + 1 = $((1+1))"
# prints 1 + 1 = 2

# a random number between 1 and 10
echo "$((RANDOM % 10 + 1))"

# by default $RANDOM will hold a random
# number between 0 and 32767
echo $RANDOM

Inside a math expression variables should NOT start with an $

Linting with shellcheck

Shellcheck is a powerful tool that can find many mistakes and problems in your scripts.

It is an external command that needs to be installed.

shellcheck my-script
# produces a list of potential problems

Fun fact: there is a version of the popular containerization tool Docker written entirely in bash

#!/bin/bash

# Menu items
options=("Install Package" "Update System" "View Logs" "Configure Settings" "Exit")

# Currently selected index
selected=0

# Function to draw the menu
draw_menu() {
  clear
  echo "=== Main Menu ==="
  echo "Use ↑/↓ arrows and Enter to select"
  echo ""

  for i in "${!options[@]}"; do
    if [ $i -eq $selected ]; then
      echo "→ ${options[$i]}" # Selected item
    else
      echo "  ${options[$i]}"
    fi
  done
}

# Function to read arrow keys
read_input() {
  read -rsn1 key
  if [[ $key == $'\x1b' ]]; then
    read -rsn2 key
  fi
  echo "$key"
}

# Main loop
while true; do
  draw_menu

  key=$(read_input)

  case "$key" in
  '[A') # Up arrow
    ((selected--))
    if [ $selected -lt 0 ]; then
      selected=$((${#options[@]} - 1))
    fi
    ;;
  '[B') # Down arrow
    ((selected++))
    if [ $selected -ge ${#options[@]} ]; then
      selected=0
    fi
    ;;
  '') # Enter key
    clear
    echo "You selected: ${options[$selected]}"

    # Handle the selection
    case $selected in
    0) echo "Installing package..." ;;
    1) echo "Updating system..." ;;
    2) echo "Viewing logs..." ;;
    3) echo "Configuring settings..." ;;
    4)
      echo "Goodbye!"
      exit 0
      ;;
    esac

    echo ""
    echo "Press any key to return to menu..."
    read -n1
    ;;
  esac
done