Scripting Basics¶

The fundamentals of writing bash scripts: structure, variables, and quoting.

Script Structure¶

A bash script is a text file containing commands:

#!/usr/bin/env bash

# This is a comment
echo "Hello, World!"

The Shebang¶

The first line tells the system how to run the script:

#!/usr/bin/env bash    # Recommended - finds bash in PATH
#!/bin/bash            # Direct path (less portable)
#!/bin/sh              # POSIX shell (more portable, fewer features)

Using env is more portable across systems where bash might be in different locations.

Making Scripts Executable¶

chmod +x script.sh
./script.sh

Or run with bash directly:

bash script.sh

File Extensions¶

The .sh extension is conventional but not required. What matters is:

The shebang line
The execute permission

Variables¶

Basic Assignment¶

name="Alice"           # No spaces around =
count=42
path=/home/user

Using Variables¶

echo $name             # Simple expansion
echo ${name}           # Explicit braces
echo "Hello, $name"    # Inside double quotes
echo "Files: ${count}" # Braces prevent ambiguity

Variable Names¶

Valid names:

Start with letter or underscore
Contain letters, numbers, underscores
Case-sensitive

valid_name="yes"
_private="yes"
NAME2="yes"
2invalid="no"          # Error: starts with number
invalid-name="no"      # Error: contains hyphen

Readonly Variables¶

readonly PI=3.14159
PI=3                   # Error: readonly variable

Unset Variables¶

unset name
echo $name             # Empty

Quoting¶

Quoting is one of the most important concepts in bash scripting.

Double Quotes¶

Variables expand, special characters mostly literal:

name="World"
echo "Hello, $name"    # Hello, World
echo "Path is $HOME"   # Path is /home/user
echo "Tab:\there"      # Tab:    here

Single Quotes¶

Everything is literal - no expansion:

name="World"
echo 'Hello, $name'    # Hello, $name
echo '$HOME'           # $HOME

No Quotes¶

Word splitting and glob expansion occur:

files="file1 file2"
rm $files              # Removes file1 AND file2 (word split)
rm "$files"            # Error: no file named "file1 file2"

When to Quote¶

Always quote variables unless you specifically want word splitting:

# Good
echo "$name"
path="$HOME/documents"
[[ -f "$file" ]]

# Bad - can break on spaces or special chars
echo $name
path=$HOME/documents
[[ -f $file ]]

Escaping¶

Backslash escapes special characters:

echo "She said \"Hello\""    # She said "Hello"
echo 'It'\''s working'       # It's working (end quote, escape, new quote)
echo "Cost: \$50"            # Cost: $50

$'...' Quoting¶

Interprets escape sequences:

echo $'Line1\nLine2'         # Two lines
echo $'Tab\there'            # Tab character

Special Variables¶

Variable	Meaning
`$0`	Script name
`$1` to `$9`	Positional arguments
`${10}`	10th argument (braces required)
`$#`	Number of arguments
`$@`	All arguments (preserves quoting)
`$*`	All arguments (as single word)
`$?`	Exit status of last command
`$$`	Current shell PID
`$!`	PID of last background job

Script Arguments¶

#!/usr/bin/env bash

echo "Script: $0"
echo "First arg: $1"
echo "Second arg: $2"
echo "All args: $@"
echo "Count: $#"

./script.sh hello world

Script: ./script.sh
First arg: hello
Second arg: world
All args: hello world
Count: 2

Difference Between $@ and $*¶

#!/usr/bin/env bash

echo "Using \$@:"
for arg in "$@"; do
    echo "  '$arg'"
done

echo "Using \$*:"
for arg in "$*"; do
    echo "  '$arg'"
done

./script.sh "hello world" foo

Using $@:
  'hello world'
  'foo'
Using $*:
  'hello world foo'

Always use "$@" when passing arguments to other commands.

Parameter Expansion¶

Default Values¶

echo ${name:-default}        # Use 'default' if unset or empty
echo ${name:=default}        # Set to 'default' if unset or empty
echo ${name:+alternate}      # Use 'alternate' if set
echo ${name:?error message}  # Error if unset or empty

String Length¶

str="hello"
echo ${#str}                 # 5

Substring¶

str="hello world"
echo ${str:0:5}              # hello (from 0, length 5)
echo ${str:6}                # world (from 6 to end)
echo ${str: -5}              # world (last 5, note space)

Substitution¶

file="document.txt"
echo ${file%.txt}            # document (remove suffix)
echo ${file#doc}             # ument.txt (remove prefix)
echo ${file%.txt}.md         # document.md (change extension)

path="/home/user/file.txt"
echo ${path##*/}             # file.txt (basename)
echo ${path%/*}              # /home/user (dirname)

Command Substitution¶

Capture command output in a variable:

# Modern syntax (preferred)
date=$(date +%Y-%m-%d)
files=$(ls *.txt)

# Old syntax (avoid)
date=`date +%Y-%m-%d`

Nest command substitutions:

dirname=$(dirname $(readlink -f "$0"))

Arithmetic¶

Basic arithmetic (covered in detail in Arithmetic):

count=$((count + 1))
total=$((5 * 10))
result=$((10 / 3))           # Integer division: 3

Comments¶

# This is a single-line comment

echo "Hello" # Inline comment

: '
This is a
multi-line comment
(actually a null command with a string argument)
'

Exit Status¶

Every command returns an exit status:

0 = success
1-255 = failure

#!/usr/bin/env bash

if command_that_might_fail; then
    echo "Success"
else
    echo "Failed with status: $?"
fi

Exit your script with a status:

exit 0           # Success
exit 1           # General error
exit 2           # Misuse of command

Debugging Shebang¶

Add flags to shebang for debugging:

#!/usr/bin/env -S bash -x    # Trace execution
#!/usr/bin/env -S bash -e    # Exit on error
#!/usr/bin/env -S bash -u    # Error on undefined variables

Or use set in the script:

#!/usr/bin/env bash
set -x           # Enable tracing
set -e           # Exit on error
set -u           # Error on undefined variables
set -o pipefail  # Pipe failure propagation

Combine them:

set -euo pipefail

Complete Example¶

#!/usr/bin/env bash
#
# greet.sh - Greet users with a personalized message
#

set -euo pipefail

# Default values
NAME="${1:-World}"
GREETING="${2:-Hello}"

# Main logic
main() {
    local message="${GREETING}, ${NAME}!"
    echo "$message"

    if [[ "$NAME" == "World" ]]; then
        echo "(Tip: Pass a name as the first argument)"
    fi

    return 0
}

main

chmod +x greet.sh
./greet.sh
./greet.sh Alice
./greet.sh Alice "Good morning"

Hello, World!
(Tip: Pass a name as the first argument)
Hello, Alice!
Good morning, Alice!

Try It¶

Create a simple script:

cat > /tmp/hello.sh << 'EOF'
#!/usr/bin/env bash
echo "Hello, ${1:-World}!"
EOF
chmod +x /tmp/hello.sh
/tmp/hello.sh
/tmp/hello.sh Alice

Practice quoting:

name="Hello World"
echo $name      # Two arguments to echo
echo "$name"    # One argument
echo '$name'    # Literal

Test parameter expansion:

file="document.backup.txt"
echo ${file%.txt}
echo ${file%%.*}
echo ${file#*.}
echo ${file##*.}

Summary¶

Concept	Syntax
Shebang	`#!/usr/bin/env bash`
Variable assignment	`name="value"`
Variable usage	`"$name"` or `"${name}"`
Command substitution	`$(command)`
Arithmetic	`$((expression))`
Default value	`${var:-default}`
Script arguments	`$1`, `$2`, `$@`, `$#`
Exit status	`$?`, `exit N`

Key rules:

No spaces around = in assignments
Always quote variables: "$var"
Use "$@" for passing arguments
Start scripts with set -euo pipefail