Beginners Guide: Overview, Features, and Learning Path

A shell script is a text file containing commands written for a shell (command interpreter). It takes the commands you would type one by one in a terminal and organizes them into a file so they can be run as a sequence of operations.

Shell scripts require no special compilation. Simply add execute permission to a script file and it runs directly.

chmod +x script.sh
./script.sh
# or
bash script.sh

sh / bash / zsh — Types of Shells

The first line of a script (the shebang line) specifies which shell to use for execution.

#!/bin/bash
# When running with bash

#!/bin/sh
# When running with POSIX sh (for portability across environments)

Use Cases Where Shell Scripts Excel

Variables

When assigning to a variable, do not add spaces around =. Reference a variable by prefixing it with $.

#!/bin/bash
name="Ayanami Rei"
echo "Hello, $name"
echo "Hello, ${name}"  # Curly braces make the variable boundary explicit

Hello, Ayanami Rei
Hello, Ayanami Rei

Conditionals and Comparisons

The if statement evaluates conditions using [ ] (POSIX) or [[ ]] (bash extension).

#!/bin/bash
score=85

if [ $score -ge 80 ]; then
  echo "Pass"
elif [ $score -ge 60 ]; then
  echo "Borderline"
else
  echo "Fail"
fi

Pass

Loops

#!/bin/bash
# Loop over a list
for member in "Ayanami Rei" "Ikari Shinji" "Soryu Asuka"; do
  echo "Pilot: $member"
done

# Numeric loop (bash extension)
for i in {1..3}; do
  echo "Count: $i"
done

Pilot: Ayanami Rei
Pilot: Ikari Shinji
Pilot: Soryu Asuka
Count: 1
Count: 2
Count: 3

Pipes and Redirection

One of the strengths of shell scripts is the ability to connect multiple commands with a pipe (|).

#!/bin/bash
# Count lines in a file
cat /var/log/nginx/access.log | wc -l

# Extract only error lines and save to a file
grep "ERROR" app.log > errors.txt

# Write both stdout and stderr to a file
./build.sh > build.log 2>&1

The following is one suggested order for working through the pages in this dictionary.

Shell scripts = Commands for a shell organized into a file. Widely used for file automation, command chaining, and deployments.

Shebang line (first line)

• #!/bin/bash — When using bash-specific features
• #!/bin/sh — When you need portability across environments (POSIX-compliant)

Variable basics

• Assignment: name=value (no spaces around =)
• Reference: $name or ${name}
• Command substitution: result=$(command)
• Arithmetic: n=$((a + b))

Commonly used special variables

• $0 — The script's own filename
• $1 through $9 — Arguments passed to the script
• $# — Number of arguments
• $@ — List of all arguments
• $? — Exit code of the last command
• $$ — Current process ID

Commonly Used bash Extensions

• [[ ]] — Supports regex matching with =~ and writing AND/OR directly
• (( )) — C-style arithmetic expression evaluation
• Arrays — arr=(a b c), ${arr[0]}
• {1..10} — Brace expansion (sequence generation)

Common Mistakes

Adding spaces around the assignment operator

This is one of the most common mistakes in shell scripting.

name = "Ayanami Rei"  # Wrong: name is interpreted as a command name
name="Ayanami Rei"    # Correct

Using variables without quotes

If a variable contains spaces, omitting quotes can cause unintended word splitting.

name="Ayanami Rei"
echo $name    # Works, but word splitting occurs for values with spaces
echo "$name"  # Safe. Make it a habit to always wrap variables in double quotes