Master Shell Scripting: From Basics to Advanced Tricks

What Is a Shell

Shell

is a layer that wraps the operating system core, providing a direct interface for users to interact with the system; it interprets user input and returns output to the screen.

It is one of the main ways users communicate with the system, interpreting commands and displaying results.

The Shell explains user commands and outputs results, whether via command‑line or graphical interfaces.

What Is a Shell Script

A Shell script is a file that contains a series of Linux commands and control statements. When executed, the commands run non‑interactively.

Windows uses *.bat batch files.

Linux commonly uses *.sh scripts.

Shell Script Rules

Shell scripts are usually edited with vim and consist of Linux commands, Bash directives, logic control statements, and comments.

First Shell Script

# vim test1.sh
# cat test1.sh
# 第一个 shell 脚本 这是注释
#!/bin/bash

echo "hello world!"

Run it with:

# bash test1.sh
hello world!

Shebang

The shebang ( #!) on the first line tells the system which interpreter to use, e.g., #!/bin/bash invokes the Bash interpreter.

If no shebang is specified, the current shell ( $SHELL) is used.

If the interpreter path is missing or not executable, an error occurs.

Common Ways to Execute Scripts

1. Use bash script.sh or sh script.sh (no execute permission needed).

$ sh helloworld.sh
Helloworld

2. Make the script executable ( chmod +x script.sh) and run it directly:

$ ./helloworld.sh
Helloworld

The first method runs the script via the Bash interpreter, while the second runs the script itself, requiring execute permission.

Shell Variables

Variables are defined without spaces, e.g., name="zfox". Bash treats all variables as strings by default.

name="zfox"
# Access
echo ${name}
# or
echo $name

Variable naming rules:

Use letters, numbers, and underscores; cannot start with a number.

Case‑sensitive.

System Variables

Common system variables include HOME, PWD, SHELL, USER, etc.

$ echo $HOME
/home/zfox

List all variables with set or env.

Custom Variables

Define with var=value, delete with unset var, make read‑only with readonly var.

Single vs Double Quotes

Single quotes prevent variable expansion; double quotes allow it. Backticks (or $(...)) capture command output.

# name="奥里给"
# echo '${name}'
${name}
# double quotes
# echo "${name}"
zfox带你学bash

Special Variables

$?

– exit status (0 = success). $# – number of positional parameters. $* – all parameters as a single string. $@ – all parameters as separate strings.

Environment Variable Management

Exported variables define the shell’s runtime environment. User‑specific files ( ~/.bash_profile, ~/.bashrc) and global files ( /etc/profile, /etc/bashrc) store them.

Commands to view or modify: set – show all variables. env – show environment variables. declare – list variables. export – set/export a variable.

Read‑Only Variables

# readonly name="super"
# name="changed"   # will fail
-bash: name: readonly variable

Environment Variable Initialization Order

Shell Substring Operations

${var}                # value
${#var}               # length
${var:offset}         # from offset
${var:offset:length}  # length chars
${var#word}           # remove shortest prefix match
${var##word}          # remove longest prefix match
${var%word}           # remove shortest suffix match
${var%%word}          # remove longest suffix match
${var/pattern/repl}   # replace first match
${var//pattern/repl}  # replace all matches

Example:

name="yuchao180"
echo ${#name}          # 9
echo ${name:3}        # hao180
echo ${name:2:4}    # chao

String Replacement

str="Hello,man,i am your brother."
echo ${str/man/boy}   # Hello,boy,i am your brother.
echo ${str//o/O}      # HellO,man,i am yOur brOther.

Batch Rename Example

# Create sample files
touch chaochao_{1..5}_finished.jpg
touch chaochao_{1..5}_finished.png

# Rename to remove "_finished"
for file in *finished.jpg; do
  mv "$file" "${file//_finished/}"
done

Conclusion

These notes cover the essentials of Shell scripting, from basic concepts to practical tricks, helping you automate tasks efficiently on Linux.