Shell進階筆記 - The Missing Semester of Your CS Education

Shell Scripting

1. Variables:

   • Assign: foo=bar
   • Access: $foo
   • Note: No spaces around = in assignment.

2. Strings:

   • Single quotes (') → Literal string (no variable substitution).
   • Double quotes (") → Variable substitution.

3. Special Variables:

   • $0 → Script name.

   • $1 to $9 → Script arguments.

   • $@ → All arguments.

   • $# → Number of arguments.

   • $? → Return code of the last command.

   • $$ → Current script PID.

   • !! → Last command (useful for sudo !!).

   • $_ → Last argument of the last command.

   • can run like this

     echo $?

4. Control Flow:

   • if, case, while, for constructs.

   • Example function:

     mcd () {
         mkdir -p "$1"
         cd "$1"
     }

5. Exit Codes & Operators:

   • 0 → Success, non-zero → Error.

   • && (AND), || (OR), ; (sequential execution).

   • Examples:

     false || echo "Oops, fail" # Oops, fail
     true && echo "Success" # Success

6. Command & Process Substitution:

   • $(CMD) → Substitute command output.
   • <(CMD) → Process substitution (temporary file).

7. Globbing:

   • Wildcards: ? (single char), * (any chars).

   • Curly braces {} for expansion:

     convert image.{png,jpg} → convert image.png image.jpg  

8. Shebang:

   • #!/usr/bin/env python → Portable script execution.

9. Scripts vs. Functions:

   • Scripts run in their own process; functions modify the current shell.

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Shell Tools

1. Finding Commands:

   • CMD -h or --help → Quick help.
   • man CMD → Full manual.
   • tldr CMD → Simplified examples.

2. Finding Files:

   • find:

     find . -name "*.txt" -type f  
     find . -mtime -1  # Modified in last day  
     find . -size +1M  # Files >1MB  
     find . -exec rm {} \;  # Delete matches  
     

   • Alternatives:

     • fd PATTERN → Faster, user-friendly.
     • locate FILE → Uses pre-built database.

3. Finding Code:

   • grep:

     grep -R "pattern" .  # Recursive search  
     grep -C 5 "pattern"  # Context (5 lines around match)  

   • Alternatives: rg (ripgrep), ag, ack.

4. Finding Shell Commands:

   • history | grep CMD → Search past commands.
   • Ctrl+R → Reverse search.
   • fzf → Fuzzy history search.

5. Directory Navigation:

   • z DIR (fasd) / j DIR (autojump) → Quick directory jumps.
   • Tools: tree, broot, nnn, ranger.

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Key Commands Summary

Category	Commands/Tools
Variables	foo=bar, $foo, $1-$9, $?
Strings	'literal', "substitute $var"
Control	if, for, while, &&, ||
Substitution	$(CMD), <(CMD)
Globbing	*, ?, {}
Finding	find, fd, grep, rg, locate
History	history, Ctrl+R, fzf
Navigation	z, j, tree, ranger