In Unix/Linux shells, the difference between single-dash (-) and double-dash (--) flags isn't arbitrary - it follows specific conventions:

# Single-letter options (can be combined)
ls -l -a      # Equivalent to
ls -la

# Full-word options (must be separate)
git commit --message "Update"  # Cannot combine with other options

The single-dash convention originated from early Unix systems where parsing efficiency mattered. Double-dash emerged later for better readability:

# Classic Unix style (single-dash)
tar -xzvf archive.tar.gz

# GNU style (allowing both)
grep --ignore-case --after-context=2 "pattern" file.txt

Most command-line parsers follow these rules:

• Single-dash: Typically followed by single letters (-h, -v)
• Double-dash: For full words (--help, --version)

#!/bin/bash
# Example argument parsing
while [[ "$#" -gt 0 ]]; do
  case $1 in
    -v|--verbose) verbose=1 ;;
    -f|--file) shift; filename="$1" ;;
    *) echo "Unknown option: $1"; exit 1 ;;
  esac
  shift
done

Modern best practices:

• Use single-dash for common, frequently used options
• Use double-dash for less common, more descriptive options
• Consider implementing both forms for important options

# Good practice example
rsync -avz --progress --delete /source/ user@host:/dest/

Watch for:

• Commands using single-dash for arguments (find -name)
• Double-dash used as argument terminator (grep -- -v file searches for "-v")

# Using -- to stop option parsing
rm -- -filename-with-dash

In Unix/Linux systems, command line flags evolved from two distinct traditions. Single-dash flags (-h) originated from early Unix commands where single letters were used for options. Double-dash flags (--help) came later with GNU utilities, allowing more descriptive option names.

# Single dash examples (traditional)
ls -l -a -h
grep -i "pattern" file.txt
tar -xzvf archive.tar.gz

# Double dash examples (GNU style)
ls --all --human-readable
grep --ignore-case "pattern" file.txt
ssh --verbose user@host

Single-dash flags can often be combined:
ls -lah is equivalent to ls -l -a -h

Double-dash flags typically require full separation:
git commit --amend --no-edit cannot be combined

# Short options (single character) use single dash
find . -name "*.txt" -type f -mtime +7

# Long options (descriptive) use double dash
docker run --detach --name my_container image:tag

Some commands implement both styles for the same option:
grep --ignore-case and grep -i do the same thing

The double-dash by itself signals end of options:
rm -- -filename-with-dash

Certain commands break these conventions. For example, the ps command uses BSD-style options without dashes:
ps aux

Meanwhile, X11 applications often use single dash for long options:
xterm -bg black -fg white

• Use single-dash for short, common options you'll type frequently
• Use double-dash for more explicit, self-documenting commands in scripts
• Always check man pages for command-specific variations

# Example showing both styles together
ffmpeg -i input.mp4 --video-codec libx264 --crf 23 output.mp4