Absolute Paths, Relative Paths, and Path Expansion

1. Pathnames identify objects through a directory hierarchy

A pathname is a sequence of components separated by /. The kernel resolves components through directories until it reaches the target object or an error. The object may be a regular file, directory, symbolic link, socket, device, or another filesystem type.

flowchart TD
  X["Typed path expression"] --> E["Shell expansions"]
  E --> A["Argument passed to command"]
  A --> B{"Begins with slash?"}
  B -->|"yes"| R["Start at root directory"]
  B -->|"no"| C["Start at process working directory"]
  R --> K["Kernel resolves components and links"]
  C --> K
  K --> O["Filesystem object or error"]

2. Absolute and relative paths

Absolute paths improve clarity but can reduce portability when a workspace moves. Relative paths improve portability inside a known project root but become dangerous when that root is not established. Robust automation combines a verified root with derived, quoted paths.

3. Special components: dot, dot-dot, and repeated slashes

4. The shell can transform a path before execution

Bash performs several expansion stages. The exact rules are detailed, but the operational model is essential:

workspace="$HOME/devops academy"

# One argument because the expansion is quoted.
printf '<%s>\n' "$workspace"

# Potentially multiple arguments because the expansion is unquoted.
# Do not use this form for path variables.
printf '<%s>\n' $workspace

# Tilde expansion occurs here:
printf 'home expression: %s\n' ~

# Quoted tilde is literal text, not a home expansion:
printf 'literal: %s\n' "~"

5. Quoting controls shell interpretation

The safest default is simple: quote every parameter expansion that should remain one argument. Exceptions such as intentional arrays, pattern expansion, and arithmetic should be explicit and reviewed.

6. Pathname expansion and wildcard patterns

lab="$HOME/devops-academy/linux/chapter03/lesson03/globs"
mkdir -p -- "$lab"
printf '%s\n' alpha beta > "$lab/app.log"
printf '%s\n' gamma > "$lab/worker.log"
printf '%s\n' hidden > "$lab/.audit.log"
printf '%s\n' text > "$lab/readme.txt"

cd -- "$lab" || exit 1
printf 'Unquoted pattern expands to matching non-hidden names:\n'
printf '  %s\n' *.log

printf 'Quoted pattern remains literal:\n'
printf '  %s\n' '*.log'

printf 'Explicit hidden-name pattern:\n'
printf '  %s\n' .*.log

By default, a leading dot must be matched explicitly. A pattern with no match may remain literal unless shell options such as nullglob or failglob change behavior. Scripts must define and verify the intended rule.

7. Canonicalization and target validation

realpath resolves components and symbolic links according to its options. readlink -f offers similar behavior on many GNU/Linux systems, but portability and missing-target behavior differ.

course_root="$HOME/devops-academy/linux"
requested="$course_root/chapter03/lesson03"

root_real=$(realpath -e -- "$course_root") || exit 1
target_real=$(realpath -m -- "$requested") || exit 1

printf 'root=%s\n' "$root_real"
printf 'target=%s\n' "$target_real"

case "$target_real" in
  "$root_real"|"$root_real"/*)
    printf 'Target is inside the approved course root.\n'
    ;;
  *)
    printf 'Refusing target outside course root: %s\n' "$target_real" >&2
    exit 1
    ;;
esac

8. Hands-on lab: observe expansion and resolution

lab="$HOME/devops-academy/linux/chapter03/lesson03"
root="$lab/path lab"
mkdir -p -- "$root/real/releases/v1" "$root/reports"
ln -sfn -- "real/releases/v1" "$root/current"
printf 'version=1\n' > "$root/real/releases/v1/app.env"

cd -- "$root" || exit 1
{
  printf '=== working directory ===\n'
  printf 'logical=%s\n' "$(pwd -L)"
  printf 'physical=%s\n' "$(pwd -P)"

  printf '\n=== path forms ===\n'
  printf 'absolute=%s\n' "$root/current/app.env"
  printf 'relative=%s\n' "current/app.env"
  printf 'canonical=%s\n' "$(realpath -e -- current/app.env)"

  printf '\n=== argument boundaries ===\n'
  printf 'quoted argument: <%s>\n' "$root"
  printf 'literal glob: <%s>\n' '*.env'

  printf '\n=== matches under current release ===\n'
  for file in current/*.env; do
    [ -e "$file" ] || continue
    printf '%s -> %s\n' "$file" "$(realpath -e -- "$file")"
  done
} > "$lab/path-report.txt"

less "$lab/path-report.txt"

9. Common path-handling mistakes

10. Knowledge check

11. Summary

Absolute paths begin at root; relative paths begin at a process’s current directory. Before the kernel resolves a pathname, Bash may expand tildes, variables, substitutions, and wildcard patterns. Quote path variables by default, canonicalize when physical location matters, and validate that resolved targets remain inside an approved root.

12. Further reading

• GNU Bash Reference Manual — shell expansions, quoting, filename expansion, and shell options.
• Linux manual pages for path_resolution, realpath, readlink, glob, and symlink.
• POSIX pathname resolution and Shell Command Language specifications.
• GNU Coreutils manuals for realpath, readlink, dirname, basename, and stat.
• Filesystem Hierarchy Standard — conventional absolute system paths.