🗂 Extended Filesystem Operations

Shell scripts frequently manipulate files and directories. This section covers safe, efficient filesystem operations.

🧭 Common Operations

Creating Directories

# Single directory
mkdir /path/to/dir

# Nested directories (no error if exists)
mkdir -p /path/to/nested/dir

# With permissions
mkdir -m 750 /path/to/secure

---

Copying Files

# Simple copy
cp source.txt dest.txt

# Recursive copy
cp -r dir1 dir2

# Preserve attributes
cp -a source dest   # Archive mode (recursive + preserve all)

# Interactive (prompt before overwrite)
cp -i source dest

# Verbose
cp -v source dest

---

Moving/Renaming

mv old_name new_name
mv file.txt /new/location/
mv -i old new   # Interactive

---

Removing

rm file.txt              # Single file
rm -r directory/         # Recursive delete
rm -rf directory/        # Force recursive (dangerous!)
rm -i file.txt           # Interactive prompt
rm -v file.txt           # Verbose

⚠️ Never use rm -rf with variables without validation!

---

🧪 Safe Deletion Patterns

Validate Before Delete

target="/path/to/delete"

if [ -z "$target" ]; then
    echo "Error: target is empty" >&2
    exit 1
fi

if [ "$target" = "/" ]; then
    echo "Error: cannot delete root" >&2
    exit 1
fi

rm -rf -- "$target"

The -- prevents options from being interpreted if filename starts with -.

---

Delete Only Matching Files

# Delete only .tmp files
find /tmp -name "*.tmp" -type f -delete

# Delete files older than 7 days
find /var/log -name "*.log" -mtime +7 -delete

# Delete empty directories
find /tmp -type d -empty -delete

---

🧠 Permissions and Ownership

Changing Permissions

chmod 755 script.sh        # rwxr-xr-x
chmod u+x,g-w,o-r file     # Add execute, remove write, remove read
chmod 644 file.txt         # rw-r--r--

Numeric format: | Digit | Owner | Group | Others | |-------|-------|-------|--------| | 7 | rwx | rwx | rwx | | 6 | rw- | rw- | rw- | | 5 | r-x | r-x | r-x | | 4 | r-- | r-- | r-- |

---

Changing Ownership

chown user:group file.txt
chown -R user:group directory/
chgrp group file.txt

---

🧪 Path Manipulation

Extract Components

path="/home/user/documents/file.txt"

dirname "$path"   # /home/user/documents
basename "$path"  # file.txt
basename "$path" .txt  # file (strip extension)

Real Paths

realpath ../relative/path   # /full/resolved/path
readlink -f symlink         # Follow symlinks

---

Building Paths Safely

# ❌ Unsafe (spaces break it)
path="$base/$filename"

# ✅ Safe
path="${base%/}/${filename#./}"   # Handle leading/trailing slashes

# Or use printf
path=$(printf '%s/%s' "$base" "$filename")

---

🧠 Temporary Files and Directories

Secure Temporary Files

# Create secure temp file
tmpfile=$(mktemp)
trap "rm -f $tmpfile" EXIT

echo "data" > "$tmpfile"
process "$tmpfile"
# Automatically cleaned up on exit

Temporary Directories

tmpdir=$(mktemp -d)
trap "rm -rf $tmpdir" EXIT

cd "$tmpdir"
# Do work...

---

🧪 File Testing

Test	Meaning	Example
-e	Exists	[ -e file.txt ]
-f	Regular file	[ -f file.txt ]
-d	Directory	[ -d /tmp ]
-r	Readable	[ -r file.txt ]
-w	Writable	[ -w file.txt ]
-x	Executable	[ -x script.sh ]
-s	Size > 0	[ -s file.txt ]
-L	Symbolic link	[ -L link ]
-nt	Newer than	[ f1 -nt f2 ]
-ot	Older than	[ f1 -ot f2 ]

---

🧾 Summary

• Always validate paths before operations.
• Use -- to prevent option injection.
• Prefer find -delete over rm -rf for safety.
• Use mktemp for temporary files.
• Trap cleanup to avoid leftover temp files.
• Test file properties before acting.

👉 Continue to: Text Processing