Shell Scripting — File & Directory Operations

# ── Create files ──────────────────────────────────────────
touch newfile.txt               # create empty or update timestamp
touch file{1,2,3}.txt           # create file1.txt file2.txt file3.txt
echo "content" > file.txt        # create with content
cat > file.txt << 'EOF'
multi-line
content here
EOF

# ── Read files ────────────────────────────────────────────
cat file.txt                    # print entire file
head -5 file.txt                # first 5 lines
tail -5 file.txt                # last 5 lines
content=$(cat file.txt)          # read into variable
first_line=$(head -1 file.txt)   # first line into variable

# ── Write / append ────────────────────────────────────────
echo "line" > file.txt           # overwrite
echo "line" >> file.txt          # append
printf "%s\n" "line" >> file.txt  # printf append (more control)

# ── Create directories ────────────────────────────────────
mkdir mydir                     # single directory
mkdir -p /deep/nested/path      # create all parents (no error if exists)
mkdir -p logs/{2026,2025}/{jan,feb,mar}  # brace expansion structure

# ── Copy ──────────────────────────────────────────────────
cp file.txt backup.txt          # copy file
cp -r srcdir/ destdir/          # copy directory recursively
cp -p file.txt backup.txt       # preserve permissions + timestamps
cp -a srcdir/ destdir/          # archive: recursive + preserve all
cp -u src dst                   # only copy if src is newer
cp -v file.txt backup.txt       # verbose — show what's happening
cp -i file.txt backup.txt       # interactive — prompt before overwrite

# ── Move / rename ─────────────────────────────────────────
mv old.txt new.txt              # rename
mv file.txt /backup/            # move to directory
mv -v file.txt /backup/         # verbose
mv -n file.txt /backup/         # no-clobber: don't overwrite

# ── Delete safely ─────────────────────────────────────────
rm file.txt                     # delete file
rm -f file.txt                  # force (no error if missing)
rm -r mydir/                    # delete directory recursively
rm -rf mydir/                   # force recursive (DANGEROUS — no undo!)
rmdir emptydir/                 # remove empty directory only

# ── Safe delete pattern — always check path is not empty ──
safe_rm() {
  local path="${1:?safe_rm: path required}"
  [[ -z "${path}" ]] && { echo "ERROR: empty path" >&2; return 1; }
  [[ "${path}" == "/" ]] && { echo "ERROR: refusing to delete /" >&2; return 1; }
  rm -rf "${path}"
}

# Never do: rm -rf "${var}/"   if $var could be empty!
# That becomes: rm -rf /

# ── Basic find ────────────────────────────────────────────
find /var/log -name "*.log"             # by name pattern
find /var/log -iname "*.LOG"           # case-insensitive
find /app -type f                       # files only
find /app -type d                       # directories only
find /app -type l                       # symlinks only

# ── By age ────────────────────────────────────────────────
find /tmp -mtime +7                     # modified > 7 days ago
find /tmp -mtime -1                     # modified within 1 day
find /var/log -mmin +60                 # modified > 60 minutes ago
find /backups -newer reference.txt      # newer than reference file

# ── By size ───────────────────────────────────────────────
find /var -size +100M                   # > 100 MB
find /tmp -size 0                       # empty files
find /logs -size +1G -size -5G         # between 1G and 5G

# ── With actions ──────────────────────────────────────────
find /tmp -name "*.tmp" -delete        # delete in one step
find /logs -mtime +30 -exec gzip {} \; # compress old logs
find /app -name "*.sh" -exec chmod +x {} \;  # make scripts executable
find /var/log -mtime +7 -print0 | xargs -0 rm -f  # safe with spaces

# ── Combine conditions ────────────────────────────────────
find /var/log -name "*.log" -size +10M -mtime +3
find /app \( -name "*.pyc" -o -name "__pycache__" \) -delete

# ── Permissions ───────────────────────────────────────────
chmod 755 script.sh              # rwxr-xr-x
chmod 640 config.env             # rw-r----- (owner rw, group r)
chmod +x script.sh               # add execute for all
chmod go-w file.txt              # remove write from group+others
chmod -R 755 /app/               # recursive
chown vriddh:www-data file.txt   # change owner:group
chown -R app:app /opt/myapp/     # recursive ownership

# ── stat — file metadata ──────────────────────────────────
stat file.txt                    # full metadata
stat -c "%s" file.txt            # file size in bytes
stat -c "%U:%G" file.txt         # owner:group
stat -c "%a" file.txt            # octal permissions
stat -c "%y" file.txt            # last modified time
stat -c "%i" file.txt            # inode number

# ── mktemp — safe temporary files ────────────────────────
tmpfile=$(mktemp)                # /tmp/tmp.XXXXXXXXXX
tmpdir=$(mktemp -d)              # temporary directory
tmpfile=$(mktemp /tmp/myapp.XXXXXX)  # custom prefix/suffix

# Always clean up temp files on exit
cleanup() { rm -f "${tmpfile}"; rm -rf "${tmpdir}"; }
trap cleanup EXIT INT TERM

# ── rsync basics ──────────────────────────────────────────
rsync -av src/ dest/             # archive + verbose
rsync -av --delete src/ dest/   # exact mirror (delete extras)
rsync -av --dry-run src/ dest/  # preview without copying
rsync -avz src/ remote:/dest/   # compress over network
rsync -avz --progress src/ remote:/dest/  # with progress bar

# ── Exclude patterns ──────────────────────────────────────
rsync -av --exclude='*.log' --exclude='.git' src/ dest/
rsync -av --exclude-from='.rsync-exclude' src/ dest/

# ── Backup script using rsync ─────────────────────────────
BACKUP_SRC="/app/data"
BACKUP_DST="backup@bkp-01:/backups/$(date +%Y/%m/%d)"

rsync -avz \
  --delete \
  --exclude='*.tmp' \
  --exclude='cache/' \
  --log-file=/var/log/rsync.log \
  "${BACKUP_SRC}/" \
  "${BACKUP_DST}/"

echo "Backup exit code: $?"