Linux Cheatsheet 02 — Files and Permissions

-rw-r--r-- 1 user group  1024 Jan 15 10:00 file
drwxr-xr-x 2 user group  4096 Jan 15 10:00 dir

r=4  w=2  x=1

owner  group  other
rwx    r-x    r--
7      5      4

chmod 755 file
chmod u+x file
chmod g-w file
chmod o=r file
chmod -R 644 dir/
chmod a+r file              # all = u+g+o
chmod u+rwx,g+rx,o-rwx file

chown user file
chown user:group file
chown -R user:group dir/
chown :group file           # group only

umask                       # default file creation mask
umask 022                   # files 644, dirs 755
umask 077                   # private (600, 700)

chmod u+s file              # setuid (run as owner)
chmod g+s dir               # setgid (new files inherit group)
chmod +t dir                # sticky (only owner can delete) /tmp uses this

chmod 4755 file             # setuid
chmod 2755 dir              # setgid
chmod 1777 dir              # sticky (like /tmp)

setfacl -m u:alice:rwx file
setfacl -m g:devs:rx file
setfacl -d -m u:alice:rwx dir       # default for new files in dir
setfacl -x u:alice file
getfacl file

chattr +i file              # immutable (can't modify or delete)
chattr -i file
chattr +a logfile           # append-only
lsattr file

(umask 077; touch private)

find /path -name "*.txt"
find . -iname "readme*"     # case-insensitive
find . -type f              # files only
find . -type d
find . -size +10M           # > 10MB
find . -size -1k
find . -mtime -7            # modified < 7 days
find . -atime +30           # accessed > 30 days
find . -newer file
find . -empty
find . -user alice
find . -group devs
find . -perm 0644
find . -perm /u+x           # any of these perms

find . -name "*.tmp" -delete
find . -name "*.log" -mtime +30 -exec rm {} \;
find . -type f -exec chmod 644 {} \;
find . -type d -exec chmod 755 {} \;

sudo updatedb
locate filename

ln -s /actual/path /link
readlink /link
realpath /link              # resolved

ln file copy                # same inode
stat file copy              # check inode

- regular file
d directory
l symlink
b block device
c character device
p pipe (FIFO)
s socket

diff file1 file2
diff -u file1 file2          # unified
diff -r dir1 dir2
cmp file1 file2              # binary

md5sum file
sha256sum file

cp -a src dst                # archive: -dR --preserve=all
cp -u src dst                # only if newer
cp -i src dst                # interactive
cp -l src dst                # hard link instead of copy
cp -s src dst                # symlink instead

rsync -a src/ dst/           # better than cp; resumable
rsync -avz src/ user@host:dst/
rsync -avz --delete src/ dst/  # mirror (deletes extras)

gzip file                    # → file.gz
gunzip file.gz
gzip -k file                 # keep original
gzip -9 file                 # max compression

xz file                      # better compression
bzip2 file
zstd file                    # fast + good

tar czf out.tar.gz dir/
tar cJf out.tar.xz dir/      # xz
tar c --zstd -f out.tar.zst dir/

du -h file
du -sh dir/                  # summary
du -sh */                    # each subdir
du -shc */                   # with grand total
du -h --max-depth=1 .

df -h                        # disk free
df -i                        # inodes
df -hT                       # type

ls /proc                     # processes by PID
cat /proc/cpuinfo
cat /proc/meminfo
cat /proc/loadavg
cat /proc/uptime
cat /proc/version

inotifywait -m -r dir        # watch for changes
inotifywait -e modify,create,delete file

nice -n 19 cmd               # low priority
ionice -c 3 cmd              # idle I/O priority