Linux File Permissions And Chmod Patterns Practice Exam

Question 1 Linux File Permissions And Chmod Patterns

Change owner and group?

/etc/apparmor.d/; check status with aa-status. Owner: rwx (7).
Group: r-x (5).
Other: r-- (4).
Octal: 754. Private key: 600. Public key: 644. .ssh dir: 700. authorized_keys: 600. chown user:group path; add -R for recursive.

Question 2 Linux File Permissions And Chmod Patterns

Default umask on Ubuntu desktop?

Programs in /tmp can't be executed even if marked +x — defense against malware dropped to /tmp. 022. /etc/apparmor.d/; check status with aa-status. FAT32 — only stores 8.3 attributes; mount options simulate.

Question 3 Linux File Permissions And Chmod Patterns

Bind-mount a directory read-only?

/etc/apparmor.d/; check status with aa-status. mount --bind /src /dst, then mount -o remount,ro /dst. setfacl -m u:alice:r file restorecon -Rv path — re-applies labels defined by policy.

Question 4 Linux File Permissions And Chmod Patterns

Convert 644 to symbolic.

mount --bind /src /dst, then mount -o remount,ro /dst. install -m 600 src dst — copy with explicit mode, owner, group in one go. Execute bit not set — chmod +x script. rw-r--r-- — owner read+write, group read, others read.

Question 5 Linux File Permissions And Chmod Patterns

Convert 600 to symbolic.

No path is blocked by chmod itself if you own the file; mounted with restrictive options (like nosuid, noexec) restricts effective behavior. find dir -type d -exec chmod 755 {} +; find dir -type f -exec chmod 644 {} +; rw------- — typical for private files like ~/.ssh/id_ed25519. Owner is root (UID 0).

Question 6 Linux File Permissions And Chmod Patterns

Best command to audit world-writable files?

getfacl file find / -type f -perm -o+w -not -path '/proc/*' -not -path '/sys/*' getcap binary /etc/passwd: 644 (world-readable, just usernames/UIDs).
/etc/shadow: 640 root:shadow (password hashes).

Question 7 Linux File Permissions And Chmod Patterns

Newer alternative to ACLs for fine-grained access?

File should be 440 owned by root:root; edit via visudo to validate syntax. chattr +i file — even root can't delete/modify until chattr -i. POSIX capabilities for binaries; SELinux/AppArmor for system-wide MAC policies. RUID: who invoked the program.
EUID: identity used for permission checks (changes with setuid).

Question 8 Linux File Permissions And Chmod Patterns

Difference between effective user ID and real user ID?

Directories: 755 with owner the deployer, group www-data. Files: 644. Writable upload dirs: 775 or 770 owned by www-data. RUID: who invoked the program.
EUID: identity used for permission checks (changes with setuid). getfacl file Immutable attribute (chattr +i), SELinux/AppArmor, or filesystem mounted read-only.

Question 9 Linux File Permissions And Chmod Patterns

Set umask 077 — meaning?

Immutable attribute (chattr +i), SELinux/AppArmor, or filesystem mounted read-only. RUID: who invoked the program.
EUID: identity used for permission checks (changes with setuid). find . -type d -exec chmod 755 {} \; -o -type f -exec chmod 644 {} \; New files: 600; new dirs: 700 — strict, others get nothing.

Question 10 Linux File Permissions And Chmod Patterns

Numeric chmod for setuid+rwx for owner, rx for group/other?

SSH refuses to use a key in a directory or with permissions that are group/world accessible — security check. 4755. Requires root — for security. chown -R user:group path — note: changes ownership of symlinks themselves; use -h to operate on symlinks.

Question 11 Linux File Permissions And Chmod Patterns

Lock a sensitive log so it can only be appended?

getfacl -R / > acls.bak; restore with setfacl --restore=acls.bak. getfacl file chattr +a logfile — append-only; even root can't truncate without removing the attr. getcap binary

Question 12 Linux File Permissions And Chmod Patterns

Quick way to copy permissions from one file to another?

chmod -R a-w path — removes write for all; or set on parent and rely on umask for new files. getcap binary Files become 664 — useful when working in a shared group; group-write enabled. chmod --reference=src dst

Question 13 Linux File Permissions And Chmod Patterns

Permission for shell scripts in `~/.local/bin`?

Private key: 600. Public key: 644. .ssh dir: 700. authorized_keys: 600. Typically 755 or 700 if private. They must have execute for the user who runs them. No path is blocked by chmod itself if you own the file; mounted with restrictive options (like nosuid, noexec) restricts effective behavior. mount --bind /src /dst, then mount -o remount,ro /dst.

Question 14 Linux File Permissions And Chmod Patterns

Why must `~/.ssh` be 700?

chgrp -R www-data dir SSH refuses to use a key in a directory or with permissions that are group/world accessible — security check. setgid on directory: new files inherit group of dir.
sticky bit: only owner can delete files in dir (like /tmp). strace -e trace=chmod chmod 755 file — shows the syscall result and the underlying error code.

Question 15 Linux File Permissions And Chmod Patterns

AppArmor profile location?

getcap binary /etc/apparmor.d/; check status with aa-status. rwx------ — typical for ~/.ssh directory. find . -type d -exec chmod 755 {} \; -o -type f -exec chmod 644 {} \;

Question 16 Linux File Permissions And Chmod Patterns

Permission for `sudo` usage in `/etc/sudoers`?

Owner: rwx (7).
Group: r-x (5).
Other: r-- (4).
Octal: 754. setgid on directory: new files inherit group of dir.
sticky bit: only owner can delete files in dir (like /tmp). mount | grep ' ro,' or check /proc/mounts. File should be 440 owned by root:root; edit via visudo to validate syntax.

Question 17 Linux File Permissions And Chmod Patterns

File owned by user 0 means?

find / -perm -4000 -type f 2>/dev/null /etc/apparmor.d/; check status with aa-status. lsattr file Owner is root (UID 0).

Question 18 Linux File Permissions And Chmod Patterns

PASSWD file perms?

/etc/passwd: 644 (world-readable, just usernames/UIDs).
/etc/shadow: 640 root:shadow (password hashes). rwxr-xr-x — typical for executables and directories. setfacl -m u:alice:r,u:bob:rw,g:devops:rwx file Same POSIX rules apply; btrfs supports POSIX ACLs and xattrs; XFS supports POSIX ACLs and DMAPI.

Question 19 Linux File Permissions And Chmod Patterns

How to make a file immutable?

chmod --reference=src dst Either file is world-readable, or group is www-data and group has read, or ACL grants it, or POSIX.1e ACL/default ACL. chattr +i file — even root can't delete/modify until chattr -i. setfacl -m u:alice:r,u:bob:rw,g:devops:rwx file

Question 20 Linux File Permissions And Chmod Patterns

Why can root read but not delete a file owned by another user?

Root can do anything filesystem-wise (unless immutable). If delete fails, suspect chattr +i or a mount option (read-only). find dir -type d -exec chmod 755 {} +; find dir -type f -exec chmod 644 {} +; FAT32 — only stores 8.3 attributes; mount options simulate. chmod u+s /usr/bin/passwd — runs as file owner (root) regardless of caller. Use with care.

Question 21 Linux File Permissions And Chmod Patterns

What does `rwxr-xr--` mean?

setfacl -b file 022. Owner: rwx (7).
Group: r-x (5).
Other: r-- (4).
Octal: 754. SSH refuses to use a key in a directory or with permissions that are group/world accessible — security check.

Question 22 Linux File Permissions And Chmod Patterns

Remove all ACLs from a file?

setfacl -b file find / -perm -4000 -type f 2>/dev/null 666 & ~022 = 644; dirs: 777 & ~022 = 755. chmod g+s shared/

Question 23 Linux File Permissions And Chmod Patterns

Setuid bit example?

getcap binary chmod u+s /usr/bin/passwd — runs as file owner (root) regardless of caller. Use with care. Directories: 755 with owner the deployer, group www-data. Files: 644. Writable upload dirs: 775 or 770 owned by www-data. 1777 (rwxrwxrwt).

Question 24 Linux File Permissions And Chmod Patterns

Filesystem ownership beyond chown — set on btrfs / xfs?

getfacl file setgid on directory: new files inherit group of dir.
sticky bit: only owner can delete files in dir (like /tmp). chattr +i file — even root can't delete/modify until chattr -i. Same POSIX rules apply; btrfs supports POSIX ACLs and xattrs; XFS supports POSIX ACLs and DMAPI.

Question 25 Linux File Permissions And Chmod Patterns

Set new files to inherit group via setgid?

POSIX capabilities for binaries; SELinux/AppArmor for system-wide MAC policies. Either file is world-readable, or group is www-data and group has read, or ACL grants it, or POSIX.1e ACL/default ACL. Adds the execute bit for the file's owner only. chmod g+s shared/

Question 26 Linux File Permissions And Chmod Patterns

How to recursively set 755 on dirs and 644 on files?

stat -c '%a %U %G %n' file rwxr-xr-x — typical for executables and directories. find dir -type d -exec chmod 755 {} +; find dir -type f -exec chmod 644 {} +; find . -type d -exec chmod 755 {} \; -o -type f -exec chmod 644 {} \;

Question 27 Linux File Permissions And Chmod Patterns

Add multiple ACL entries at once?

Typically 755 or 700 if private. They must have execute for the user who runs them. Same POSIX rules apply; btrfs supports POSIX ACLs and xattrs; XFS supports POSIX ACLs and DMAPI. setfacl -m u:alice:r,u:bob:rw,g:devops:rwx file lsattr file

Question 28 Linux File Permissions And Chmod Patterns

Permission error 'Operation not permitted' but you own the file — likely cause?

stat -c '%a %U %G %n' file Immutable attribute (chattr +i), SELinux/AppArmor, or filesystem mounted read-only. rwx------ — typical for ~/.ssh directory. getfacl -R / > acls.bak; restore with setfacl --restore=acls.bak.

Question 29 Linux File Permissions And Chmod Patterns

View ACLs?

666 & ~022 = 644; dirs: 777 & ~022 = 755. 4755. getfacl file lsattr file

Question 30 Linux File Permissions And Chmod Patterns

Set group of a directory tree?

chgrp -R www-data dir RUID: who invoked the program.
EUID: identity used for permission checks (changes with setuid). POSIX capabilities for binaries; SELinux/AppArmor for system-wide MAC policies. find / -perm -4000 -type f 2>/dev/null

Question 31 Linux File Permissions And Chmod Patterns

View capabilities of a binary?

getcap binary install -m 600 src dst — copy with explicit mode, owner, group in one go. chattr +i file — even root can't delete/modify until chattr -i. chown user:group path; add -R for recursive.

Question 32 Linux File Permissions And Chmod Patterns

Filesystem that doesn't support POSIX ACLs?

chmod -R a-w path — removes write for all; or set on parent and rely on umask for new files. rw-r--r-- — owner read+write, group read, others read. chgrp -R www-data dir FAT32 — only stores 8.3 attributes; mount options simulate.

Question 33 Linux File Permissions And Chmod Patterns

Tell what's blocking a syscall when chmod fails?

strace -e trace=chmod chmod 755 file — shows the syscall result and the underlying error code. /etc/passwd: 644 (world-readable, just usernames/UIDs).
/etc/shadow: 640 root:shadow (password hashes). Requires root — for security. restorecon -Rv path — re-applies labels defined by policy.

Question 34 Linux File Permissions And Chmod Patterns

Find SUID binaries?

getfacl -R / > acls.bak; restore with setfacl --restore=acls.bak. SSH refuses to use a key in a directory or with permissions that are group/world accessible — security check. 022. find / -perm -4000 -type f 2>/dev/null

Question 35 Linux File Permissions And Chmod Patterns

Set permissions on a brand-new file at creation in shell?

Either file is world-readable, or group is www-data and group has read, or ACL grants it, or POSIX.1e ACL/default ACL. install -m 600 src dst — copy with explicit mode, owner, group in one go. 666 & ~022 = 644; dirs: 777 & ~022 = 755. setcap cap_net_bind_service=+ep /path/binary — grants specific capability without making whole binary setuid root.

Question 36 Linux File Permissions And Chmod Patterns

Why `./script` says Permission denied even when readable?

RUID: who invoked the program.
EUID: identity used for permission checks (changes with setuid). setfacl -m u:alice:r,u:bob:rw,g:devops:rwx file chattr +i file — even root can't delete/modify until chattr -i. Execute bit not set — chmod +x script.

Question 37 Linux File Permissions And Chmod Patterns

Why can normal users write to /tmp but only delete their own files?

rw-r--r-- — owner read+write, group read, others read. File should be 440 owned by root:root; edit via visudo to validate syntax. setfacl -m u:alice:r file Sticky bit prevents users from deleting/renaming files they don't own — even if the directory itself is world-writable.

Question 38 Linux File Permissions And Chmod Patterns

ls output column for permissions starts with d, l, -. What do they mean?

d: directory.
l: symlink.
-: regular file.
c/b: char/block device.
s/p: socket/pipe. chgrp -R www-data dir Owner: rwx (7).
Group: r-x (5).
Other: r-- (4).
Octal: 754. 4755.

Question 39 Linux File Permissions And Chmod Patterns

Reset a directory tree to standard perms?

4755. rwx------ — typical for ~/.ssh directory. Programs in /tmp can't be executed even if marked +x — defense against malware dropped to /tmp. find dir -type d -exec chmod 755 {} +; find dir -type f -exec chmod 644 {} +;

Question 40 Linux File Permissions And Chmod Patterns

View extended attributes?

getfacl file lsattr file rw------- — typical for private files like ~/.ssh/id_ed25519. chmod 700 file or chmod u=rwx,g=,o= file

Question 41 Linux File Permissions And Chmod Patterns

noexec on /tmp — effect?

4755. d: directory.
l: symlink.
-: regular file.
c/b: char/block device.
s/p: socket/pipe. Programs in /tmp can't be executed even if marked +x — defense against malware dropped to /tmp. chown -R user:group path — note: changes ownership of symlinks themselves; use -h to operate on symlinks.

Question 42 Linux File Permissions And Chmod Patterns

Effect of umask 002?

4755. setcap cap_net_bind_service=+ep /path/binary — grants specific capability without making whole binary setuid root. rwx------ — typical for ~/.ssh directory. Files become 664 — useful when working in a shared group; group-write enabled.

Question 43 Linux File Permissions And Chmod Patterns

Convert 755 to symbolic.

chattr +a logfile — append-only; even root can't truncate without removing the attr. chmod -R a-w path — removes write for all; or set on parent and rely on umask for new files. rwxr-xr-x — typical for executables and directories. chattr +i file — even root can't delete/modify until chattr -i.

Question 44 Linux File Permissions And Chmod Patterns

Convert 700 to symbolic.

SSH refuses to use a key in a directory or with permissions that are group/world accessible — security check. rwx------ — typical for ~/.ssh directory. Adds the execute bit for the file's owner only. rw-r--r-- — owner read+write, group read, others read.

Question 45 Linux File Permissions And Chmod Patterns

Default ACL on a directory?

chattr +i file — even root can't delete/modify until chattr -i. Private key: 600. Public key: 644. .ssh dir: 700. authorized_keys: 600. setfacl -d -m u:alice:rx dir — applied to new files created inside. Directories: 755 with owner the deployer, group www-data. Files: 644. Writable upload dirs: 775 or 770 owned by www-data.

Question 46 Linux File Permissions And Chmod Patterns

What does `chmod u+x file` do?

rwx------ — typical for ~/.ssh directory. d: directory.
l: symlink.
-: regular file.
c/b: char/block device.
s/p: socket/pipe. find dir -type d -exec chmod 755 {} +; find dir -type f -exec chmod 644 {} +; Adds the execute bit for the file's owner only.

Question 47 Linux File Permissions And Chmod Patterns

Quick perms for SSH key files?

Private key: 600. Public key: 644. .ssh dir: 700. authorized_keys: 600. find / -perm -4000 -type f 2>/dev/null Owner: rwx (7).
Group: r-x (5).
Other: r-- (4).
Octal: 754. Owner is root (UID 0).

Question 48 Linux File Permissions And Chmod Patterns

Shorthand to set rwx for owner only?

/etc/passwd: 644 (world-readable, just usernames/UIDs).
/etc/shadow: 640 root:shadow (password hashes). SSH refuses to use a key in a directory or with permissions that are group/world accessible — security check. chmod -R a-w path — removes write for all; or set on parent and rely on umask for new files. chmod 700 file or chmod u=rwx,g=,o= file

Question 49 Linux File Permissions And Chmod Patterns

How are symlink permissions interpreted?

Always 777 on the link; actual checks happen on the target. chmod u+s /usr/bin/passwd — runs as file owner (root) regardless of caller. Use with care. setfacl -b file 666 & ~022 = 644; dirs: 777 & ~022 = 755.

Question 50 Linux File Permissions And Chmod Patterns

Read-only a directory tree?

rw-r--r-- — owner read+write, group read, others read. Files become 664 — useful when working in a shared group; group-write enabled. chmod -R a-w path — removes write for all; or set on parent and rely on umask for new files. mount --bind /src /dst, then mount -o remount,ro /dst.