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Use chmod to change read, write, and execute permissions; chown to change a file’s user owner and optionally its group; and chgrp to change only the group. These are separate controls, so a Permission denied error can require an ownership fix, a permission fix, or both. Check the current state first, make the smallest change that solves the problem, and use recursive options only when you have verified the path.
How Linux permissions and ownership work
Every filesystem object has permission bits for three classes: the owner user, the owner group, and everyone else (often called others). Each class can have read (r), write (w), and execute (x) permission. The owner and group are separate metadata fields. Changing one does not automatically change the other.
For a regular file, read permits opening its contents, write permits modifying or truncating it, and execute permits running it as a program. A directory uses the bits differently: read allows listing names, write allows creating or removing entries, and execute allows traversing the directory and accessing entries when the rest of the path also permits it. A directory that is readable but not executable can still produce access errors.
Inspect a path before changing it:
ls -l /path/to/file
id
stat /path/to/fileThe first field from ls -l shows the permission bits, followed by the owning user and group. id shows your user ID and group memberships, which matters when you are trying to use a group you do not belong to.
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chmod, chown, and chgrp compared
| Command | Changes | Typical form | Who can use it |
|---|---|---|---|
chmod |
Read, write, and execute permission bits | chmod 644 file |
The owner or a privileged administrator can change mode bits |
chown |
User owner, and optionally group | chown user:group file |
Only root can change a file’s owner; changing the group is restricted to groups you are allowed to use |
chgrp |
Group owner only | chgrp group file |
A user can select a group they belong to; root can select other groups |
Use chmod when the correct user and group already own the path but access is too broad or too narrow. Use chown when a service, account, or project needs a different owner. Use chgrp when the user owner should remain unchanged and only the collaborating group needs to change.
Change permissions with chmod
Numeric (octal) mode
A numeric mode supplies one digit for the owner, group, and others, in that order. Read is represented by 4, write by 2, and execute by 1; add the values you want for each class. Thus, 6 means read plus write, 4 means read only, and 5 means read plus execute.
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chmod 600 secrets.env
chmod 755 deploy.sh644 gives the owner read/write and everyone else read-only access. 600 restricts a file to its owner. 755 makes the owner able to read, write, and execute while group and others can read and execute. Select modes according to the file’s purpose; do not use a convenient number without considering who should be able to modify it.
Symbolic mode
Symbolic mode identifies the class and operation explicitly. The class letters are u (user owner), g (group), o (others), and a (all three). The operators are + to add, - to remove, and = to set exactly the listed permissions.
chmod u+x script.sh
chmod go-w shared.txt
chmod a=r,u+w notes.txt
chmod u=rw,g=r,o= file.txtMultiple comma-separated clauses let you make a targeted change without rewriting unrelated bits. For example, chmod a=r,u+w notes.txt gives everyone read permission and then adds owner write permission, resulting in owner read/write and group/others read.
Recursive changes without making every file executable
-R applies a command below a directory. A blanket chmod -R 755 directory gives execute permission to regular documents, scripts, images, and data files alike. That is usually unnecessary and can create a security or operational problem.
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For a tree in which directories should be traversable and existing executable files should retain execution, use the GNU manual’s pattern:
chmod -R a=,+rwX project/The capital X adds execute permission only to directories and to files that already have an execute bit. Review the result with find or repeated ls -l before exposing the tree to other users or services.
Change the owner or group with chown
Set a user owner
sudo chown alice report.txtThe command changes the user owner and leaves the group as it was. Changing a file’s owner is a privileged operation: only root can do it. On systems where you administer through sudo, put sudo before the command and confirm the path carefully.
Set both user and group
sudo chown alice:developers report.txtThis sets the user owner to alice and the group owner to developers. Naming both fields is the clearest choice for deployments because it avoids depending on a user’s default login group.
Change only the group with chown
sudo chown :developers report.txtA leading colon with no user changes only the group, similar to chgrp developers report.txt. A user followed by a colon and no group has a different meaning on GNU systems: it sets the user owner and sets the group to that owner’s login group. If you need predictable cross-system behavior, specify the group explicitly.
Change only the group with chgrp
chgrp is useful when ownership by the current service account or individual must remain intact. Basic usage is:
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chgrp developers report.txt
sudo chgrp -R developers project/Non-root users can change a file’s group only to a group they belong to. The GNU form also supports a reference file:
chgrp --reference=template.txt new.txtThis copies the group ownership from template.txt to new.txt. Check the resulting group with ls -l; a successful command can still leave an application unable to access the file if directory traversal or permission bits are wrong.
Recursive operations and symbolic links
Recursive ownership changes are powerful and easy to over-apply. Before running chown -R, chgrp -R, or chmod -R, verify the directory with pwd, list its immediate contents, and use an absolute path when practical. A typo in a recursive path can alter a much larger tree than intended.
Symbolic links need special care. By default, chown changes the link’s target. To change the ownership of the symbolic-link object itself, use -h:
sudo chown -h alice:developers currentFor chgrp, the manual documents -R, -H, and --no-dereference. Use --no-dereference when the group change must apply to the link itself rather than the object it points to:
sudo chgrp --no-dereference developers currentDecide first whether the application follows the link or needs the link metadata changed. A link-only change does not repair permissions on its target.
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Shared directories: SGID and ACLs
Keep a shared group on new files
For a team directory, set the directory’s group and the SGID bit so newly created entries inherit the directory’s group:
sudo chown root:developers /srv/teamwork
sudo chmod 2775 /srv/teamworkThe leading 2 sets SGID on the directory; 775 gives the owner and group read/write/execute and gives others read/execute. The exact mode should match your policy. SGID helps maintain group ownership, but it does not by itself grant group write permission if the mode does not include it.
Grant access to named users with ACLs
Owner/group/other bits provide only three access classes. When one or more individual users need additional access without changing the primary owner or broadening access for everyone, use POSIX ACL tools:
setfacl -m u:alice:rwx /srv/teamwork
getfacl /srv/teamworkDefault ACLs can define permissions inherited by new entries in a shared project. ACLs add another layer to diagnose: inspect them with getfacl after checking ordinary mode bits and ownership.
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Ubuntu 25.10 and newer: verify which chown you are running
Ubuntu 25.10 and newer ship the Rust-based uutils implementation of chown by default. A tutorial testing Ubuntu 26.04 reported that chown USER: changed the owner but left the group unchanged, unlike the GNU behavior described above. This was a single-system report, so verify your release and implementation rather than assuming the result.
When both fields matter, avoid the ambiguous shorthand and name the group:
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sudo chown USER:GROUP fileThe GNU implementation remains available as gnuchown on the cited Ubuntu setup. Check your command’s documentation and test on a noncritical file before scripting behavior that depends on an omitted group.
Diagnose “Permission denied” methodically
1. Confirm the path and every directory in it
Read and write permission on the final file are not enough if a parent directory cannot be traversed. Check each directory component, especially when a service uses a home directory, mounted volume, or deployment path.
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2. Check user, group, and mode together
ls -ld /path /path/to /path/to/file
id serviceuser
stat /path/to/fileCompare the process user and its supplementary groups with the file’s owner and group. A file can show a seemingly permissive mode while still being inaccessible because a parent directory lacks execute permission or because the process is running as a different account.
3. Fix ownership before widening permissions
In web-server and container deployments, wrong ownership is frequently the underlying cause. If a service should own generated files, assign the correct service account and group with chown or chgrp, then set the narrowest mode that permits the required operation. Avoid solving an ownership mistake with world-writable permissions.
4. Account for symlinks and mounts
If the path is a symbolic link, determine whether your command changed the link or its target. If it crosses into a mounted filesystem, verify that the mounted object supports the Unix ownership and mode semantics you expect.
5. Re-test as the real user
Running a command with sudo proves only that root can access the path. Re-run the application action as the service account or log in as the intended user, then inspect the newly created file’s owner, group, and mode.
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Safe command patterns
| Goal | Command | What to verify |
|---|---|---|
| Private configuration file | chmod 600 config.env |
Only the intended owner needs to read it |
| Read-only document for a group | chmod 640 report.txt |
The group is correct and others do not need access |
| Make one script runnable | chmod u+x deploy.sh |
The script’s interpreter and directory traversal also work |
| Assign a service file | sudo chown app:app data.db |
The service account and group names are correct |
| Prepare a collaborative tree | sudo chown -R root:developers project/sudo chmod -R a=,+rwX project/ |
Review the tree; add SGID or ACLs if group inheritance or named-user access is required |
Common mistakes to avoid
- Confusing permission and ownership:
chmodcannot change the user owner, andchowncannot grant write permission that the mode denies. - Using
chmod -R 755indiscriminately: it marks every regular file executable. Prefer a targeted mode orX. - Assuming root is required for every change: owners can change mode bits, and users can select groups they belong to; only changing the user owner requires root.
- Forgetting directory execute permission: users need
xon each directory they must traverse. - Relying on an omitted group in scripts: use
USER:GROUP, particularly on Ubuntu 25.10 and newer where uutils behavior may differ. - Changing a symlink target unintentionally: use
chown -horchgrp --no-dereferencewhen the link itself is the object you intend to modify. - Making data world-writable: broad write access can allow unauthorized modification or replacement. Correct the owner, group, directory mode, or ACL instead.
A practical change procedure
- Identify the actual path: use
pwd, absolute paths, andls -ldto avoid acting on the wrong directory. - Record the current state: save the output of
ls -l,stat, and, where applicable,getfacl. - Identify the process user: check the account and groups with
id; do not assume your interactive login is the account that needs access. - Select one control: use
chmodfor bits,chownfor user ownership, andchgrpfor group ownership. - Apply the narrowest non-recursive change first: test it on one file or a staging directory.
- Handle inheritance deliberately: use SGID for a shared group and ACLs for named-user exceptions.
- Use recursion only after review: confirm the path, account for symlinks, and use
Xrather than making every regular file executable. - Test as the intended user or service: verify both the requested operation and the ownership/mode of anything it creates.
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