The Tool Desk
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The three questions to answer for every path
When a process cannot open, create, delete or execute something, identify these facts before changing anything:
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- Who owns it? Every inode has a user owner and a group owner.
- What do the mode bits allow? Linux evaluates read, write and execute/search rights separately for the owner, group and everyone else.
- Is another control involved? An extended ACL, a special attribute, a read-only mount or a security policy can override what the nine familiar bits appear to say.
Ownership and permissions are different controls. chown changes the user owner and, optionally, group owner. chgrp changes only the group. chmod changes mode bits and special bits; it does not transfer ownership.
How to read ls -l output
A typical line such as -rw-r-----+ 1 alice web 1842 Oct 15 12:10 settings.conf can be read from left to right:
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- The first character identifies the file type. A hyphen is a regular file;
dis a directory; other characters identify objects such as symbolic links or devices. - The next nine characters are three groups of three: owner, group and other.
- The username and group name after the link count are the inode’s user and group owners.
- A trailing
+after the mode means extended ACL data is present on ACL-aware systems. Usegetfaclrather than assuming the nine mode characters tell the whole story.
Read the permissions in class order: owner, group, other. In -rw-r-----, the owner has read and write, the group has read, and everyone else has no rights. A capital letter such as S or T can indicate a special bit whose corresponding execute bit is not set.
What read, write and execute mean
Regular files
- Read (
r) permits obtaining file contents. - Write (
w) permits changing contents; it does not by itself permit deleting the directory entry that names the file. - Execute (
x) permits running the file as a program, subject to the file format and interpreter requirements.
Directories
- Read permits listing names in the directory.
- Write permits creating and removing directory entries, but practical use normally also requires execute/search.
- Execute/search permits traversing the directory and accessing an entry when its name is known.
Directory access is cumulative along the entire path. A service may have permission on /srv/app/config.yml but still fail if it lacks search permission on /srv or /srv/app. To inspect each component, use namei -l /srv/app/config.yml where available, or run ls -ld on each parent.
Converting chmod numbers into rights
Each classic permission class uses a three-bit value: read is 4, write is 2 and execute is 1. Add the values for owner, group and other to form a three-digit mode.
| Digit | Rights | Meaning |
|---|---|---|
| 0 | — | No rights |
| 1 | –x | Execute/search only |
| 2 | -w- | Write only |
| 3 | -wx | Write and execute/search |
| 4 | r– | Read only |
| 5 | r-x | Read and execute/search |
| 6 | rw- | Read and write |
| 7 | rwx | Read, write and execute/search |
For example, chmod 640 file sets owner read/write (6), group read (4), and no rights for other (0). chmod 750 program gives the owner full access, the group read and execute, and other execute only.
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A fourth, leading octal digit controls special bits:
| Digit | Bit | Typical use |
|---|---|---|
| 4 | setuid | An executable runs with the file owner’s effective user identity. |
| 2 | setgid | An executable runs with the file group’s effective group identity; on most directories, new entries inherit the directory’s group. |
| 1 | sticky | In a directory, users generally may remove or rename only entries they own (or an administrator may); commonly used on shared temporary directories. |
Thus chmod 2770 /srv/team sets setgid plus rwx for owner and group and no rights for other. A mode such as 1777 is appropriate only for a deliberately world-writable directory that also needs sticky deletion protection; using it on ordinary application data is unsafe.
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Use symbolic mode when you need to change one dimension while preserving unrelated bits. The form is who operator permissions, where u, g, o and a mean owner, group, other and all; operators are +, - and =.
chmod g+rX directoryadds group read and adds execute only to directories and files that already have an execute bit.chmod o-rwx fileremoves all rights for other.chmod u=rw,go= fileestablishes a private regular-file mode without changing ownership.chmod g-w configremoves group write while retaining any existing group read or execute bit.
Use an explicit octal mode when establishing a complete, known policy. Before applying either form, inspect a sample with ls -l and confirm that the path is the intended one.
Changing owners and groups
chown: user and group ownership
chown appuser:appgroup /srv/app/config.yml sets both owners in one operation. Omitting the colon and group changes only the user: chown appuser /srv/app/config.yml. Use a narrowly scoped path first, then verify with stat or ls -l.
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chgrp: group only
chgrp appgroup /srv/team/report.csv changes the group while leaving the user owner intact. This is useful when ownership is correct but collaboration should be granted through a different Unix group.
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Recursive changes
chown -R and chmod -R are high-impact operations. They can modify service files, nested directories and symbolic-link targets, and a recursive mode applied uniformly can remove required directory search bits or add execute bits to data files. Review the exact path, preview a representative listing, and prefer separate commands for directories and regular files when their policies differ.
Setgid and sticky directories for shared workspaces
A common team-directory design is a dedicated group, membership for every collaborating account, and a setgid directory such as 2770. The setgid bit causes newly created entries to inherit the directory’s group on most Linux filesystems, reducing group drift. It does not automatically make files group-writable; the creating process’s mode and umask still matter.
Verify group membership with the system’s account tools and have users start a new login session after membership changes where required. For a directory where users may create files but must not delete one another’s files, add the sticky bit, for example chmod 3770 shared when both setgid and sticky behavior are intentionally required.
How umask changes newly created objects
The process umask removes permission bits during creation. It affects system calls such as open and mkdir; it is not a repair command for existing files.
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For a regular file requested with mode 0666 and a 022 umask, the resulting mode is 0644: the owner can read and write, while group and other can read. Directories are commonly requested with execute bits (for example 0777), so the same umask commonly yields 0755. The creating program can request a different base mode, and a default ACL can further change the result.
Check the effective setting in the shell or service context with umask. A shell’s umask does not automatically describe a daemon launched by a service manager, container runtime or supervisor; inspect the environment that actually creates the object.
POSIX ACLs when three classes are not enough
Classic modes can express one owner, one owning group and other. POSIX ACLs add named-user and named-group entries and can define default entries on directories for inheritance. They are appropriate when one additional account needs access without placing every user in the owning group, or when a shared tree needs an inherited policy that mode bits alone cannot represent.
Use getfacl path to display access ACL entries and, for directories, default ACL entries. A mask entry limits the effective rights of named users (other than the owner), named groups and the owning-group entry. getfacl may show an effective-rights comment when the mask reduces what an entry appears to grant.
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For example, an ACL entry can name user backup with read access while leaving the file’s owner unchanged. Set ACLs deliberately with setfacl, then immediately run getfacl to verify both the entry and its mask. A + in ls -l is a signal to perform this check, not proof that a particular user is allowed.
| Criterion | Classic mode bits | POSIX ACLs |
|---|---|---|
| Principals | One owner, one owning group and other | Named users and groups in addition to the owner and owning group |
| Audit method | Compact output from standard ls, stat and chmod |
Requires ACL-aware inspection such as getfacl |
| Inheritance | Directory setgid handles group inheritance; mode bits alone do not provide named-user defaults | Directory default ACLs provide inherited named entries |
| Portability | Compact and broadly portable across Unix filesystems | Support and behavior vary by filesystem and Unix implementation |
| Common trap | Cannot express a second independent group policy | The ACL mask can silently limit effective rights |
A safe permission-change workflow
- Inspect first. Run
ls -l path,stat path, andnamei -l path(or inspect every parent directory). Ifls -lshows+, rungetfacl path. - Classify the failure. Decide whether the wrong account owns the object, the mode is too restrictive, a parent directory blocks traversal, or an ACL/special control is involved.
- Fix ownership only when ownership is wrong. Use a scoped
chown user:group pathorchgrp group path; do not use ownership changes as a substitute for a missing permission bit. - Choose the narrowest mode edit. Use symbolic
chmodto add or remove one right; use octal when you are intentionally defining all three classes and any special bit. - Account for creation policy. Check the creating process’s umask and any parent default ACL before assuming future files will match the current file.
- Verify as the service identity. Test the actual account, not only an administrator’s shell. Confirm traversal through every parent and inspect the resulting path with
statandgetfacl. - Apply recursion last. If a tree-wide change is unavoidable, review the path, test on a small subset, and separate directory and file rules where necessary.
Diagnosing a permission denied error
Work from the outside of the path inward:
- Parent traversal: every directory component needs search permission for the process’s relevant class.
- Owner and group selection: confirm the process’s effective user and supplementary groups; Linux does not simply add owner, group and other rights together.
- Mode bits: check the required operation—read, write, execute, directory listing, creation or deletion.
- ACL and mask: inspect access and default ACLs, including the mask that may reduce an apparently permissive entry.
- Special attributes: immutable or append-only attributes can block writes even when mode bits allow them.
- Filesystem and policy: a read-only mount, container boundary, mandatory security policy or other service restriction can deny access outside the classic permission model.
Do not “solve” an unexplained denial with chmod 777. That grants read, write and execute/search to everyone in the classic mode and still may not overcome an ACL mask, immutable attribute, read-only mount or security policy. It also expands the blast radius of a compromised process. Identify the failing layer and grant only the required right to the required principal.
Practical patterns
Private configuration readable by a service group
Assign the intended service account and group with chown service:app /etc/app/settings.conf, then use chmod 640 /etc/app/settings.conf. The owner can edit, the group can read, and other users have no mode-bit access. Verify that the daemon actually runs with membership in app.
Team directory with inherited group
Create or select a group, grant the directory to that group, and set chmod 2770 /srv/team. Check that every collaborator belongs to the group. If a named user outside that group needs inherited access, use a default ACL and verify its mask with getfacl.
One-time access for a backup account
Keep the file’s owner and normal group unchanged, add a named-user ACL for the backup account, and inspect the result with getfacl. Remove the entry when the job no longer needs it; otherwise future administrators may overlook the extra principal because the basic ls -l view only shows a plus sign.
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