chmod

 linux chmod

change file modes or Access Control Lists (ACL)

chmod [-fv ] [-R [-H|-L|-P] mode file …
chmod [-fv ] [-R [-H|-L|-P] [-a|+a|=a] ACL file …
chmod [-fhv] [-R [-H|-L|-P] [-E] file …
chmod [-fhv] [-R [-H|-L|-P] [-C] file …
chmod [-fhv] [-R [-H|-L|-P] [-N] file …

Modifies the file mode bits and Access Control Lists (ACLs)

-f force (?)Do not display failure message.
-R Recursively change the modes of the file hierarchies rooted in files .

-H, -L and -P are ignored without -R , override each other and the results are the last specified.
•Symbolic link behavor ( with -R)
-H links on the command line are followed.
links encountered in the tree traversal are not followed by default.
-L Links are followed .
-P links are NOT followed.Default.
-h change the mode of the link itself rather than the target .
-v
-v -v		 verbose showing filenames as the mode is modified.
Specified more than once, the old and new modes of the file will also be displayed, in both octal and symbolic notation. 
chmod -v -v a+r shop.txt
shop.txt:0100700 [-rwx------ ] -> 0100744 [-rwxr--r-- ]

Only the owner or the super-user is permitted to change the mode.

DIAGNOSTICS

Exits with return code: 0 on success
1 if target is inaccessible or syntax is incorrect.

Modes

Absolute which is the octal sum of:
4000 set-user-ID-on-execution.
Executable with this bit set will run with effective uid set to the uid of the file owner.
Directories with the set-user-id bit set will force all files and sub-directories created in them to be owned by the directory owner and not by the uid of the creating process, if the underlying file system supports this feature: see chmod(2) and the suiddir option to mount(8).
2000 set-group-ID-on-execution.
Executable with this bit set will run with effective gid set to the gid of the file owner.
1000 sticky bit See chmod(2) and sticky(8).
0400 read by owner.
0200 write by owner.
0100 execution by owner, for files.
allow search by owner , for directories.
0040 read by group members.
0020 write by group members.
0010 execution by group members.
Search group members in the directory.
0004 read by others.
0002 write by others.
0001 execution by others.
others search in the directory.

For example, the absolute mode that permits read, write and execute by the owner, read and execute by group members, read and execute by others, and no set-uid or set-gid behaviour is 755 (400+200+100+040+010+004+001>).

Symbolic
    mode ::= clause[,clause…]
    clause ::= [who…][action …]
    who ::= a|u|g|o|␀
    action ::= op[permission…]
    op ::= +|-| =
    permission ::= r|w|x|X|s|t

Where who is user, group, other (not owner) and all (equivalent to ugo).

The permission flags:
r read
w write
x executable for file ; searchable for directory
X eXecute/search if the file is a directory or any of the execute/search flags are set in the original (unmodified) mode.
X is only meaningful with +
u user
g group
o others not owner!
null all who not prohibited by umask
s set-user-ID-on-execution and set-group-ID-on-execution bits.
t sticky bit. 

ls -l 
    -rw-------  1   5595 Feb 22 12:31 .bash_history 
    -rwxr-----  1     27 Dec 15 16:31 .profile
+ If no value is supplied for who, permission is set.
If no value is supplied for permission + has no effect.
Otherwise, mode for who and permision are set.
- If no value is supplied for who
Otherwise, the mode specified who and perm values are cleared.
= mode bits specified by who are cleared
If no who is specified the user, group and other mode bits are cleared. if no who supplied for each permission bit specified in perm, for which the corresponding bit in the file mode creation mask is clear, is set.
Otherwise, the mode bits represented by the specified who and permssion values are set.
Each clause specifies one or more operations to be performed on the mode bits, and each operation is applied to the mode bits in the order specified.

Operations of other only with s or t, are ignored.

Examples of Valid Modes

644 make a file readable by anyone and writable by the user(owner) only.
go-w deny write permission to group and others.
=rw,+X set the read and write permissions to the usual defaults, but retain any execute permissions that are currently set.
+X make a directory or file searchable/executable by everyone if it is already searchable/executable by anyone.
755
u=rwx,go=rx
u=rwx,go=u-w 		make a file readable/executable by everyone and writable by the user only.
go= clear all mode bits for group and others.
g=u-w set the group bits equal to the user bits, but clear the group write bit.

Errors

chmod: Unable to change file mode on com.teamviewer.teamviewer.plist: Operation not permitted
File may be owned by root and System Integrity Protection is enabled.

Access Control List

The Access Control List is an extension to symbolic mode.

A file or directory may have an ACL, containing an ordered list of entries.
Each entry refers to a user or group, and allows or denies a set of actions.

  • ACL ::= group|user allow|deny action

    Actions

    For all filesystem objects:
    delete Deletion of an object may be allowed|denied by either this permission or
    the delete_child right on the containing directory.
    readattr Implicitly allowed if the object can be looked up and not explicitly denied.
    readextattr
    writeattr
    writeextattr
    readsecurity
    writesecurity
    chown Change ownership (root only)
    Operations applicable to directories:
    list
    search Look up files by name.
    add_file
    add_subdirectory
    delete_child Delete a contained object. See the file delete permission.
    for non-directory objects:
    read
    write
    append
    execute
    Inheritance is controlled on directories with
    file_inherit Inherit to files.
    directory_inherit Inherit to directories.
    limit_inherit Clears directory_inherit in the entry that is inherited, preventing further nested subdirectories from also inheriting the entry. only relevant to entries inherited by subdirectories
    only_inherit The entry is inherited by created items but not considered when processing the ACL.

    If the user and group are the same (example mail mail) , prefix the user/group with "user:" or "group:"

    +a ACL Adds into the canonical location.
          local deny, local allow, inherited deny, inherited allow
    By default, chmod adds entries to the top of the local deny and local allow .

    Entries refering to an existing user/group are combined.

    Examples 

    > /bin/chmod +a "everyone deny delete" sacredFile      # ( root can still delete!)

    ls -l  displays + when ACL entries are present.
    ls -le displays the ACL entries.

    drwx------+ 108 dgerman  staff  3672 Feb  9 20:09 Documents
 0: group:everyone deny delete

 ls -dle Documents    # Can not click on finder 
drwxr-xr-x+ 31 dgerman  staff  992 Jun 23 14:14 Documents/
 0: group:admin allow list,search,readattr,readextattr,readsecurity 

       > ls -dle Documents     #list the directory, long format, including ACL
  drwx------+ 108   3672 Feb  9 20:09 Documents
    0: group:everyone deny delete


  > ls -le     # file1 initially has no ACL
  -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1

  > chmod +a "admin allow write" file1
  # ls -le
  -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
    owner: juser
    1: admin allow write

  > chmod +a "guest deny read" file1   # Add in the proper order
  > ls -le
  -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
    owner: juser
    1: guest deny read
    2: admin allow write

  > chmod +a "admin allow delete" file1       # Add delete to existing  admin entry
  > ls -le
  -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
    owner: juser
    1: guest deny read
    2: admin allow write,delete
    +a# i
        ^-index     		Adds entry at a specific location

    Examples 

    > ls -le
-rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
owner: juser
1: guest deny read
2: admin allow write
> chmod +a# 2 "others deny read" file1
> ls -le
-rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
owner: juser
1: guest deny read
2: others deny read
3: admin allow write
    +ai user permission access Adds Inherited entries

    Examples 

    > ls -le
-rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
owner: juser
1: guest deny read
2: admin allow write,delete
3: juser inherited deny delete
4: admin inherited allow delete
5: backup inherited deny read
6: admin inherited allow write-security

> chmod +ai "others allow read" file1

> ls -le
-rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
owner: juser
1: guest deny read
2: admin allow write,delete
3: juser inherited deny delete
4: others inherited allow read
5: admin inherited allow delete
6: backup inherited deny read
7: admin inherited allow write-security
    +ai# i
              ^-index     		Add inherited entries at a specific location.
    These modes allow non-canonical ACL ordering to be constructed.
    -a user premission actionREMOVES specified permission from user Example:
     # ls -le
 -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
 owner: juser
 1: admin allow write,delete
 # chmod -a "admin allow write" file1
 # ls -le
 -rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
 owner: juser
    -a# i user premission action
        ^-index     		REMOVES entries at a specific location.
    If the entry lists a subset of rights allowed by an entry, only the rights listed are removed.
    Inheritance is not considered .

    Examples 

    > ls -le
-rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
owner: juser
1: guest deny read
2: admin allow write,delete
> chmod -a# 1 file1 # remove entry 1 ,i.e guest deny read
> ls -le
-rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
owner: juser
1: admin allow write,delete
> chmod -a "admin allow write" file1
> ls -le
-rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
owner: juser
1: admin allow delete
    =a# i
        ^-index    		 Set Individual entries at a specific location, don't add any.
    Examples 
    > ls -le
-rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
owner: juser
1: admin allow delete
> chmod =a# 1 "admin allow write,chown"
> ls -le
-rw-r--r--+ 1 juser  wheel  0 Apr 28 14:06 file1
owner: juser
1: admin allow write,chown
    Replace
    -E Existing ACL is replaced with lines of ACL information from stdin.
    Remove
    -i file [… Removes inherited flag from all entries.
    -I file [… Removes inherited entries .
    -N file [… Removes the ACL
     
    -C file [… Checks if any of files have ACLs in non-canonical order.Returns false

    Errors

    chmod: No ACL currently associated with file 'xxxxx'

    • Compatibility

    -v is non-standard
    t is not included in IEEE Std 1003.2 (POSIX.2)

    SEE ALSO

    chflags(1), fsaclctl(1), install(1), chmod(2), stat(2), umask(2), fts(3), setmode(3), symlink(7), chown(8), mount(8), sticky(8)