chmod

wikipedia:chmod command is used to change the access permissions including special mode flags (sticky) of files and directories

• 400 user read only
• 600 user read and write only

chmod examples

• chmod a+x /path/to/file.txt
• chmod og-r ~/.ssh/authorized_keys
• chmod og-rxw ~/.ssh/

chmod ug+s /path/to/file.txt

Before:
ls -la file.txt
-rwxr-xr-x   1 root         root    31 Nov 27 11:34 file.txt

stat file.txt
  File: file.txt
  Size: 31        	Blocks: 8          IO Block: 4096   regular file
Device: fd00h/64768d	Inode: 13          Links: 1
Access: (0755/-rwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2019-11-27 11:34:54.610196393 +0400
Modify: 2019-11-27 11:34:49.662193415 +0400
Change: 2019-11-27 11:34:49.666193418 +0400
 Birth: -

chmod ug+s file.txt 

After:
ls -la file.txt
-rwsr-sr-x 1 root root 31 Nov 27 11:34 file.txt 

stat file.txt
  File: file.txt
  Size: 31        	Blocks: 8          IO Block: 4096   regular file
Device: fd00h/64768d	Inode: 13          Links: 1
Access: (6755/-rwsr-sr-x)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2019-11-27 11:34:54.610196393 +0400
Modify: 2019-11-27 11:34:49.662193415 +0400
Change: 2019-11-27 12:11:20.637362248 +0400
 Birth: -

Related terms

• Sticky bit
• 0644
• 0600
• 0755

install -m

See also

• chmod: 400, 0600, 0644
• UID, GID, id, chown, chgrp, chmod, groups, sticky bit, stat, User ID, USER
• sudo, id, visudo, useradd, userdel, usermod, groups, passwd, chown, chmod, chgrp, groupadd, groupdel, Passwordless sudo, passwd (package), sudo --help
• file, File system, directory, touch, mkdir, ls, ln truncate, fallocate, split, stat, inode, File descriptor, superblock, block size (blockdev), fuser, lsof, scrub, chattr, ulimit (nofiles), cp, mv, file (command), mkfifo, chmod