chmod

chmod is a command in Unix-like operating systems that changes the file mode bits of files and directories. The mode controls whether a file can be read, written, or executed, and it is defined separately for three classes of users: the owner (user), the group, and others. The execute bit is also used to allow entering directories or executing programs.

Modes can be specified either numerically (octal) or symbolically. An octal mode uses digits representing the permissions for user, group, and others, with each digit equal to the sum of read (4), write (2), and execute (1). Special bits can be included by using a leading digit: setuid (4), setgid (2), and the sticky bit (1). For example, 755 gives the owner full permissions and read/execute for group and others, while 4755 sets the setuid bit in addition to 755 permissions. A common variant is 1777 for a world-writable directory with the sticky bit. Symbolic modes use expressions like u+rw, g-w, o+x, or a+rX, and can be combined with the target file or directory.

Common usage and options include: chmod mode file... to change one or more items, and -R to apply recursively to directories. Other options such as -v (verbose), -c (changes only), and -f (suppress errors) are available on many systems. Examples: chmod 644 file.txt for read/write by owner and read by others; chmod u+x script.sh to allow the owner to execute; chmod -R 755 /path/to/dir to set permissions for a directory tree.

ACLs and other extended permissions may interact with the basic mode bits, providing additional or overriding access in some environments.