chmod Command Reference

The chmod (change mode) command is used to change the access permissions of files and directories in Unix and Linux systems. Permissions determine who can read, write, or execute a file or directory. This reference provides an overview of how to use chmod with both symbolic and numeric modes, along with examples to help you manage file permissions effectively.

Understanding File Permissions

Each file and directory has three types of permissions for three categories of users:

User (u): The owner of the file.
Group (g): Users who are members of the file's group.
Others (o): All other users.

The permissions are:

Read (r): Permission to read the file or list the directory contents.
Write (w): Permission to modify the file or directory.
Execute (x): Permission to execute the file or access the directory.

Symbolic Mode

In symbolic mode, you specify permissions using letters and symbols.

Syntax

chmod [references][operator][modes] file

References

u - User/Owner
g - Group
o - Others
a - All (user, group, and others)

Operators

+ - Adds the specified modes.
- - Removes the specified modes.
= - Sets the specified modes and removes others.

Modes

r - Read permission
w - Write permission
x - Execute permission
s - Set user or group ID on execution
t - Sticky bit

Examples

Command	Description
`chmod u+x file`	Add execute permission for the owner.
`chmod g-w file`	Remove write permission for the group.
`chmod o=r file`	Set read-only permission for others.
`chmod a+rw file`	Add read and write permissions for all.
`chmod u=rw,g=r,o= file`	Set read/write for owner, read for group, no permissions for others.
`chmod +x file`	Add execute permission for all (if no reference is specified, `a` is assumed).

Numeric (Octal) Mode

In numeric mode, permissions are represented by a three-digit octal number, with each digit corresponding to user, group, and others.

Permission Values

Octal Digit	Permission	Description
`0`	`---`	No permissions
`1`	`--x`	Execute only
`2`	`-w-`	Write only
`3`	`-wx`	Write and execute
`4`	`r--`	Read only
`5`	`r-x`	Read and execute
`6`	`rw-`	Read and write
`7`	`rwx`	Read, write, and execute

Syntax

chmod [mode] file

Examples

Command	Permissions	Description
`chmod 755 file`	`rwxr-xr-x`	Owner can read, write, execute; group and others can read and execute.
`chmod 644 file`	`rw-r--r--`	Owner can read and write; group and others can read.
`chmod 700 file`	`rwx------`	Owner can read, write, execute; group and others have no permissions.
`chmod 600 file`	`rw-------`	Owner can read and write; group and others have no permissions.
`chmod 666 file`	`rw-rw-rw-`	All can read and write.
`chmod 444 file`	`r--r--r--`	All can read only.

Special Permissions

Special permissions include the setuid, setgid, and sticky bits.

Setuid (Set User ID)

When set on an executable file, users executing the file will have the permissions of the file owner during execution.

# Set setuid bit
chmod u+s file

# Numeric mode (4XXX)
chmod 4755 file

Setgid (Set Group ID)

When set on a file, users executing the file will have the permissions of the file's group during execution. When set on a directory, new files created within inherit the directory's group.

# Set setgid bit
chmod g+s directory

# Numeric mode (2XXX)
chmod 2755 directory

Sticky Bit

When set on a directory, files within can only be deleted or renamed by their owner, the directory owner, or the root user.

# Set sticky bit
chmod +t directory

# Numeric mode (1XXX)
chmod 1755 directory

Combining Special Bits

Special bits are represented by an additional digit in the numeric mode:

Special Bit	Numeric Value
Setuid	`4`
Setgid	`2`
Sticky Bit	`1`

Example:

# Set setuid and setgid bits
chmod 6755 file  # (4+2)=6, then regular permissions

# Permissions: rwxr-sr-x

Changing Permissions Recursively

Use the -R option to change permissions of directories and their contents recursively.

Examples

# Set permissions recursively
chmod -R 755 /path/to/directory

# Add execute permission for all directories recursively
find /path/to/directory -type d -exec chmod 755 {} \;

View Current Permissions

Use ls -l to view file permissions:

ls -l file
-rw-r--r-- 1 user group  1234 Oct 12 12:34 file

The first 10 characters represent the file type and permissions:

- or d - File type (- for regular file, d for directory)
rwx - User permissions
rwx - Group permissions
rwx - Others permissions

Common Permission Settings

Permission	Numeric Mode	Description
`rwxr-xr-x`	`755`	Common for executable files and directories.
`rw-r--r--`	`644`	Default for text files; owner can read/write, others can read.
`rwx------`	`700`	Private files and directories; only owner has full access.
`rw-rw-r--`	`664`	Owner and group can read/write; others can read.
`rwsr-xr-x`	`4755`	Setuid executable; runs with owner's permissions.
`rwxr-sr-x`	`2755`	Setgid executable; runs with group's permissions.
`rwxrwxrwt`	`1777`	Sticky bit set on directory; e.g., `/tmp`.

Advanced Examples

Remove All Permissions for Others

chmod o= file

Give Execute Permission to User and Group

chmod ug+x file

Set Permissions Based on Another File

chmod --reference=source_file target_file

Using chmod with Find

Modify permissions for files matching certain criteria:

# Find all .sh files and make them executable
find /path -type f -name "*.sh" -exec chmod +x {} \;

# Change permissions of all directories to 755
find /path -type d -exec chmod 755 {} \;

# Change permissions of all files to 644
find /path -type f -exec chmod 644 {} \;

Security Considerations

Avoid setting permissions to 777 (full access for everyone) as it poses security risks.
Use the least privilege principle; grant only necessary permissions.
Be cautious when setting setuid and setgid bits; they can be exploited if not managed properly.
Regularly audit file permissions, especially in shared environments.

Summary of chmod Options

Option	Description
`-R`	Recursively change permissions of directories and their contents.
`-v`	Verbose output; show files as permissions are modified.
`-c`	Like verbose but reports only when changes are made.
`-f`	Suppress most error messages.
`--reference=FILE`	Set permissions to match those of FILE.

Additional Tips

Always check current permissions using ls -l before making changes.
Use symbolic links cautiously; changing permissions on a symbolic link does not affect the target file.
Consider using chown and chgrp to change ownership and group if needed.
Use umask to set default permission for new files and directories.

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