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In a Unix-like system, 600, 644 and 755 are numeric permission modes: each digit sets read, write and execute/search access for the file’s owner, its group and everyone else. 600 keeps access with the owner, 644 lets others read, and 755 also lets the group and others execute a file or search a directory. Choose a mode based on what the object is and who needs to use it—not because a number is a universal security label.
How to read a three-digit permission mode
The digits always correspond to three user classes, in order: owner, group and others. Within each digit, read is worth 4, write is worth 2, and execute is worth 1. Add the values for the permissions you want that class to have. GNU Coreutils documents this numeric structure in its file-permissions guide.
7= 4 + 2 + 1: read, write and execute/search.6= 4 + 2: read and write.5= 4 + 1: read and execute/search.4= read only.0= no permissions.
For example, 644 breaks down as owner 6 (read and write), group 4 (read), and others 4 (read). The number is a compact encoding of permissions, not a built-in classification such as “secure” or “public.”
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| Mode | Owner | Group | Others | Typical interpretation |
|---|---|---|---|---|
600 |
Read, write | None | None | Owner-only access to a regular file |
644 |
Read, write | Read | Read | Owner can edit; group and others can read |
755 |
Read, write, execute | Read, execute | Read, execute | An executable file or a directory that others should be able to traverse |
These are decoding examples, not universal recommendations. Use the least access that still allows the intended work. In particular, 755 grants execute/search to group and others, which is often unnecessary for an ordinary data file.
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Why directory permissions work differently
On a regular file, read allows access to its contents, write allows changes, and execute allows it to run as a program. On a directory, the same bits have directory-specific effects: read allows listing names, write allows creating or removing entries, and execute means search—accessing an entry by name and traversing the directory path. Execute on a directory does not mean running the directory as a program.
That distinction matters when selecting a mode. A directory may need execute/search permission for users to reach files inside it, even if those users should not run any program. A text file may need read access without execute permission. Access also depends on the relevant parent directories and any special permission bits.
Why Linux permissions use numbers
Read, write and execute are represented as three bits, with values 4, 2 and 1. Because their possible combinations add up to a single octal digit, each user class can be represented compactly by one digit. Three such digits express owner, group and others—for instance, 600 or 755. Symbolic modes can express the same choices more readably: u=rw,go=r means the owner can read and write while the group and others can read.
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Start with the target and the access people or processes need. A mode should not give execute/search or write access merely because another file uses the same number.
- Identify the object. Decide whether it is a regular file or a directory; execute has a different practical meaning for each.
- Identify the user classes. Determine whether access is needed by the owner, members of the file’s group, or other users.
- Choose only the required operations. Use read, write and execute/search bits that match the job.
- Check the path and target before changing permissions. A numeric mode sets the ordinary permission bits on the named target. For example,
chmod 600 filenamesets owner read/write and removes group and other access;chmod 644 filenameadds read access for group and others. GNU Coreutils gives further chmod usage and behavior details.
Only the file’s owner or a suitably privileged process can change its mode. On typical systems, invoking chmod on a command-line symlink changes the target’s permissions rather than the symlink’s; symlink permission handling is system-specific and often ignored.
Why a new file may not be 644 or 755
Creation permissions depend on the mode requested by the creating program and the process’s umask, which removes permissions from that request. A default ACL can restrict the resulting permissions further. The umask varies by system and environment, so there is no universal creation default; inspect the actual mode on the system you are using. GNU explains the interaction in its guide to umask and protection.
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Special bits and limits of ordinary modes
The three-digit modes here describe ordinary owner, group and other permissions. An additional leading octal digit can represent set-user-ID, set-group-ID and the sticky bit, with values 4, 2 and 1 respectively. These bits change behavior and should not be added casually. On directories, setgid commonly makes new entries inherit the directory’s group; sticky behavior on a shared writable directory limits who can remove or rename entries. Exact behavior can vary by platform. GNU documents the mode structure and notes implementation-specific details.
Permission bits are only one layer of access control. They do not, by themselves, guarantee secrecy or safety against privileged users, ACLs, filesystem attributes, accessible parent directories or application-level controls.
Use care with recursive changes
A recursive permission change affects many targets, so verify the path and intended result before applying it. GNU documents symlink traversal options for recursive chmod and warns that following symlinks during recursive traversal can create a security risk if an attacker can introduce a symlink during the operation. GNU also documents differences between chmod 755 dir and chmod =755 dir involving set-ID bits; do not assume every numeric form has identical special-bit behavior on every platform.
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