What Is chmod? Changing File Permissions

chmod (change mode) is the Unix command that modifies a file's permission bits, controlling who can read, write, or execute it.

chmod is how you change file permissions on Unix. You might make a script runnable, lock a secrets file down, or open a directory up. In CI, the most common use is chmod +x to add the execute bit to a script so it can be run directly, fixing a frequent "permission denied" error.

What chmod does

chmod sets the read, write, and execute bits on a file for the owner, group, and others. It accepts either symbolic notation (like +x) or numeric octal notation (like 755).

Symbolic notation

chmod +x script.sh adds the execute bit for all classes.
chmod u+w file adds write for the owner only.
chmod go-r secret removes read for group and others.

Numeric notation

Numeric mode sets all bits at once: chmod 755 script.sh gives rwx to the owner and r-x to group and others, while chmod 600 key restricts a private key to owner read and write only.

Recursive changes

Adding -R applies chmod to a directory tree. Use it carefully: chmod -R 777 on a project is a security smell, granting everyone full access to every file.

chmod in CI

When a step runs ./build.sh and hits "permission denied," adding chmod +x build.sh first usually fixes it. Better still, commit the file with the execute bit set so the chmod is unnecessary.

Fixing permissions on managed runners

On Latchkey runners, chmod +x reliably makes a script executable within a step. For sensitive files like keys written during a job, use restrictive modes such as 600 to limit access.

Key takeaways

chmod changes a file's read, write, and execute permission bits.
It accepts symbolic (+x) or numeric (755) notation.
In CI, chmod +x is the common fix for a non-executable script.