chmod +x

Answer

chmod +x file adds the execute bit so the file can be run directly, as ./file. With no u, g, o or a in front, GNU chmod adds execute for everyone except where your umask blocks it, so a 644 file usually becomes 755.

`chmod +x` on common starting modes

Computed with the umask at 022, the usual default. Checked against GNU chmod on real files.

Before	Type	After
`644`	file	`755` `rwxr-xr-x`
`600`	file	`711` `rwx--x--x`
`755`	directory	`755` `rwxr-xr-x`
`700`	directory	`711` `rwx--x--x`

What to know

When the clause has no who-letter, chmod behaves like a+x but leaves alone any bit that is set in the umask. With the common umask 022, execute is not masked, so all three classes get it: 644 becomes 755. With umask 077, only the owner gets it: 644 becomes 744.

To be explicit, name the class: chmod u+x (only you), chmod ug+x, or chmod a+x (everyone, regardless of umask).

Execute alone is not enough for a script to run: it also needs a first line such as #!/bin/sh or #!/usr/bin/env python3, or the shell will not know which interpreter to use.

Where it fits

Shell, Python or other scripts you want to run as ./script.sh
Downloaded binaries and installers (after verifying them)
Git hooks in .git/hooks/, which are ignored unless executable

Gotchas

"Permission denied" right after chmod +x often means the file is on a filesystem mounted noexec (common for /tmp and some home or USB mounts). Run it as sh script.sh or move it.
Git records the executable bit. On systems where it is not preserved, use git update-index --chmod=+x script.sh so the bit is committed.
The umask filter also applies to chmod -w and chmod =r with no who-letter. GNU chmod prints a warning when the result differs from what you typed.

Related explainers

← chmod calculator