chmod X (capital X)

Answer

A capital X in a chmod operation adds execute permission only to directories and to files that already have execute set for someone. Ordinary data files are left non-executable, which makes X the safe choice with chmod -R.

`chmod a+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	`644` `rw-r--r--`
`600`	file	`600` `rw-------`
`755`	directory	`755` `rwxr-xr-x`
`700`	directory	`711` `rwx--x--x`

What to know

chmod -R a+X dir makes every directory in the tree enterable without marking every data file executable: a 644 file stays 644, a 644 directory becomes 755, and a 700 script becomes 711.

The common recipe for a web or project tree is chmod -R u=rwX,go=rX dir. Directories end up 755, plain files 644, and files that were already executable 755.

Where it fits

Resetting a whole tree after an accidental chmod -R 777 or a bad archive extraction
Making a copied directory tree readable by other users (chmod -R go+rX dir)
Private trees: chmod -R u=rwX,go= dir gives directories 700 and files 600

Gotchas

X looks at whether a file has any execute bit at the moment it is applied. A data file that was wrongly made executable (for example by a previous 777) keeps execute; fix those separately with find dir -type f -exec chmod a-x {} + if needed.
X has no octal form. Numeric modes like chmod -R 755 cannot tell files and directories apart.
chmod +X with no who-letter is filtered by the umask, the same way as chmod +x. Write a+X to be explicit.

Related explainers

← chmod calculator