'\" t
.\"     Title: subuid
.\"    Author: [FIXME: author] [see http://docbook.sf.net/el/author]
.\" Generator: DocBook XSL Stylesheets v1.78.1 <http://docbook.sf.net/>
.\"      Date: 05/19/2026
.\"    Manual: File Formats and Conversions
.\"    Source: shadow-utils 4.6
.\"  Language: English
.\"
.TH "SUBUID" "5" "05/19/2026" "shadow\-utils 4\&.6" "File Formats and Conversions"
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.SH "NAME"
subuid \- the subordinate uid file
.SH "DESCRIPTION"
.PP
Each line in
/etc/subuid
contains a user name and a range of subordinate user ids that user is allowed to use\&. This is specified with three fields delimited by colons (\(lq:\(rq)\&. These fields are:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
login name or UID
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
numerical subordinate user ID
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
numerical subordinate user ID count
.RE
.PP
This file specifies the user IDs that ordinary users can use, with the
\fBnewuidmap\fR
command, to configure uid mapping in a user namespace\&.
.PP
Multiple ranges may be specified per user\&.
.PP
When large number of entries (10000\-100000 or more) are defined in
/etc/subuid, parsing performance penalty will become noticeable\&. In this case it is recommended to use UIDs instead of login names\&. Benchmarks have shown speed\-ups up to 20x\&.
.SH "FILES"
.PP
/etc/subuid
.RS 4
Per user subordinate user IDs\&.
.RE
.PP
/etc/subuid\-
.RS 4
Backup file for /etc/subuid\&.
.RE
.SH "SEE ALSO"
.PP
\fBlogin.defs\fR(5),
\fBnewgidmap\fR(1),
\fBnewuidmap\fR(1),
\fBnewusers\fR(1),
\fBsubgid\fR(5),
\fBuseradd\fR(8),
\fBuserdel\fR(8),
\fBusermod\fR(8),
\fBuser_namespaces\fR(7)\&.
