YUM(Yellowdog Updater Modified)是一個基於RPM(Redhat Package Manager紅帽包管理器)包的前端軟體管理工具，是C/S架構下的一種rpm的前端工具，YUM透過分析rpm header資料，自動處理依賴關係，從指定的repositories倉庫源一次性自動下載安裝所有依賴的rpm包，無須繁瑣地一次次下載、安裝。

repo就是repositories(倉庫)的縮寫，repositories其實就是存放rpm包的路徑，比如file://本地路徑 或ftp://伺服器路徑 或http://網站url路徑

我們要使用YUM，必須要先找到存放rpm包的Repository倉庫源，這些Repository倉庫源的資訊存放在目錄為/etc/yum.repos.d/下的副檔名為repo的配置檔案中，通常一個repo配置檔案定義了一個或者多個Repository倉庫源的細節內容，其中可能會多個repo配置檔案，每一個repo配置檔案中都可以配置一個或多個Repository，但是最終所有Repository倉庫源會被合併為一個交給系統(可以透過yum repolist all顯示出來)，所以多個repo配置檔案只是為了方便管理。repo配置檔案中的設定內容將被yum命令讀取和應用，這樣yum命令就從這些Repository倉庫源下載需要安裝或者升級的rpm包

檢視擁有的repo倉庫列表：yum repolist all
查詢軟體XX：yum info xx
搜尋軟體XX：yum search xx 
安裝軟體包xx.rpm：yum install xx.rpm
刪除軟體包xx.rpm：yum erase xx.rpm
升級軟體XX：yum upgrade xx或者yum update xx
區別：yum upgrade會刪除舊版本的package，而yum update則會保留(obsoletes=0)，建議使用yum update，防止舊版本包被刪除從而導致其他舊的軟體包依賴出現問題。

repo配置檔案內容的解釋，示例如下
```shell
[base]
name=CentOS-$releasever - Base
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
```
[base]:repo的id，必須是獨一無二的，不能和其他原始檔衝突，一旦衝突，可能這個repo id對應的repo name就不顯示了
name:repo的名稱
mirrorlist:為rpm包對應的一大堆路徑或baseurl，yum根據使用者客戶端的IP地址和mirrorlist中資訊來選取在地理位置上靠近客戶端的repo倉庫源，如mirrorlist不工作時可以使用baseurl。
baseurl:為rpm包對應的目錄或路徑，如果我們想把掛載在本地光碟/mnt為rpm路徑，可以使用baseurl來配置成baseurl=file:///mnt/Server ，如果我們想把阿里雲rpm倉庫為rpm路徑，可以使用baseurl來配置成baseurl=http://mirrors.aliyun.com/centos/6/os/x86_64
示例中，我們確定repo配置檔案中$releasever和$basearch變數的值的方法：使用cat命令開啟/etc/yum.conf檔案中看到一行資訊cachedir=/var/cache/yum/$basearch/$releasever，我們可以進入/var/cache/yum/資料夾下，發現 $basearch 其實就是 x86_64，$releasever就是7，然後代入mirrorlist中發現mirrorlist就是http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=os&infra=$infra 網頁，該網頁下面一堆的url，代入baseurl發現baseurl就是http://mirror.centos.org/centos/7/os/x86_64/
enabled=0/1：只有1和0兩個值，為1時表示repo可以獲取，0表示關閉，預設是1
gpgcheck=0/1：只有1和0兩個值，為1時表示進行gpg校驗，預設是1
gpgkey:數字證書公鑰檔案所在位置，使用者下載安裝RPM包時，可以透過這個公鑰來驗證RPM包是不是官方簽名的，示例中我們可以開啟/etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7檔案看看這個公鑰資訊

實驗1
```shell
[root@FRSPGSQLDEV3]# ll /etc/yum.repos.d/
total 8
-rw-r--r--. 1 root root 633 Feb  9 21:46 CentOS-Base.repo
-rw-r--r--. 1 root root 213 Feb  9 21:47 pgdg-redhat-all.repo

[root@FRSPGSQLDEV3]# cat /etc/yum.repos.d/CentOS-Base.repo
[base]
name=CentOS-$releasever - Base
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

[centosplus]
name=CentOS-$releasever - Plus
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

[root@FRSPGSQLDEV3]# cat /etc/yum.repos.d/pgdg-redhat-all.repo
[pgdg12]
name=PostgreSQL 12 $releasever - $basearch
baseurl=https://download.postgresql.org/pub/repos/yum/12/redhat/rhel-$releasever-$basearch
enabled=0
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-PGDG

[root@FRSPGSQLDEV3]# yum repolist all
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base: la.mirrors.clouvider.net
repo id                    repo name                       status
base/7/x86_64              CentOS-7 - Base                 enabled: 10,072
centosplus/7/x86_64        CentOS-7 - Plus                 disabled
pgdg12/7/x86_64            PostgreSQL 12 7 - x86_64        disabled
repolist: 10,072
```
結果：只顯示了/etc/yum.repos.d/CentOS-Base.repo中的repo id為的base有些，其他的repo id都是無效,因為其他的repo id對應的enabled=0

實驗2
pgdg-redhat-all.repo新增內容[centosplus]，如下,該內容和CentOS-Base.repo中[centosplus]一樣
```shell
[root@FRSPGSQLDEV3]# vim /etc/yum.repos.d/pgdg-redhat-all.repo
[root@FRSPGSQLDEV3]# cat /etc/yum.repos.d/pgdg-redhat-all.repo
[pgdg12]
name=PostgreSQL 12 $releasever - $basearch
baseurl=https://download.postgresql.org/pub/repos/yum/12/redhat/rhel-$releasever-$basearch
enabled=0
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-PGDG

[centosplus]
name=CentOS-$releasever - Plus
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

[root@FRSPGSQLDEV3]# yum repolist all
Loaded plugins: fastestmirror
Repository centosplus is listed more than once in the configuration
Loading mirror speeds from cached hostfile
 * base: la.mirrors.clouvider.net
repo id                    repo name                       status
base/7/x86_64              CentOS-7 - Base                 enabled: 10,072
centosplus/7/x86_64        CentOS-7 - Plus                 disabled
pgdg12/7/x86_64            PostgreSQL 12 7 - x86_64        disabled
repolist: 10,072
```
結果：只顯示了/etc/yum.repos.d/CentOS-Base.repo中的repo id為的base有些，其他的repo id都是無效,因為其他的repo id對應的enabled=0

實驗3
需要改實驗2中pgdg-redhat-all.repo內容,把centosplus修改成pgdg12
```shell
[root@FRSPGSQLDEV3]# vim /etc/yum.repos.d/pgdg-redhat-all.repo
[root@FRSPGSQLDEV3]# cat /etc/yum.repos.d/pgdg-redhat-all.repo
[pgdg12]
name=PostgreSQL 12 $releasever - $basearch
baseurl=https://download.postgresql.org/pub/repos/yum/12/redhat/rhel-$releasever-$basearch
enabled=0
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-PGDG

[pgdg12]
name=CentOS-$releasever - Plus
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

[root@FRSPGSQLDEV3]# yum repolist all
Loaded plugins: fastestmirror
Repository centosplus is listed more than once in the configuration
Loading mirror speeds from cached hostfile
 * base: la.mirrors.clouvider.net
repo id                    repo name                       status
base/7/x86_64              CentOS-7 - Base                 enabled: 10,072
centosplus/7/x86_64        CentOS-7 - Plus                 disabled
pgdg12/7/x86_64            PostgreSQL 12 7 - x86_64        disabled
repolist: 10,072
```
結果：只顯示了/etc/yum.repos.d/CentOS-Base.repo中的repo id為的base有些，其他的repo id都是無效,因為其他的repo id對應的enabled=0

實驗4
pgdg-redhat-all.repo新增內容[updates],如下
```shell
[root@FRSPGSQLDEV3]# vim /etc/yum.repos.d/pgdg-redhat-all.repo
[root@FRSPGSQLDEV3]# cat /etc/yum.repos.d/pgdg-redhat-all.repo
[pgdg12]
name=PostgreSQL 12 $releasever - $basearch
baseurl=https://download.postgresql.org/pub/repos/yum/12/redhat/rhel-$releasever-$basearch
enabled=0
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-PGDG

[pgdg12]
name=CentOS-$releasever - Plus
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

[updates]
name=CentOS-$releasever - Updates
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

[root@FRSPGSQLDEV3]# yum repolist all
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base: repos.lax.layerhost.com
 * updates: mirror.sfo12.us.leaseweb.net
repo id                    repo name                       status
base/7/x86_64              CentOS-7 - Base                 enabled: 10,072
centosplus/7/x86_64        CentOS-7 - Plus                 disabled
pgdg12/7/x86_64            PostgreSQL 12 7 - x86_64        disabled
updates/7/x86_64           CentOS-7 - Updates              enabled:  4,691
repolist: 14,763
```
結果：只顯示了/etc/yum.repos.d/CentOS-Base.repo中的repo id為的base有效和/etc/yum.repos.d/pgdg-redhat-all.repo中的updates有效，其他的repo id都是無效,因為其他的repo id對應的enabled=0

實驗5，修改pgdg-redhat-all.repo內容中的[pgdg12]中兩個enable=1
```shell
[root@FRSPGSQLDEV3]# vim /etc/yum.repos.d/pgdg-redhat-all.repo
[root@FRSPGSQLDEV3]# cat /etc/yum.repos.d/pgdg-redhat-all.repo
[pgdg12]
name=PostgreSQL 12 $releasever - $basearch
baseurl=https://download.postgresql.org/pub/repos/yum/12/redhat/rhel-$releasever-$basearch
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-PGDG

[pgdg12]
name=CentOS-$releasever - Plus
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

[updates]
name=CentOS-$releasever - Updates
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

[root@FRSPGSQLDEV3]# yum repolist all
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base: mirrors.ocf.berkeley.edu
 * pgdg12: repos.lax.layerhost.com
 * updates: mirrors.oit.uci.edu
repo id                          repo name                       status
base/7/x86_64                    CentOS-7 - Base                 enabled: 10,072
centosplus/7/x86_64              CentOS-7 - Plus                 disabled
pgdg12/7/x86_64                  CentOS-7 - Plus                 enabled:    255
updates/7/x86_64                 CentOS-7 - Updates              enabled:  4,691
repolist: 15,018
```
結果：
/etc/yum.repos.d/CentOS-Base.repo中的repo id為的base有效
/etc/yum.repos.d/pgdg-redhat-all.repo中的updates有效
/etc/yum.repos.d/pgdg-redhat-all.repo中的repo id為pgdg12但是repo name為CentOS-7 - Plus有效
/etc/yum.repos.d/pgdg-redhat-all.repo中的repo id為pgdg12但是repo name為PostgreSQL 12 7 - x86_64沒顯示
/etc/yum.repos.d/CentOS-Base.repo中的repo id為的centosplus無效，因為enable=0

實驗6，修改pgdg-redhat-all.repo內容中的,再增加一個repo id 為updates，但是repo name為name=CentOS-$releasever - Updates1
```shell
[root@FRSPGSQLDEV3]# vim /etc/yum.repos.d/pgdg-redhat-all.repo
[root@FRSPGSQLDEV3]# cat /etc/yum.repos.d/pgdg-redhat-all.repo
[pgdg12]
name=PostgreSQL 12 $releasever - $basearch
baseurl=https://download.postgresql.org/pub/repos/yum/12/redhat/rhel-$releasever-$basearch
enabled=0
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-PGDG

[pgdg12]
name=CentOS-$releasever - Plus
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

[updates]
name=CentOS-$releasever - Updates
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

[updates]
name=CentOS-$releasever - Updates1
mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra
#baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

[root@FRSPGSQLDEV3 yum.repos.d]# yum repolist all
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base: mirrors.ocf.berkeley.edu
 * pgdg12: centos.mirror.ndchost.com
 * updates: centos.mirror.ndchost.com
repo id                         repo name                        status
base/7/x86_64                   CentOS-7 - Base                  enabled: 10,072
centosplus/7/x86_64             CentOS-7 - Plus                  disabled
pgdg12/7/x86_64                 CentOS-7 - Plus                  enabled:    255
updates/7/x86_64                CentOS-7 - Updates1              enabled:  4,691
repolist: 15,018
```
結果：
/etc/yum.repos.d/CentOS-Base.repo中的repo id為的base有效
/etc/yum.repos.d/pgdg-redhat-all.repo中的repo id為updates但是repo name為CentOS-7 - Updates1的有效
/etc/yum.repos.d/pgdg-redhat-all.repo中的repo id為updates但是repo name為CentOS-7 - Updates的沒有顯示
/etc/yum.repos.d/pgdg-redhat-all.repo中的repo id為pgdg12但是repo name為CentOS-7 - Plus有效
/etc/yum.repos.d/pgdg-redhat-all.repo中的repo id為pgdg12但是repo name為PostgreSQL 12 7 - x86_64沒顯示
/etc/yum.repos.d/CentOS-Base.repo中的repo id為的centosplus無效，因為enable=0
結論：當兩個同樣的repo id的出現時，其中的一個可能會不顯示

Linux之Yum_rpm_repo詳解

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