部署服務器時 yum list 給我上了一課

今天新部署一臺服務器，將之前的項目copy一份過去，想要跑起來。在安裝composer後使用install時報錯

[root@ecs-1ada-0317666 yedan_school]# composer install
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - phpunit/phpunit 5.7.9 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.8 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.7 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.6 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.5 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.4 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.3 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.27 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.26 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.25 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.24 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.23 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.22 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.21 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.20 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.2 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.19 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.18 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.17 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.16 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.15 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.14 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.13 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.12 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.11 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.10 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.1 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - phpunit/phpunit 5.7.0 requires ext-dom * -> the requested PHP extension dom is missing from your system.
    - Installation request for phpunit/phpunit ~5.7 -> satisfiable by phpunit/phpunit[5.7.0, 5.7.1, 5.7.10, 5.7.11, 5.7.12, 5.7.13, 5.7.14, 5.7.15, 5.7.16, 5.7.17, 5.7.18, 5.7.19, 5.7.2, 5.7.20, 5.7.21, 5.7.22, 5.7.23, 5.7.24, 5.7.25, 5.7.26, 5.7.27, 5.7.3, 5.7.4, 5.7.5, 5.7.6, 5.7.7, 5.7.8, 5.7.9].

  To enable extensions, verify that they are enabled in your .ini files:
    - /etc/php.ini
    - /etc/php.d/bcmath.ini
    - /etc/php.d/bz2.ini
    - /etc/php.d/calendar.ini
    - /etc/php.d/ctype.ini
    - /etc/php.d/curl.ini
    - /etc/php.d/exif.ini
    - /etc/php.d/fileinfo.ini
    - /etc/php.d/ftp.ini
    - /etc/php.d/gd.ini
    - /etc/php.d/gettext.ini
    - /etc/php.d/gmp.ini
    - /etc/php.d/iconv.ini
    - /etc/php.d/json.ini
    - /etc/php.d/mbstring.ini
    - /etc/php.d/mysqli.ini
    - /etc/php.d/pdo.ini
    - /etc/php.d/pdo_mysql.ini
    - /etc/php.d/pdo_sqlite.ini
    - /etc/php.d/phar.ini
    - /etc/php.d/shmop.ini
    - /etc/php.d/simplexml.ini
    - /etc/php.d/sockets.ini
    - /etc/php.d/sqlite3.ini
    - /etc/php.d/tokenizer.ini
    - /etc/php.d/xml.ini
    - /etc/php.d/zip.ini
  You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

在網上找了問題所在，知道是composer包裏有擴展依賴與php擴展，然後這個PHP擴展（yum install php71w-xml）沒裝。（php版本7.1）

但是很不幸的是我自以爲對yum很瞭解，用了yum list | grep xml後發現有php71w-xml，以爲是我本地已經有這個擴展了，所以一直沒有管這個答案，一直找其他原因。後來才發現yum list的意思是列出所有可安裝的軟件包。真正列出本地已安裝的軟件包的其實是命令yum list installed。因此要牢記一下免得下次再犯同樣的錯誤！！

附（網址來源https://www.cnblogs.com/AganRun/p/8118339.html）：

1.使用YUM查找軟件包 
命令：yum search 
2.列出所有可安裝的軟件包 
命令：yum list 
3.列出所有可更新的軟件包 
命令：yum list updates 
4.列出所有已安裝的軟件包 
命令：yum list installed