composer update foo/bar

Warning: The lock file is not up to date with the latest changes in composer.json, you may be getting outdated dependencies, run update to update them.

$ composer update nothing
Loading composer repositories with package information
Updating dependencies
Nothing to install or update
Writing lock file
Generating autoload files

composer update --lock

composer require "foo/bar:1.0.0"

$ composer init --require=foo/bar:1.0.0 -n
$ cat composer.json
{
  "require": {
    "foo/bar": "1.0.0"
  }
}

composer create-project doctrine/orm path 2.2.0

$ composer init --require="twig/twig:1.*" -n --profile
Memory usage: 3.94MB (peak: 4.08MB), time: 0s

$ composer install --profile
Loading composer repositories with package information
Installing dependencies
 - Installing twig/twig (v1.12.2)
  Downloading: 100%

Writing lock file
Generating autoload files
Memory usage: 10.13MB (peak: 12.65MB), time: 4.71s

$ rm -rf vendor

$ composer install --profile
Loading composer repositories with package information
Installing dependencies from lock file
 - Installing twig/twig (v1.12.2)
  Loading from cache

Generating autoload files
Memory usage: 4.96MB (peak: 5.57MB), time: 0.45s

composer update symfony/yaml --prefer-source

composer status -v
You have changes in the following dependencies:
/path/to/app/vendor/symfony/yaml/Symfony/Component/Yaml:
  M Dumper.php

$ composer update
Loading composer repositories with package information
Updating dependencies
 - Updating symfony/symfony v2.2.0 (v2.2.0- => v2.2.0)
  The package has modified files:
  M Dumper.php
  Discard changes [y,n,v,s,"htmlcode">

composer dump-autoload --optimize



安装包的时候可以同样使用--optimize-autoloader。不加这一选项，你可能会发现20%到25%的性能损失。
如果你需要帮助，或者想要了解某个命令的细节，你可以阅读官方文档，或者查看JoliCode做的这个交互式备忘单。