本博客系列翻译自 Bigbinary 的 Ruby 2.6 系列, 已得到作者允许。Ruby 2.6.0-preview2 现已发布。
平时我们会写一些自动化设置和部署 Rails 应用的脚本。在这些脚本中,我们会时常使用系统命令,例如 bundle install
,抑或是 rake db:create
等等。
我们假设一下:我们需要在一个Rails 项目里头跑 rake db:migrate
,我们可以使用 Kernel#system 方法。
irb> system('rake db:migrate')
Ruby 2.5.0
执行system
方法会返回 true
或者 false
(Ruby 2.5 下)。system
其中一个Feature(థ౪థ) 就是他会把异常抛出给吃掉。
我们假设这个命令会成功,那么我们会得到返回值 true
。
irb> system('rake db:migrate')
=> true
那我们继续假如:这个migration 尝试加一个字段到一个不存在的表里头。这个情况下,我们会得到返回值 false
。
irb> system('rake db:migrate')
== 20180311211836 AddFirstNameToAdmins: migrating =============================
-- add_column(:admins, :first_name, :string)
rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:
PG::UndefinedTable: ERROR: relation "admins" does not exist
: ALTER TABLE "admins" ADD "first_name" character varying
.
.
Tasks: TOP => db:migrate
(See full trace by running task with --trace)
=> false
如我们所见,即使执行系统命令失败,方法返回值还是会是 false
。Ruby 不会因此抛出异常。尽管如此,我们可以用这个方法来抛出异常。
irb> system('rake db:migrate') || raise('Failed to run migrations')
== 20180311211836 AddFirstNameToAdmins: migrating =============================
-- add_column(:admins, :first_name, :string)
rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:
PG::UndefinedTable: ERROR: relation "admins" does not exist
: ALTER TABLE "admins" ADD "first_name" character varying
.
.
.
Tasks: TOP => db:migrate
(See full trace by running task with --trace)
Traceback (most recent call last):
2: from /Users/amit/.rvm/rubies/ruby-2.5.0/bin/irb:11:in `<main>'
1: from (irb):4
RuntimeError (Failed to run migrations)
Ruby 2.6.0-preview1
Ruby 2.6 给 system
方法提供了一个新的option, 也就是 exception ,你可以通过设置 exception: true 来让方法可以抛出异常。
irb> system('rake db:migrate', exception: true)
== 20180311211836 AddFirstNameToAdmins: migrating =============================
-- add_column(:admins, :first_name, :string)
rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:
PG::UndefinedTable: ERROR: relation "admins" does not exist
: ALTER TABLE "admins" ADD "first_name" character varying
.
.
.
Tasks: TOP => db:migrate
(See full trace by running task with --trace)
Traceback (most recent call last):
3: from /Users/amit/.rvm/rubies/ruby-2.6.0-preview1/bin/irb:11:in `<main>'
2: from (irb):2
1: from (irb):2:in `system'
RuntimeError (Command failed with exit 1: rake)
当然,你只要设置exception 为false,那么这个函数的行为就和以前一样,返回布尔值。
irb> system('rake db:migrate', exception: false)
== 20180311211836 AddFirstNameToAdmins: migrating =============================
-- add_column(:admins, :first_name, :string)
rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:
PG::UndefinedTable: ERROR: relation "admins" does not exist
: ALTER TABLE "admins" ADD "first_name" character varying
.
.
.
Tasks: TOP => db:migrate
(See full trace by running task with --trace)
=> false
system
也不是唯一可以执行这些语句的方法,我(作者)六年前写了一个博客讨论这些(执行系统命令)的方法的异同:backtick
, exec
, sh
, popen3
, popen2e
and Process.spawn
.