Ruby On Rails in UA/Code Review, теоретические вопросы/Как можно организовать в программе запуск файла?/RE: Как можно организовать в программе запуск файла?

An important thing to notice is that it replaces the current process. To immediately see what is meant by 'replaces', try running exec from irb:

focus:~/work/eaa jay$ irb
irb(main):001:0> exec 'svn st'
focus:~/work/eaa jay$

A more realistic example could be, assume you wanted to execute an external command from rake and you tried to use exec. Your external command would be executed, but rake would never finish. Not exactly desired results.

Kernel.system behaves very similarly, but does not replace the current process. The documentation states the following:

Executes cmd in a subshell, returning true if the command was found and ran successfully, false otherwise. An error status is available in $?. The arguments are processed in the same way as for Kernel::exec.

system("echo *")
system("echo", "*")

produces:

config.h main.rb
*

The same irb test shows us that system will return true or false, and $? can be used to get a Process::Status instance.

irb(main):014:0> s = system 'uptime'
10:56 up 3 days, 23:10, 2 users, load averages: 0.17 0.17 0.14
=> true
irb(main):015:0> s.class
=> TrueClass
irb(main):016:0> $?.class
=> Process::Status

Though I do like knowing my external application failed, it is nice to know why it failed. If you can redirect the output to a file this isn't a problem; however, you have 2 options if the execution output is simply dumped to stdout. The first option is to use system and redirect output to a file

irb(main):003:0> system 'uptime > results.log'
=> true
irb(main):004:0> exit
focus:~/work/eaa jay$ more results.log
13:09 up 4 days, 1:23, 2 users, load averages: 0.21 0.24 0.19

Another option is to use %x[..] and save the results. This option is more often used when the results need to be captured and used to determine further execution flow. To capture the results of a %x[..] you need to assign a variable to the return value.

irb(main):001:0> result = %x[uptime]
=> "13:16 up 4 days, 1:30, 2 users, load averages: 0.39 0.29 0.23\n"
irb(main):002:0> p result
"13:16 up 4 days, 1:30, 2 users, load averages: 0.39 0.29 0.23\n"

Using system allows you to deal with the output when false is returned. However, if you are always going to open the file and process the results you are likely better off using %x[..] in the first place.

Thanks for the post. If you need STDOUT and STDERR too, you can always pipe all output using &> instead:

Kernel.system("cmd &> output.txt")