system command and echo strangeness

rubyonrails-talk
1

Hi,

> Which implies that when I do not specify the path it is using a > different version of echo.

It is - google "bash builtin commands" (assuming you're using bash as your default shell).

it's actually more than that:

irb(main):003:0> system "echo -e hello" hello => true irb(main):004:0> system "echo -e 'hello'" -e hello => true irb(main):005:0> $ sudo mv /bin/echo /bin/echo.1 ... $ irb irb(main):001:0> system "echo -e hello" => nil irb(main):002:0> system "echo -e 'hello'" -e hello => true irb(main):003:0>

Looks like adding quote to the command string causes it to be executed differently.

My guess says that if there's quote in the string at all it is executed by spawning a subshell while if there's none it's directly executed through execve() or whatever libc function for it.

observe:

irb(main):009:0> system 'bogus command' => nil irb(main):010:0> system 'bogus "command"' sh: 1: bogus: not found => false

If my quick lookup is correct, it's handled by rb_exec_fillarg function in process.c - it decides whether or not to spawn shell or just exec immediately (use_shell) by checking the string against various shell special keywords and characters.

2

Hi,

> Which implies that when I do not specify the path it is using a > different version of echo.

It is - google "bash builtin commands" (assuming you're using bash as your default shell).

it's actually more than that:

irb(main):003:0> system "echo -e hello" hello => true irb(main):004:0> system "echo -e 'hello'" -e hello => true irb(main):005:0> $ sudo mv /bin/echo /bin/echo.1 ... $ irb irb(main):001:0> system "echo -e hello" => nil irb(main):002:0> system "echo -e 'hello'" -e hello => true irb(main):003:0>

Looks like adding quote to the command string causes it to be executed differently.

My guess says that if there's quote in the string at all it is executed by spawning a subshell while if there's none it's directly executed through execve() or whatever libc function for it.

observe:

irb(main):009:0> system 'bogus command' => nil irb(main):010:0> system 'bogus "command"' sh: 1: bogus: not found => false

If my quick lookup is correct, it's handled by rb_exec_fillarg function in process.c - it decides whether or not to spawn shell or just exec immediately (use_shell) by checking the string against various shell special keywords and characters.

Thanks for that explanation, that all seems to make sense. Also running sh in a terminal shows that the sh builtin echo does not support -e. It can be a liitle confusing that there are three versions of echo on the system, builtin commands in sh and bash, and also /bin/echo.

Thanks again

Colin