What does this Linux command do?

Question

g++ -Wall -I/usr/local/include/thrift *.cpp -lthrift -o something

I am new to Linux and am trying to learn "as I go along"....This is from the Apache Thrift website.

Also is the -I/usr supposed to be -I /usr?

Thanks for your help!

Answer 1

Here is a breakdown of the command. First the original command, for reference

g++ -Wall -I/usr/local/include/thrift *.cpp -lthrift -o something

Now, for the breakdown.

g++

This is the actual command command, g++. It is the program that is being executed. Here is what it is, from the man page:

gcc - GNU project C and C++ compiler

This is a compiler for programs written in C++ and C. It takes C or C++ code and turns it into a program, basically.

-Wall

This part makes it display all warnings when compiling. (Warn All)

-I/usr/local/include/thrift

This part tells g++ to use /usr/local/include/thrift as the directory to get the header files from. And with the question about whether to put a space after the I or not. You can do it either way. The way the options (options are things in a command after - signs. -Wall and -I are options) are parsed allows you to put a space or not. It depends on your personal preference.

*.cpp

This part passes every .cpp file in the current directory to the g++ command.

-lthrift

This can also be -l thrift. It tells g++ to search the thrift library when linking.

-o something

This tells it that when everything is compiled to place the executable in the file something.

I hope this helps and please comment if anything is unclear!

Answer 2

Generally you should look for the documentation of a command in its man page: man g++. In the case of GNU software such as GCC, you'll usually find more complete documentation in info format, or in HTML on the software home page.

C (and C++) compilers have a somewhat peculiar syntax that doesn't heed the usual conventions for options (options come before operand, there's an optional space between an option and its argument, …). The space after -I is actually optional, but -Wall needs to be one word.

Here's a quick overview of that command (look in the documentation for details):

• g++ run a C++ compiler
• -Wall emit warnings on suspicious code (-Wall actually means important warnings only, not all possible warnings)
• -I/usr/local/include/thrift look in that directory for include files (#include directives)
• *.cpp compile these files
• -lthrift link with this library (this searches libthrift.a). Note that -l must come after the files you're compiling, because -l means “if there are any undefined symbols at this point, look for definitions in the specificed library”
• -o something put the compiled and linked output into the specified file