go ldflags usage

Using ldflags with go build

ld stands for linker, the program that links together the different pieces of the compiled source code into the final binary.

ldflags, stands for linker flags. It passes a flag to the underlying Go toolchain linker, cmd/link, that allows you to change the values of imported packages at build time from the command line.

使用方式为：

# 格式
go build -ldflags="-flag"
# 示例
go build -ldflags="-X 'package_path.variable_name=new_value'"
# 实例
go build -ldflags="-X 'main.Version=v1.0.0'"
#复杂实例
go build -v -ldflags="-X 'main.Version=v1.0.0' -X 'app/build.User=$(id -u -n)' -X 'app/build.Time=$(date)'"

• 外层使用双引号，确保传递的flag中的内容即使包含空格也不截断命令；
• key-value值使用单引号
• 要改变的变量需要是包级别的string类型变量。不能是const类型
• 变量是否export都可以（大小写开头的变量都支持）

进一步可以使用nm工具查找编译文件中的symbols。（包名中不能包含非ASCII码，引号"和百分号%）

在make文件中使用——

main.go

package main

var (
    version string
    date    string
)

func init() {
    if version == "" {
        version = "no version"
    }
    if date == "" {
        date = "(Mon YYYY)"
    }
}

func main() {
    println(version, date)
}

makefile:

version=0.0.1
date=$(shell date -j "+(%b %Y)")
exec=a.out

.PHONY: all

all:
    @echo " make <cmd>"
    @echo ""
    @echo "commands:"
    @echo " build          - runs go build"
    @echo " build_version  - runs go build with ldflags version=${version} & date=${date}"
    @echo ""

build: clean
    @go build -v -o ${exec}

build_version: check_version
    @go build -v -ldflags '-X "main.version=${version}" -X "main.date=${date}"' -o ${exec}_${version}

clean:
    @rm -f ${exec}

check_version:
    @if [ -a "${exec}_${version}" ]; then \
        echo "${exec}_${version} already exists"; \
        exit 1; \
    fi;

Setting Go variables from the outside

在go run命令中也可以直接使用（因为会默认先执行go build)

go run -ldflags="-X main.who CloudFlare" hello.go