英文:
Shell Script question mark (?) issue in its uses
问题
我正在尝试运行以下脚本，但遇到了一个问题。
我找到了解决方案，但仍然不清楚为什么这个解决方案有效。
我想了解其中的概念：
我的脚本：
#!/bin/bash
# 运行脚本
echo "请使用：nice nohup ./run_script 运行"
# 脚本存储的工作目录
WORKING_DIR=$(pwd)
# 用于构建和运行cmake ctest的临时目录
BUILD_DIR?=/localtemp/build
# 清理并创建构建目录
rm -rf $BUILD_DIR
mkdir -p $BUILD_DIR
mkdir -p $BUILD_DIR/../result
遇到错误：
BUILD_DIR?=/localtemp/build: 没有那个文件或目录
mkdir: 缺少操作数
请运行 'mkdir --help' 以获取更多信息。
mkdir: 无法创建目录 '/../result': 权限被拒绝
但是，如果我从BUILD_DIR?=/localtemp/build这行中删除?，将其改为BUILD_DIR=/localtemp/build，那么脚本可以正常运行。
我的假设：
我想在BUILD_DIR这一行中使用**"?"**，以便如果有人想从脚本外部设置BUILD_DIR，它可以采用外部路径。否则，默认情况下，它应该使用我脚本中提到的上面路径。
有人能够解释一下在Shell脚本中使用**"?"**的用法吗？
英文:
I am trying to run below script but facing an issue .
I got the solution but still why this solution is working that I Am not clear .
I want to know the concept behind this :
My script:
#!/bin/bash
# RUN script
echo "please run with: nice nohup ./run_script"
# working directory where script is stored
WORKING_DIR=$(pwd)
# temp directory to build and run the cmake ctest
BUILD_DIR?=/localtemp/build
# clean and make build directory
rm -rf $BUILD_DIR
mkdir -p $BUILD_DIR
mkdir -p $BUILD_DIR/../result
Getting error as :
BUILD_DIR?=/localtemp/build: No such file or directory
mkdir: missing operand
Try 'mkdir --help' for more information.
mkdir: cannot create directory ‘/../result’: Permission denied
But if I remove the "?" from the line BUILD_DIR?=/localtemp/build and making it BUILD_DIR=/localtemp/build, the scripts run fine .
My assumptions:
I want "?" with BUILD_DIR line as if some one wants to set the BUILD_DIR from outside the script it has to take the outside path. otherwise by default it has to go with the above path mentioned in my script .
Can anyone clear my doubt in the shell script about the "?" use
答案1
得分: 4
看起来你把Makefile的语法和bash的语法混淆了。表达式 ?= 不是有效的bash表达式。你可以改成这样写：
BUILD_DIR=${BUILD_DIR:-/localtemp/build}
这意味着如果BUILD_DIR已设置，就将BUILD_DIR设置为其值，否则设置为/localtemp/build。
英文:
It looks like you're confusing Makefile syntax with bash syntax. The expression ?= isn't a valid bash expression. You can instead write something like this:
BUILD_DIR=${BUILD_DIR:-/localtemp/build}
Which means set BUILD_DIR to the value of BUILD_DIR, if it's set, otherwise to /localtemp/build.
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