Shell脚本用Sed修改Makefile默认目标失败及术语疑问

Hey there! Let's tackle your two questions one by one—first why your Sed command isn't working on Ubuntu, then clear up the confusion about Make's default target terminology.

一、为什么Sed命令在Ubuntu中没修改Makefile？ #

Even if your regex works on regex testing tools, there are a few common pitfalls with GNU Sed (the version Ubuntu ships with) that might be tripping you up:

• Regex syntax differences: Most online regex tools use JavaScript-style regex, but GNU Sed defaults to POSIX Basic Regular Expressions (BRE). This means characters like +, (), and | need to be escaped with a backslash (\+, \(\), \|) to behave as you expect. If you want to use syntax closer to what you tested, add the -E flag to enable POSIX Extended Regular Expressions (ERE) in Sed.
• Missing the in-place edit flag: By default, Sed only prints modified content to your terminal—it won't write changes back to the file automatically. To edit the Makefile directly, add the -i flag (GNU Sed doesn't require an empty string after -i, unlike BSD Sed). For example:

  sed -i 's/your-regex/your-replacement/' Makefile
  

• Hidden whitespace mismatches: Makefiles are picky about whitespace—target lines might use tabs instead of spaces, or have trailing spaces your regex isn't accounting for. Run cat -A Makefile to see invisible characters like tabs (^I) and newlines, then adjust your regex to match exactly.
• Regex targeting the wrong line: The default Make target is the first non-comment, non-empty line in the file. If your regex skips over comments/blank lines or matches later targets, it won't modify the actual default target.

二、Make的默认目标是main还是main.c？ #

Great question—let's break down the terminology clearly:

• The default target is the first non-comment, non-empty target defined in your Makefile. The target name is whatever you write before the colon.
  • If your Makefile starts with main: main.c, the default target is main (this is the standard case—main is the executable you're building).
  • If your first target was something like main.c: generated-header.h, the default target would be main.c (though this is uncommon, since you rarely generate source files as a primary target).
• A common best practice is to define an all target as the first entry, which depends on your final executable(s). This makes your Makefile more flexible—for example:

  all: main
  main: main.c
      gcc -o main main.c
  clean:
      rm -f main
  

  Here, running make without arguments will build main (since all is the default target and depends on main).

As a quick working example, if you want to replace the first default target with new_target, this GNU Sed command (using ERE for cleaner syntax) should do the trick:

sed -i -E '1,/^[^#[:space:]]/{s/^([^#[:space:]]+):.*/new_target:/}' Makefile

内容的提问来源于stack exchange，提问作者Athylus