在文件中尚未以“＃”开头的行的第一个字符添加＃

Question

I am writing a simple shell script that will comment out any lines of code that aren't already commented out. I am having trouble determining the correct conditional sed command. The sed command to add a # to any file works but now I need to make it conditional so that I don't end up with two ## on lines that were already commented out.

I have been trying different sed commands but am new to Bash and cant figure out the cryptic looking syntax!

sed -i -e "/^#/! 's/^/#/' $file

The first part of the sed command is wrong. I am getting an error, and without the first part of the sed command all lines of the file will have a # added to it even if one already exists on that line. Any help would be greatly appreciated!

Thanks!

Answer 1

I couldn't find the previous answers to close this as a dup of and no-one else has done it yet so here y'go:

$ cat file
foo
#bar

POSIX:

$ sed 's/^/#/; s/^##/#/' file
#foo
#bar

$ sed 's/^#\{0,1\}/#/' file
#foo
#bar

$ sed 's/^\([^#]\|$\)/#\1/' file
#foo
#bar

GNU:

$ sed 's/^#\?/#/' file
#foo
#bar

There are countless other alternatives...

Answer 2

awk版本

awk '{print (/^#/?"":"#")$0}' file