问题
"The Unix Programming Environment" states that '$' used in regular expression in sed means end-of-the line which is fine for me, because
cat file | sed 's/$/\n/'
is interpretted as "add newline at the end of each line".
The question arises, when I try to use the command:
cat file | sed '$d'
Shouldn't this line remove each line instead of the last one? In this context, dollar sign means end of the LAST line. What am I getting wrong?
回答1:
Note that regex in sed must be inside the delimiters(;,:, ~, etc) other than quotes.
/regex/
ex:
sed '/foo/s/bar/bux/g' file
or
~regex~
ex:
sed 's~dd~s~' file
but not 'regex'. So $ in '$d' won't be considered as regex by sed. '$d' acts like an address which points out the last line.
回答2:
In the second usage, there is no regular expression. The $ there is an address, meaning the last line.
回答3:
$ is treated as regex anchor when used in pattern in s command e.g.
s/$/\n
However in $d, $ is not a regex anchor, it is address notation that means the last line of the input, which is deleted using the d command.
Also note that cat is unnecessary in your last command. It can be used as:
sed '$d' file
来源：https://stackoverflow.com/questions/31370580/sed-and-dollar-sign