关于sed的命令'1!G;h;$!d'

We can do the same thing with the following sed script:

$ sed -e '1!G;h;$!d' forward.txt > backward.txt

You'll find this sed script useful if you're logged in to a FreeBSD system, which doesn't happen to have a "tac" command. While handy, it's also a good idea to know why this script does what it does. Let's dissect it.

---

 

First, this script contains three separate sed commands, separated by semicolons: '1!G', 'h' and '$!d'. Now, it's time to get an good understanding of the addresses used for the first and third commands. If the first command were '1G', the 'G' command would be applied only to the first line. However, there is an additional '!' character -- this '!' character negates the address, meaning that the 'G' command will apply to all but the first line. For the '$!d' command, we have a similar situation. If the command were '$d', it would apply the 'd' command to only the last line in the file (the '$' address is a simple way of specifying the last line). However, with the '!', '$!d' will apply the 'd' command to all but the last line. Now, all we need to to is understand what the commands themselves do.

When we execute our line reversal script on the text file above, the first command that gets executed is 'h'. This command tells sed to copy the contents of the pattern space (the buffer that holds the current line being worked on) to the hold space (a temporary buffer). Then, the 'd' command is executed, which deletes "foo" from the pattern space, so it doesn't get printed after all the commands are executed for this line.

Now, line two. After "bar" is read into the pattern space, the 'G' command is executed, which appends the contents of the hold space ("foo\n") to the pattern space ("bar\n"), resulting in "bar\n\foo\n" in our pattern space. The 'h' command puts this back in the hold space for safekeeping, and 'd' deletes the line from the pattern space so that it isn't printed.

For the last "oni" line, the same steps are repeated, except that the contents of the pattern space aren't deleted (due to the '$!' before the 'd'), and the contents of the pattern space (three lines) are printed to stdout.

Now, it's time to do some powerful data conversion with sed.