linux - Ignore spaces, tabs and new line in SED

Question

I tried to replace a string in a file that contains tabs and line breaks. the command in the shell file looked something like this:

FILE="/Somewhere"
STRING_OLD="line 1[ 	
]*line 2"
sed -i 's/'"$STRING_OLD"'/'"$STRING_NEW"'/' $FILE

if I manually remove the line breaks and the tabs and leave only the spaces then I can replace successfully the file. but if I leave the line breaks then SED is unable to locate the $STRING_OLD and unable to replace to the new string

thanks in advance

Kobi

Answer 1

sed reads lines one at a time, and usually lines are also processed one at a time, as they are read. However, sed does have facilities for reading additional lines and operating on the combined result. There are several ways that could be applied to your problem, such as:

FILE="/Somewhere"
STRING_OLD="line 1[ 	
]*line 2"
sed -n "1h;2,$H;${g;s/$STRING_OLD/$STRING_NEW/g;p}"

That that does more or less what you describe doing manually: it concatenates all the lines of the file (but keeps newlines), and then performs the substitution on the overall buffer, all at once. That does assume, however, either that the file is short (POSIX does not require it to work if the overall file length exceeds 8192 bytes) or that you are using a sed that does not have buffer-size limitations, such as GNU sed. Since you tagged Linux, I'm supposing that GNU sed can be assumed.

In detail:

• the -n option turns off line echoing, because we save everything up and print the modified text in one chunk at the end.
• there are multiple sed commands, separated by semicolons, and with literal $ characters escaped (for the shell):
  • 1h: when processing the first line of input, replace the "hold space" with the contents of the pattern space (i.e. the first line, excluding newline)
  • 2,$H: when processing any line from the second through the last, append a newline to the hold space, then the contents of the pattern space
  • ${g;s/$STRING_OLD/$STRING_NEW/g;p}: when processing the last line, perform this group of commands: copy the hold space into the pattern space; perform the substitution, globally; print the resulting contents of the pattern space.

That's one of the simpler approaches, but if you need to accommodate seds that are not as capable as GNU's with regard to buffer capacity then there are other ways to go about it. Those start to get ugly, though.