Wednesday, August 25, 2010

Where there is awk, there is sed

I couldn't do a post on awk without also quickly covering sed. Bruce Barnett has written a great tutorial for sed that is worth reading.

Sed is your friend for applying regex's to files (well, streams really, since it is the 'stream editor'). The regex syntax is the same as for vim, and since that is my primary editor I only tend to use sed where the files are large and will take ages to load into vim.

Some quick examples to illustrate what I'm talking about:
sed s'/user root/user nothingtoseehere/g' < /var/log/auth.log

sed s'!session closed for user \([^ ]*\)!\1 closed a session!g' < \

sed s'!session closed for user \([^ ]*\)!&, allegedly!g' < \
Escaped parenthesis
capture a value, and
refers to the whole match. You can also use sed like grep. By default it prints every line, but that can be disabled with "-n" and you can cause matching lines to be printed by appending a "p" option:
sed -n 's/pattern/&/p' < file.txt
OR, if we aren't making a substitution
sed -n '/pattern/p' < file.txt
OR, you can just use grep -e, which is less confusing:
grep -e "pattern" file.txt
Rewriting a file in-place, making a backup with the .bak extension:
sudo sed -i.bak s'!XKBOPTIONS=""!XKBOPTIONS="ctrl:nocaps"!' /etc/default/keyboard

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