How can I remove the first line of a text file using bash/sed script?
I need to repeatedly remove the first line from a huge text file using a bash script.
Right now I am using sed -i -e "1d" $FILE - but it takes around a minute to do the deletion.
Is there a more efficient way to accomplish this?
Try GNU tail:
tail -n +2 "$FILE"
-nx : Just print the last x lines. tail -n 5 would give you the last 5 lines of the input. The + sign kind of inverts the argument and make tail print anything but the first x-1 lines. tail -n +1 would print the whole file, tail -n +2 everything but the first line, etc.
GNU tail is much faster than sed . tail is also available on BSD and the -n +2 flag is consistent across both tools. Check the FreeBSD or OS X man pages for more.
The BSD version can be much slower than sed , though. I wonder how they managed that; tail should just read a file line by line while sed does pretty complex operations involving interpreting a script, applying regular expressions and the like.
Note: You may be tempted to use
# THIS WILL GIVE YOU AN EMPTY FILE!
tail -n +2 "$FILE" > "$FILE"
but this will give you an empty file . The reason is that the redirection ( > ) happens before tail is invoked by the shell:
$FILE tail tail process to $FILE tail reads from the now empty $FILE If you want to remove the first line inside the file, you should use:
tail -n +2 "$FILE" > "$FILE.tmp" && mv "$FILE.tmp" "$FILE"
The && will make sure that the file doesn't get overwritten when there is a problem.
You can use -i to update the file without using '>' operator. The following command will delete the first line from the file and save it to the file.
sed -i '1d' filename
对于那些非GNU的SunOS，以下代码将有所帮助：
sed '1d' test.dat > tmp.dat
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