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问题:
Is there a way in Linux to ask for the Head or Tail but with an additional offset of records to ignore.
For example if the file example.lst contains the following:
row01 row02 row03 row04 row05
And I use head -n3 example.lst I can get rows 1 - 3 but what if I want it to skip the first row and get rows 2 - 4?
I ask because some commands have a header which may not be desirable within the search results. For example du -h ~ --max-depth 1 | sort -rh will return the directory size of all folders within the home directory sorted in descending order but will append the current directory to the top of the result set (i.e. ~).
The Head and Tail man pages don't seem to have any offset parameter so maybe there is some kind of range command where the required lines can be specified: e.g. range 2-10 or something?
回答1:
From man tail:
-n, --lines=K output the last K lines, instead of the last 10; or use -n +K to output lines starting with the Kth
You can therefore use ... | tail -n +2 | head -n 3 to get 3 lines starting from line 2.
Non-head/tail methods include sed -n "2,4p" and awk "NR >= 2 && NR <= 4".
回答2:
To get the rows between 2 and 4 (both inclusive), you can use:
head -n4 example.lst | tail -n+2
or
head -n4 example.lst | tail -n3
回答3:
It took make a lot of time to end-up with this solution which, seems to be the only one that covered all usecases (so far):
command | tee full.log | stdbuf -i0 -o0 -e0 awk -v offset=${MAX_LINES:-200} \ '{ if (NR <= offset) print; else { a[NR] = $0; delete a[NR-offset]; printf "." > "/dev/stderr" } } END { print "" > "/dev/stderr"; for(i=NR-offset+1 > offset ? NR-offset+1: offset+1 ;i<=NR;i++) { print a[i]} }'
Feature list:
- live output for head (obviously that for tail is not possible)
- no use of external files
- progressbar on stderr, one dot for each line after the MAX_LINES, very useful for long running tasks.
- avoids possible incorrect logging order due to buffering (stdbuf)
