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I have a mini-system with only limited number of binaries (bash,cp,cat,sed, ...). I don't have tail command, and I am wondering if I could emulate tail functionality with sed

cat foo.txt | tail -n 10

I know that I can print lines 1-10 with sed cat foo.txt | sed -n '1,10p', but how would I print the last 10 lines?

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There is many useful sed scripts in one command line that you may need : sed one-line –  Slyx yesterday
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3 Answers

up vote 10 down vote accepted

You could do something like this:

sed -e :a -e '$q;N;11,$D;ba'
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sed -e :a -e '$q;N;11,$D;ba' foo to match the question. –  Slyx yesterday
 
I have no idea how it works, but it works perfectly. Thanks a lot. –  Martin Vegter yesterday
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up vote 0 down vote 
tac foo |  sed -n '1,10p' | tac

but if you don't have tac, you can use only sed this way :

sed -n '1!G;h;$p' foo | sed -n '1,10p' | sed -n '1!G;h;$p'
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up vote 0 down vote

Is expr available with your system? Then you can try evaluating the desired line number after you have counted the total number of lines in the file.

I created a file named tmp containing the numbers from 1 to 20 in each line.

nlines=$(cat tmp | sed -n '$=')
cat tmp | sed -n $(expr $nlines - 9),"$nlines"p
11
12
13
14
15
16
17
18
19
20

Of course, you can use wc -l, if available.

The command to compute the total number of lines in the file is taken from sed one liners. However, I haven't checked if it works with empty files too.

P.S. If expr is not available, you can use subtract numbers in Bash as shown below:

cat tmp | sed -n $((nlines-9)),"$nlines"p
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