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head and tail — line extraction

The head and tail utilities can be used to obtain only the first or last parts of a file respectively. Both will read from the files named on the commandline, or stdin if no files are provided.

The head utility takes a single argument, -n, which must be followed by an integer indicating the desired number of lines to be displayed.

For full details, see IEEE Std 1003.1-2017-head. Note that head(1) on GNU systems describes many non-portable options.

The tail utility is similar, but takes lines from the end of the file. The -n argument specifies how many lines to display.

To specify "the last five lines", use tail -n 5. To specify "all but the first five lines", use tail -n +6. 

# bad: drop first five lines, clumsily computing line count
tail -n $(($(wc -l in.txt | awk '{print $1}') - 5)) in.txt > out.txt

# good: let tail count lines from the beginning of the file
tail -n +6 in.txt > out.txt

For full details, see IEEE Std 1003.1-2017-tail. Note that tail(1) on GNU systems describes many non-portable options.

Chaining head or tail with sed

Chaining head or tail with sed is usually unnecessary. Use of addresses and early exit can do the same thing with a single sed call: 

# bad: get the first five lines of input.txt with all 'foo'
# replaced with 'bar'
head -n 5 input.txt | sed -e 's/foo/bar/g' > output.txt

# good: use sed's address ranges and command groups to do
# the same thing with only one fork
sed -n -e '1,5{ s/foo/bar/g ; p }' input.txt > output.txt

# good: another way is with an extra command which exits
# on line 5
sed -n -e 's/foo/bar/gp ; 5q' input.txt > output.txt

# bad: set foo to the first line containing somestring
foo=$(sed -n -e '/somestring/p' input.txt | head -n 1 )

# good: use early exit to do the same thing in pure sed
foo=$(sed -n -e '/somestring/{ p ; q }' input.txt )

# bad: output the last line matching 'somestring'
sed -n -e '/somestring/p' input.txt | tail -n 1

# good: do this in pure sed using the hold space
sed -n -e '/somestring/h ; $ {x;p}'

tail -n X where X is larger than one is possible to do in pure sed but tricky. Using chained commands here is probably simplest.

Finally, to extract a single specific line, use sed instead: 

sed -n -e '123p'