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= How can I print the n'th line of a file? = The dirty (but not quick) way would be: |
== How can I print the n'th line of a file? == One dirty (but not quick) way is: |
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| but this reads the whole input file, even if you only wanted the third line. This one avoids that problem: |
But this reads the entire file even if only the third line is desired, which can be avoided by printing line {{{$n}}} using the "p" command, followed by a "q" to exit the script: |
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| At line $n the command "p" is run, printing it, with a "q" afterwards: quit the program. Another way, more obvious to some, is to grab the last line from a listing of the first ''n'' lines: |
Another method is to grab the last line from a listing of the first {{{n}}} lines: |
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| If you want more than one line, it's pretty easy to adapt any of the previous methods: | If more than one line is needed, it's easy to adapt any of the previous methods: |
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| In Bash 4, this can also be done with mapfile builtin: | In Bash 4, a pure-bash solution can be achieved succinctly using the {{{mapfile}}} builtin. More than one line can be read into the array {{{MAPFILE}}} by adjusting the argument to mapfile's -n option: |
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| mapfile -ts $((n-1)) -n 1 echo "$MAPFILE" |
mapfile -ts $((n-1)) -n 1 < "$file" echo "${MAPFILE[0]}" |
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| By changing the value of argument for -n, you can get more than one line in the array MAPFILE. === Note === In most cases, you should sanitize your variable n to be sure, that it's not containing any of non-digits, before feeding it to sed or awk. You can do it with such simple code: |
{{{mapfile}}} can also be used similarly to {{{head}}} while avoiding buffering issues in the event input is a pipe: |
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| # Bash n=${n//[!0-9]/} |
{ mapfile -n $n; head -n 1; } <"$file" }}} |
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| # POSIX n=$(printf "%s" "$n"|tr -cd '0-9') }}} . One can argue that an `n` value of `Five brown horses with 3 feet and 9 little piggies.` should yield an error rather than silently behave as though it was really `39`. If you want to code safe, quote your expansions and be done with it. --[[Lhunath]] |
=== See Also === * [[BashFAQ/001]] * http://wiki.bash-hackers.org/commands/builtin/mapfile |
How can I print the n'th line of a file?
One dirty (but not quick) way is:
sed -n ${n}p "$file"But this reads the entire file even if only the third line is desired, which can be avoided by printing line $n using the "p" command, followed by a "q" to exit the script:
sed -n "$n{p;q;}" "$file"Another method is to grab the last line from a listing of the first n lines:
head -n $n $file | tail -n 1
Another approach, using AWK:
awk "NR==$n{print;exit}" fileIf more than one line is needed, it's easy to adapt any of the previous methods:
x=3 y=4
sed -n "$x,${y}p;${y}q;" "$file" # Print lines $x to $y; quit after $y.
head -n $y "$file" | tail -n $((y - x + 1)) # Same
head -n $y "$file" | tail -n +$x # If your tail supports it
awk "NR>=$x{print} NR==$y{exit}" "$file" # SameIn Bash 4, a pure-bash solution can be achieved succinctly using the mapfile builtin. More than one line can be read into the array MAPFILE by adjusting the argument to mapfile's -n option:
mapfile -ts $((n-1)) -n 1 < "$file"
echo "${MAPFILE[0]}"mapfile can also be used similarly to head while avoiding buffering issues in the event input is a pipe:
{ mapfile -n $n; head -n 1; } <"$file"