README.md

Select the first N lines of a file

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Classic use-cases:
Get the first line (header) of a file (csv, etc.)
Get 10 first lines of a command output ( ls -lt, etc.)

with head command

Complexity::white_check_mark:	Efficiency::white_check_mark:
The classic method:

head -n 15 <file>

Argument syntax variants:

head -15 <file>
head -n +15 <file>
head -n+15 <file>

Input variant:

head -n 15 < <file>

Unefficient variants:

cat <file> | head -n 15
cat < <file> | head -n 15

More than needed lines can be read be the cat command and passed into the pipe | and another process is created for the cat command. The SIGPIPE (Broken pipe) signal is send to the cat command when the head have exited.

with sed command

Complexity::warning:	Efficiency::white_check_mark:
Sed stop reading the input and exit when the 15th line is reached.

sed '15q' <file>

Input variant:

sed '15q' < <file>

Unefficient variants:

sed -n '1,15p' <file>
sed '1,15!d' <file>

All the content of the file is read by the sed command.

with awk command

Complexity::warning:	Efficiency::white_check_mark:
Awk stop reading the input and exit when the 15th line is reached.

awk 'NR > 15 { exit } { print $0 }' <file>

Syntax variants:

awk '{ print $0 ; if( NR >= 15 ) exit }' <file>
awk '{ if( NR > 15 ) exit ; print $0 }' <file>
awk '{ if( NR > 15 ) { exit } print $0 }' <file>
awk 'NR >= 15 { print $0 ; exit } { print $0 }' <file>

Input variant:

awk 'NR >= 15 { print $0 ; exit } { print $0 }' < <file>

Unefficient variants:

awk 'NR <= 15 { print $0 }' <file>
awk '{ if( NR <= 15 ) print $0 }' <file>

All the content of the file is read by the awk command.

with perl command

Complexity::warning:	Efficiency::warning:
All the content of the file is read by the perl command.

perl -ne '1..15 and print' <file>

Input variant:

perl -ne '1..15 and print' < <file>

with shell scripting

Complexity::warning:	Efficiency::warning:

Loop and read one line of the file and print it within the while, exit the when the 15th line is reached.

typeset -i lineNumber=0
while read line
do
	echo -E "${line}"
	(( lineNumber ++ ))
	if [[ "${lineNumber}" -ge 15 ]]
	then
		break
	fi
done < <file>

Interpreted shell script is less efficient than a single command.

Script variant:

exec 3< <file>
typeset -i lineNumber=0
for lineNumber in $( seq 15 )
do
    read line <&3
    echo -E "${line}"
done
exec 3<&-