CODEWITHSUNDEEP
shellunixawkposix
Rinke
Rinke

Reputation: 6332

Reverse line order except header

With a shell command I want to reverse the lines of a text file, excluding the header line.

Number,Letter,Person
1,a,Alice
2,b,Bob
3,c,Claire

should become

Number,Letter,Person
3,c,Claire
2,b,Bob
1,a,Alice

I'm looking for both an "in place" solution (change original file) and a streaming solution (read original file from stdin and print result to stdout).

Ideally, the command or script would be POSIX compatible, but other solutions could also be interesting. A oneliner would be nice.

I'm also interested in the performance/complexity of any solution.

Upvotes: 2

Views: 260

Answers (3)

Ed Morton
Ed Morton

Reputation: 203597

$ awk -v OFS='\t' '{print (NR==1), NR, $0}' file | sort -snr -k1 -k2 | cut -f3-
Number,Letter,Person
3,c,Claire
2,b,Bob
1,a,Alice

Upvotes: 1

Cyrus
Cyrus

Reputation: 88646

With GNU sed:

sed -n '1p;2h;3,${G;h};$p' file

Output:

Number,Letter,Person
3,c,Claire
2,b,Bob
1,a,Alice

If you want to edit your file "in place" use sed's option -i.

See: man sed

Upvotes: 3

anubhava
anubhava

Reputation: 785186

Here is an awk command to do this:

awk '{a[NR]=$0} END{print a[1]; for (i=NR; i>1; i--) print a[i]}' file

Number,Letter,Person
3,c,Claire
2,b,Bob
1,a,Alice

Here a[1] represents header record. Remaining entries in associative array a represent rows with the index as record number.

Alternatively, using head, tail, cat:

{ head -1 f && tail -n +2 file | tac; } > file._tmp && mv file._tmp file

Number,Letter,Person
3,c,Claire
2,b,Bob
1,a,Alice

Upvotes: 7

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