update the json file

Question

I have a file, actually json file. How I can add a comma to the end of every but the last line ? For example:

{"a":1, "b":1, "c":2}
{"a":3, "b":3, "c":1}
{"a":1, "b":2, "c":3}

What I want:

{"a":1, "b":1, "c":2},
{"a":3, "b":3, "c":1},
{"a":1, "b":2, "c":3}

Answer 1

You can use awk:

awk 'NR > 1{print line","}{line=$0;}END{print $0}' jsonfile

Answer 2

You can use sed:

sed '$n;s/$/,/' file

• The $n; means if it's the last line just continue.
• Else it adds a , at the end of the line.
• If you add the -i flag (not POSIX) the file will be edited inplace.

If it must be awk:

awk 'NR>1{print p} {p=$0","}END{print}' file1

• Stores the content of the line in a variable p.
• The variable is printed, except in the last line.
• When the input comes to an end, the last line is print as it is.

Answer 3

It seems that the real goal here is to produce a JSON array from a stream of JSON entities. A great tool for anything to do with JSON is

The task of converting a file (say input.txt) of JSON entities to a JSON array is as simple as:

$ jq -s . input.txt

jq can also take input from STDIN.

Answer 4

How about in perl:

#!/usr/bin/perl

use strict;
use warnings;

use JSON;

my $json_list; 
while ( my $json_str = <> ) { 
    push ( @$json_list, from_json ( $json_str ) );
}

print to_json ( $json_list, { pretty => 1, canonical => 1 } );

Takes data on STDIN or myscript.pl somefilename and takes the input you've designated, and builds a JSON array out of it.

Result:

[
   {
      "a" : 1,
      "b" : 1,
      "c" : 2
   },
   {
      "a" : 3,
      "b" : 3,
      "c" : 1
   },
   {
      "a" : 1,
      "b" : 2,
      "c" : 3
   }
]

Answer 5

The original file is actually Newline delimited JSON (NDJSON).

Instead of using GNU tools, I recommend installing json from NPM.

# say the file is test.log
$ json -f test.log -g
[
  {
    "a": 1,
    "b": 1,
    "c": 2
  },
  {
    "a": 3,
    "b": 3,
    "c": 1
  },
  {
    "a": 1,
    "b": 2,
    "c": 3
  }
]

Answer 6

Pure bash:

while read line
do
    echo "$line,"
done < <(head -n-1 testfile)
echo `tail -n1 testfile`

This script appends ',' to every line except the last, and then prints the last line unchanged.