Split the even and the odd lines of one column into two separate columns

Let’s start with the requirement in the title. You have this:

# file 1

1
A
2
B
3
CCopy

And you want this:

1 A
2 B
3 CCopy

In other words, you want odd rows (#1, #3, #5, etc) in column 1 and even rows (#2, #4, #6, etc) in column 2.

You can use the paste command like this:

$ paste --serial --delimiters '\t\n' <(echo '1
A
2
B
3
C')

1       A
2       B
3       CCopy

That seemed like a hack to me. Let’s break down the command above step by step:

1. <( command_to_run ): This is called process substitution. It allows the output of a command to be treated as a file-like object. In this case, the output of the echo command will be used as input “file” for the paste command.
2. paste: This is a command that merges lines from multiple files. In this case, it will merge lines from the file-like object provided by the process substitution, we have only 1 file.
3. --delimiters '\t\n': This option specifies the delimiters to be used between merged lines. In this case, it alternates between a tab character (\t) and a newline character (\n) as delimiters.
4. --serial: I left this option last because it’s a bit tricky. It tells paste to paste one file at a time instead of in parallel. Lets see some examples first.

# file 1
1
2
3

# file 2
a
b
cCopy

Default paste:

$ paste <( echo -e '1\n2\n3' ) <( echo -e 'a\nb\nc' )

1       a
2       b
3       cCopy

With serial option:

$ paste --serial <( echo -e '1\n2\n3' ) <( echo -e 'a\nb\nc' )

1       2       3
a       b       c

# or with even more rows for file 1:
$ paste --serial <( echo -e '1\n2\n3\n4\n5' ) <( echo -e 'a\nb\nc' )

1       2       3       4       5
a       b       cCopy

Also, an example if the files had 2 columns:

# file 1
1 Name1
2 Name2
3 Name3

# file 2
a 10
b 20
c 39Copy

$ paste  <( echo -e '1 Name1\n2 Name2\n3 Name3')  <( echo 'a 10\nb 20\nc 39' )

1 Name1 a 10
2 Name2 b 20
3 Name3 c 39Copy

And with 2 columns and --serial:

$ paste --serial <( echo -e '1 Name1\n2 Name2\n3 Name3' ) <( echo -e 'a\nb\nc' )

1 Name1 2 Name2 3 Name3
a       b       cCopy

So, when you run paste, by default:

1. It will go to the 1st line of the resulting file, and it will print the whole 1st line from file 1.
2. In the same line, it will add a delimiter and it will print the whole 1st line from file 2.
3. In the same line, it will add a delimiter and it will print the whole 1st line from file 3, and so on, for each file.
4. It will go the 2nd line of the resulting file, and it will print the whole 2nd line from file 1.
5. In the same line, it will add a delimiter and it will print the whole 2nd line from file 2.
6. In the same line, it will add a delimiter and it will print the whole 2nd line from file 3, and so on, for each file.
7. And so on, for each line.

Hence the merges lines from multiple files you saw before.

If you run paste with the --serial option, it will do the following:

1. It will go to the 1st line of the resulting file, and it will print the whole 1st line from file 1.
2. In the same line, it will add a delimiter and it will print the whole 2nd line from file 1.
3. In the same line, it will add a delimiter and it will print the whole 3rd line from file 1, and so on, for each line of file 1.
4. It will go the 2nd line of the resulting file, and it will print the whole 1st line from file 2.
5. In the same line, it will add a delimiter and it will print the whole 2nd line from file 2.
6. In the same line, it will add a delimiter and it will print the whole 3rd line from file 2, and so on, for each line of file 2.
7. And so on, for each file.

In other words, with the --serial option, the paste command will print all the lines of each file, separated by the delimiter, in the same line. And each file will be in a different line.

Without the --serial option, the paste command will print each line of each file in order, separated by the delimiter, in the same line. And each line of the input files will be in a different line.

Let’s see some examples with the --delimiters option to understand what’s going on:

$ paste --delimiters '^' <( echo -e '1\n2\n3' ) <( echo -e 'a\nb\nc' )

1^a
2^b
3^cCopy

Ok, it replaces the default tab delimiter with the ^ character, this makes sense.

What if we add another character in the --delimiters because man curl says that: -d, --delimiters=LIST, reuse characters from LIST instead of TABs

$ paste --delimiters '^#' <( echo -e '1\n2\n3' ) <( echo -e 'a\nb\nc' )

1^a
2^b
3^cCopy

So, nothing changed here. That is because it did not have the need for an additional delimiter, the files were only two.

But if you use the --serial option, there is room for 2 delimiters because each file has 3 lines:

$ paste --delimiters '^#' --serial <( echo -e '1\n2\n3' ) <( echo -e 'a\nb\nc' )

1^2#3
a^b#cCopy

And now let’s add again the tab and the new line as delimiters, the same as the original solution:

$ paste --delimiters '\t\n' --serial <( echo -e '1\n2\n3' ) <( echo -e 'a\nb\nc' )

1       2
3
a       b
cCopy

Not the result we wanted to achieve in the title, but, in our defense, the input is different! And I also hope that the original solution makes more sense now.

If you use serial, and you limit the input to 1 file, you’ll get the same result:

paste --delimiters '\t\n' --serial <( echo -e '1\na\n2\nb\n3\nc' )

1       a
2       b
3       cCopy

Quiz

You have the following file:

1
a
A
2
b
B
3
c
CCopy

How do you transform it to:

1 a A
2 b B
3 c CCopy

# notice the x2 spaces in the delimiters
paste --delimiters '  \n' --serial \
  <( echo -e '1\na\nA\n2\nb\nB\n3\nc\nC' )

1 a A
2 b B
3 c CCopy

Use awk if you want more flexibility

You can achieve the same result with awk and the NR built-in variable (it stores the row number):

$ awk '{ if (NR % 2) { printf "%s", $1 } else { printf "\t%s\n", $1}}' \
  <(echo "1
A
2
B
3
C")

1       A
2       B
3       CCopy

The above says, if the row number is odd, print the first column, else (the row number is even), print a tab character, the first column, and a newline character (change the line). Repeat this for every line in the file.

Links

• Explain shell web application.
• Bash Guide for Beginners from The Linux Documentation Project
• Advanced Bash-Scripting Guide from The Linux Documentation Project