remove new line if next line does not begin with a number

Question

I have a file that is like

    1 test
    test

How can I remove the new line from the so the final output becomes:

1 test test

Answer 1

That is pretty simple to accomplish with sed. Consider the file below

$ cat numbered                      
1 abc
def
ghi
2 jkl
mno
3 pqr
4 stu
vxw

The command sed '/^[0-9]/{N; s/\n/ /;}' numbered will do the trick:

$ sed '/^[0-9]/{N; s/\n/ /;}' numbered
1 abc def
ghi
2 jkl mno
3 pqr 4 stu
vxw

How it works: first, it verifies if the current line does start with a digit:

/^[0-9]/

The /^[0-9]/ is an address and matches only lines which starts with a digit. Iff some line matches it, the the command following the address will be executed.

In this case, this command is a {, which opens a function list. The function list will aggregate two or more commands as if they where only one. In this case, we have two commands in the function list. The first one is the N command:

N

This command appends a newline and the next line to the pattern space. The next line will not be considered anymore after the current cycle. The next command is s/// which replaces newlines by a space:

s/\n/ /

So, the newline appended by N will be replaced by a space. Then, we need just to close the function list with }. The result will be:

/^[0-9]/{
    N
    s/\n/ /
}

Since I've put all commands in one line for brevity, the commands inside the function list should be separated by semicolons:

/^[0-9]/{N;s/\n/ /;}

Note that if a line starting by a digit follows another line starting by a digit, the following line will be joined to the previous one and the command will apply to it.