why in an 'if' statement 'then' has to be in the next line in bash?

Question

if is followed by then in bash but I don't understand why then cannot be used in the same line like if [...] then it has to be used in the next line. Does that remove some ambiguity from the code? or bash is designed like that? what is the underlying reason for it?

I tried to write if and then in the same line but it gave the error below:

./test: line 6: syntax error near unexpected token `fi'
./test: line 6: `fi'

the code is:

#!/bin/bash
if [ $1 -gt 0 ] then
echo "$1 is positive" 
fi

Answer 1

It has to be preceded by a separator of some description, not necessarily on the next line^(a). In other words, to achieve what you want, you can simply use:

if [[ $1 -gt 0 ]] ; then
    echo "$1 is positive"
fi

As an aside, for one-liners like that, I tend to prefer:

[[ $1 -gt 0 ]] && echo "$1 is positive"

But that's simply because I prefer to see as much code on screen as possible. It's really just a style thing which you can freely ignore.

---

^(a) The reason for this can be found in the Bash manpage (my emphasis):

RESERVED WORDS: Reserved words are words that have a special meaning to the shell. The following words are recognized as reserved when unquoted and either the first word of a simple command (see SHELL GRAMMAR below) or the third word of a case or for command:

! case coproc do done elif else esac fi for function if in select then until while { } time [[ ]]

Note that, though that section states it's the "first word of a simple command", the manpage seems to contradict itself in the referenced SHELL GRAMMAR section:

A simple command is a sequence of optional variable assignments followed by blank-separated words and redirections, and terminated by a control operator. The first word specifies the command to be executed, and is passed as argument zero.

So, whether you consider it part of the next command or a separator of some sort is arguable. What is not arguable is that it needs a separator of some sort (newline or semicolon, for example) before the then keyword.

The manpage doesn't go into why it was designed that way but it's probably to make the parsing of commands a little simpler.