Why is a shell script giving syntax errors when the same code works elsewhere? [duplicate]

I have a simple shell script I copied from a working script. It works if I copy-paste it to a terminal:

if true
then
  true
fi 

However, when I run the script with bash myscript, I get various syntax errors as if some of the keywords are missing.

• myscript: line 4: syntax error near unexpected token `fi' , as if then isn't there.
• myscript: line 6: syntax error: unexpected end of file , as if fi isn't there.
• myscript: line 4: syntax error near unexpected token `$'\r' .. what?

Why does this happen in this particular script, but not on my command line or in the script I copied from?

TL;DR: Your script has Windows style CRLF line endings, aka \r\n.

Convert to Unix style \n by deleting the carriage returns.

How do I check if my script has carriage returns?

They're detectable as ^M in the output of cat -v yourscript:

$ cat -v myscript
if true^M
then^M
  true^M
...

How do I remove them?

Set your editor to save the file with Unix line endings, aka "line terminators" or "end-of-line characters", and resave it.

You can also remove them from a command line with dos2unix yourscript or cat yourscript | tr -d '\r' > fixedscript.

Why do carriage returns cause syntax errors?

The carriage return character is just another character to bash. then is not the same as then\r, so bash doesn't recognize it as a keyword and assumes it's a command. It then keeps looking for a then and fails

If there happens to be a trailing space after then, you get a similar problem for fi.