syntax error near unexpected token `do' in bash script

For me, it was due to CRLF. It should be LF for linux env.

There are a bunch of syntax errors here. Let's start with the line:

if[  ( ${file: -4} == "$1" ) || ( ${file: -4 } == "$2" )  ]{

• You need a space between if and [ (or whatever comes after it). As written, the shell is treating "if[" as the name of a command, which isn't what you want at all.

• The [ ... ] style of conditional doesn't understand || (it uses -o instead), requires that all shell metacharacters (like parentheses) be escaped or quoted, might not understand == (just = is the standard), and will get very confused if any of the unquoted variable/parameter references are blank.

• if conditionals end with then (either on the next line, or after a ;) not {

You could fix it like this:

if [  \( "${file: -4}" = "$1" \) -o \( "${file: -4}" = "$2" \) ]; then

Or, since you're using bash (instead of a more basic shell), you can use the [[ ... ]] conditional style, which has much cleaner syntax:

if [[ "${file: -4}" = "$1" || "${file: -4}" = "$2" ]]; then

Next, remove the do before ffmpeg. do is part of the syntax for for and while loops; you already have one above (where it belongs), and this one just doesn't make sense. This is what's causing the error you see.

Next, the way you're replacing the file's extension won't work right. The variable reference "${file%.extension}"$3 will try to remove ".extension" (not the variable, just the string) from the end of $file. It also has $3 outside the quotes, which can cause trouble. You could fix it by using "${file%$extension}$3" instead, but frankly I'd just use "${file%.*}$3" to remove the extension no matter what length it is (and I'd also redo the if comparisons similarly, but that's more complicated).

Finally, you need a fi (after the ffmpeg line) to end the conditional. Every if needs a then and a fi.

And just as a stylistic thing, you don't need ; at the end of a line in shell; it's only needed when you're putting multiple commands (or things like do and then) on the same line. Anyway, here's my quick rewrite:

#!/bin/bash

# $1 is the first parameter passed
# $2 is the second parameter passed 
# $3 is the third parameter passed

for file in *.*; do
    #comparing the file types in the directory to the first 2 parameters passed
    if [[  "${file: -4}" = "$1" || "${file: -4}" = "$2"  ]]; then
        #converting such files to the type of the first parameter using the FFMPEG comand
        ffmpeg -i "$file" "${file%.*}$3"
    fi
done