The function takes a string input and shifts the first character of the string by a certain number of places in the alphabet, leaving the other characters unchanged. If the first character is not an alphabet letter, the input string remains unchanged.

[CODE]: 
```python
def f(x):
    if x[0].isalpha():
        shift = 7
        new_char = chr(((ord(x[0].lower()) - ord('a') + shift) % 26) + ord('a'))
        return (new_char if x[0].islower() else new_char.upper()) + x[1:]
    else:
        return x
``` 

Let's run another test to confirm the behavior.

COMMAND: PYTHON(./temp/function.py "abcdef" "uvwxyz" "testing123" "!@#")