在Linux系統下，使用gfortran編譯器進行Fortran程序開發時，可以通過以下方法進行字符串處理：

1. 字符串拼接：

program main
    character(len=20) :: str1, str2, result_str

    str1 = "Hello, "
    str2 = "World!"
    result_str = str1 // str2

    print *, result_str
end program main

2. 字符串長度：

program main
    use iso_fortran_env, only: int32
    implicit none
    character(len=20) :: str
    integer(int32) :: str_length

    str = "Hello, World!"
    str_length = len_trim(str)

    print *, "String length: ", str_length
end program main

3. 字符串比較：

program main
    character(len=20) :: str1, str2
    logical :: is_equal

    str1 = "Hello, World!"
    str2 = "Hello, World!"
    is_equal = str1 == str2

    print *, "Strings are equal: ", is_equal
end program main

4. 字符串截取：

program main
    character(len=20) :: str, substr

    str = "Hello, World!"
    substr = str(1:5)

    print *, "Substring: ", substr
end program main

5. 字符串查找：

program main
    use iso_fortran_env, only: int32
    implicit none
    character(len=20) :: str, target
    integer(int32) :: pos

    str = "Hello, World!"
    target = "World"
    pos = index(str, target)

    print *, "Position of target: ", pos
end program main

6. 字符串替換：

program main
    character(len=20) :: str, old_substr, new_substr, result_str
    integer :: pos

    str = "Hello, World!"
    old_substr = "World"
    new_substr = "Fortran"

    pos = index(str, old_substr)
    if (pos > 0) then
        result_str = str(1:pos-1) // new_substr // str(pos+len(old_substr):)
    else
        result_str = str
    end if

    print *, "Replaced string: ", result_str
end program main

這些示例展示了如何在Linux系統下使用gfortran編譯器進行基本的字符串處理。你可以根據需要修改和擴展這些代碼來滿足你的實際需求。