Convert relative paths to absolute paths using shell Functions

There are many methods to convert relative paths to absolute paths. I don't know if there are any ready-made bash commands. I wrote a script first, which is also counted as practice shell programming.

At that time, I thought of the first method, that is, to determine whether it is a directory. If it is a directory, call the CD command, and then take the return value of the PWD command. If it is a file, remove the file name, then, call the CD command to obtain its directory value and then combine it. However, a condition for calling the CD command here is that the directory must exist. therefore, I have optimized this method:

Call method: get_fullpathRelative_path

Get_fullpath ()
{

# Determine whether a parameter exists. For example, if the current path is/home/user1/workspace, the parameter is./../DIR/Hello. C.
If [-Z $1]
Then
Return 1
Fi
Relative_path = $1

# Retrieve the first part of the Directory, such. /.. /.. /,.. /.. /, etc. Here, call the CD command to obtain the absolute path of this part of the path, because the upper-level directory of the current path certainly exists.

# Tmp_path1 is ./..

# Tmp_fullpath1/home/user1

Tmp_path1 =h1 (echo $ relative_path | sed-e "s =/[^ \.] * $ = ")
Tmp_fullpath1 = $ (CD $ tmp_path1; PWD)

# Obtain the following path

# Tmp_path2: Dir/Hello. c

Tmp_path2 = $ (echo $ relative_path | sed-e "s = \. [\./] * [/| $] = ")

# Return the patchwork path
Echo $ {tmp_fullpath1}/$ {tmp_path2}
Return 0
}

I can see that KSh has a whence command on the Internet, but I haven't found it in bash.