Is there a one-liner that allows me to create a directory and move into it at the same time?

There's no built-in command, but you can easily write a function that calls mkdir then cd:

mkcd () {
  mkdir "$1"
  cd "$1"
}

Put this code in your ~/.bashrc file (or ~/.kshrc for ksh users, or ~/.zshrc for zsh users). It defines a function called mkcd. "$1" will be replaced by the argument of the function when you run it.

This simple version has several defects:

• You cannot create a chain of subdirectories at once. Fix: pass the -p option to mkdir. (This may or may not be desirable, as it increases the risk of a typo going undetected, e.g. mkcd mydierctory/newsub will happily create mydierctory and mydierctory/newsub when you meant to create newsub inside the existing mydirectory.)
• If the argument begins with - but isn't just -, then mkdir and cd will interpret it as an option. If it's just -, then cd will interpret it to mean $OLDPWD. If it's + followed by 0 or more digits, then cd in zsh will interpret it as an index in the directory stack. You can fix the first problem, but not the other two, by passing -- before the argument. You can fix all of these problems by prepending ./ to the argument if it's a relative path.
• mkdir doesn't follow CDPATH, but cd does, so if you've set CDPATH to a value that doesn't begin with . (an admittedly somewhat unusual configuration), then cd may bring you to a different directory than the one that was just created. Prepending ./ to relative paths fixes this¹ (it causes CDPATH to be ignored).
• If mkdir fails, it tries to execute cd. Fix: use && to separate the two commands.

Still fairly simple:

mkcd () {
  case "$1" in /*) :;; *) set -- "./$1";; esac
  mkdir -p "$1" && cd "$1"
}

This version still has the potential to make cd go into a different directory from the one that mkdir just created in one edge case: if the argument to mkcd contains .. and goes through a symbolic link. For example, if the current directory is /tmp/here and mylink is a symbolic link to /somewhere/else, then mkdir mylink/../foo creates /somewhere/else/foo whereas cd mylink/../foo changes into foo. It's not enough to look for symbolic links in the argument, because the shell also tracks symbolic links in its own current directory, so cd /tmp/mylink; mkdir ../foo; cd ../foo does not change into the new directory (/somewhere/else/foo) but into /tmp/foo. A fix for this is to let the cd builtin resolve all .. path components first (it doesn't make sense to use foo/.. if foo doesn't exist, so mkdir never needs to see any ..).

We come to this robust if slightly gory version:

mkcd () {
  case "$1" in
    */..|*/../) cd -- "$1";; # that doesn't make any sense unless the directory already exists
    /*/../*) (cd "${1%/../*}/.." && mkdir -p "./${1##*/../}") && cd -- "$1";;
    /*) mkdir -p "$1" && cd "$1";;
    */../*) (cd "./${1%/../*}/.." && mkdir -p "./${1##*/../}") && cd "./$1";;
    ../*) (cd .. && mkdir -p "${1#.}") && cd "$1";;
    *) mkdir -p "./$1" && cd "./$1";;
  esac
}

(Exercise: why am I using a subshell for the first cd call?)

If mkdir fails, I want to be sure not to change the current directory. Changing back with cd - or $OLDPWD isn't good enough if the shell doesn't have permission to change into its current directory. Also, calling cd updates OLDPWD, so we only want to do it once (or restore OLDPWD).

---

There are also less specialized ways to not have to retype the word from the previous line:

• Type cd , then Esc . (or Alt+.) to insert the last argument from the previous command.
• cd !$ executes cd on the last argument of the previous command.
• Press Up to recall the previous command line, then edit it to change mkdir into cd.

---

^{¹ beware however that it doesn't work in ksh93 since the u+ version, fixed in 93u+m/1.0.0-alpha+d1483150 2021-01-05}

This is the one-liner that you need. No other config needed:

mkdir longtitleproject && cd $_

The $_ variable, in bash, is the last argument given to the previous command. In this case, the name of the directory you just created. As explained in man bash:

_         At  shell  startup,  set to the absolute pathname used to invoke
          the shell or shell script being executed as passed in the  envi‐
          ronment  or  argument  list.   Subsequently, expands to the last
          argument to the previous command, after expansion.  Also set  to
          the  full  pathname  used  to  invoke  each command executed and
          placed in the environment exported to that command.  When check‐
          ing  mail,  this  parameter holds the name of the mail file cur‐
          rently being checked."$_" is the last argument of the previous command.

Use cd $_ to retrieve the last argument of the previous command instead of cd !$ because cd !$ gives the last argument of previous command in the shell history:

cd ~/
mkdir folder && cd !$

you end up home (or ~/ )

cd ~/
mkdir newfolder && cd $_

you end up in newfolder under home !! ( or ~/newfolder )

It would never have occurred to me to script up this behaviour because I enter the following on a near-hourly basis ...

$ mkdir someDirectory<ENTER>
$ cd !$

where bash kindly substitutes !$ with the last word of the last line; i.e. the long directory name that you entered.

In addition, filename completion is your friend in such situations. If your new directory was the only file in the folder a quick double TAB would give you the new directory without re-entering it.

Although it's cool that bash allows you to script up such common tasks as the other answers suggest I think it is better to learn the command line editing features that bash has to offer so that when you are working on another machine you are not missing the syntactic sugar that your custom scripts provide.