docs/reference: exec: update some examples

Use /bin/sh in the examples, as it's more likely to be present in a
container than bash (some users got confused by this, so using plain
"sh" in the examples could lead to less confusion).

Also added some extra wording around defaults, and how they're inherited
by the exec'd process.

It's definitely not "perfect" yet (lots to do in this document to improve
it), but it's a start :)

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 60833d2046)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/commandline/exec.md

@@ -32,13 +32,13 @@ The command started using `docker exec` only runs while the container's primary
 process (`PID 1`) is running, and it is not restarted if the container is
 restarted.
 
-COMMAND will run in the default directory of the container. If the
-underlying image has a custom directory specified with the WORKDIR directive
-in its Dockerfile, this will be used instead.
+COMMAND runs in the default directory of the container. If the underlying image
+has a custom directory specified with the WORKDIR directive in its Dockerfile,
+this directory is used instead.
 
-COMMAND should be an executable, a chained or a quoted command
-will not work. Example: `docker exec -ti my_container "echo a && echo b"` will
-not work, but `docker exec -ti my_container sh -c "echo a && echo b"` will.
+COMMAND must be an executable. A chained or a quoted command does not work.
+For example, `docker exec -it my_container sh -c "echo a && echo b"` works,
+work, but `docker exec -it my_container "echo a && echo b"` does not.
 
 ## Examples
 
@@ -47,70 +47,91 @@ not work, but `docker exec -ti my_container sh -c "echo a && echo b"` will.
 First, start a container.
 
 ```console
-$ docker run --name ubuntu_bash --rm -i -t ubuntu bash
+$ docker run --name mycontainer -d -i -t alpine /bin/sh
 ```
 
-This will create a container named `ubuntu_bash` and start a Bash session.
+This creates and starts a container named `mycontainer` from an `alpine` image
+with an `sh` shell as its main process. The `-d` option (shorthand for `--detach`)
+sets the container to run in the background, in detached mode, with a pseudo-TTY
+attached (`-t`). The `-i` option is set to keep `STDIN` attached (`-i`), which
+prevents the `sh` process from exiting immediately.
 
 Next, execute a command on the container.
 
 ```console
-$ docker exec -d ubuntu_bash touch /tmp/execWorks
+$ docker exec -d mycontainer touch /tmp/execWorks
 ```
 
-This will create a new file `/tmp/execWorks` inside the running container
-`ubuntu_bash`, in the background.
+This creates a new file `/tmp/execWorks` inside the running container
+`mycontainer`, in the background.
 
-Next, execute an interactive `bash` shell on the container.
+Next, execute an interactive `sh` shell on the container.
 
 ```console
-$ docker exec -it ubuntu_bash bash
+$ docker exec -it mycontainer sh
 ```
 
-This will create a new Bash session in the container `ubuntu_bash`.
+This starts a new shell session in the container `mycontainer`.
+
+### <a name=env></a> Set environment variables for the exec process (--env, -e)
 
 Next, set environment variables in the current bash session.
 
+By default, the `docker exec` command, inherits the environment variables that
+are set at the time the container is created. Use the `--env` (or the `-e` shorthand)
+to override global environment variables, or to set additional environment variables
+for the process started by `docker exec`.
+
+The example below creates a new shell session in the container `mycontainer` with
+environment variables `$VAR_A` and `$VAR_B` set to "1" and "2" respectively.
+These environment variables are only valid for the `sh` process started by that
+`docker exec` command, and are not available to other processes running inside
+the container.
+
 ```console
-$ docker exec -it -e VAR_A=1 -e VAR_B=2 ubuntu_bash bash
+$ docker exec -e VAR_A=1 -e VAR_B=2 mycontainer env
+PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+HOSTNAME=f64a4851eb71
+VAR_A=1
+VAR_B=2
+HOME=/root
 ```
 
-This will create a new Bash session in the container `ubuntu_bash` with environment
-variables `$VAR_A` and `$VAR_B` set to "1" and "2" respectively. Note that these
-environment variables will only be valid on the current Bash session.
+### <a name=workdir></a> Set the working directory for the exec process (--workdir, -w)
 
-By default `docker exec` command runs in the same working directory set when container was created.
+By default `docker exec` command runs in the same working directory set when 
+the container was created.
 
 ```console
-$ docker exec -it ubuntu_bash pwd
+$ docker exec -it mycontainer pwd
 /
 ```
 
-You can select working directory for the command to execute into
+You can specify an alternative working directory for the command to execute 
+using the `--workdir` option (or the `-w` shorthand):
 
 ```console
-$ docker exec -it -w /root ubuntu_bash pwd
+$ docker exec -it -w /root mycontainer pwd
 /root
 ```
 
 
 ### Try to run `docker exec` on a paused container
 
-If the container is paused, then the `docker exec` command will fail with an error:
+If the container is paused, then the `docker exec` command fails with an error:
 
 ```console
-$ docker pause test
-
-test
+$ docker pause mycontainer
+mycontainer
 
 $ docker ps
 
-CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS                   PORTS               NAMES
-1ae3b36715d2        ubuntu:latest       "bash"              17 seconds ago      Up 16 seconds (Paused)                       test
+CONTAINER ID   IMAGE     COMMAND     CREATED          STATUS                   PORTS     NAMES
+482efdf39fac   alpine    "/bin/sh"   17 seconds ago   Up 16 seconds (Paused)             mycontainer
 
-$ docker exec test ls
+$ docker exec mycontainer sh
 
-FATA[0000] Error response from daemon: Container test is paused, unpause the container before exec
+Error response from daemon: Container mycontainer is paused, unpause the container before exec
 
 $ echo $?
 1