What does -c mean as an argument to bash?

Question

  docker run -d ubuntu:14.04 /bin/bash -c "while true; do echo hello world;done"

  docker run -d ubuntu:14.04 /bin/bash "while true; do echo hello world; done"

I tried both.

In case 2, the container stopped immediately. So docker ps returns nothing. And docker ps -a returns just itself.

In case 1, docker ps lists the container. It is not stopped.

So what does -c flag do?

Answer 1

From the bash manual page:

bash interprets the following options when it is invoked:

-c string

If the -c option is present, then commands are read from string. If there are arguments after the string, they are assigned to the positional parameters, starting with $0.

Without the -c, the "while true..." string is taken to be a filename for bash to open.

Answer 2

The -c flag tells Bash that the next argument is a string of commands to run instead of the filename of a script. In the first case, Bash executes the next argument after -c. In the second case, where the -c flag isn't passed, Bash tries to run a script called while true; do echo hello world; done but can't find that script, so it quits.

From man bash:

-c string

If the -c option is present, then commands are read from string. If there are arguments after the string, they are assigned to the positional parameters, starting with $0.