I'm trying to understand what this Docker entrypoint does.
It seems to me that's a very common pattern when writing Dockerfiles, but my bash skills are limited and I have no idea of all the special bash symbols kung fu.
Also, it's hard to google for "--", "$!" etc. What are these called in bash world?
To summarize, what is the line bellow trying to do?
if [ "${1#-}" != "$1" ]; then
set -- haproxy "$@"
fi
The set command (when not setting options) sets the positional parameters
eg
$ set a b c
$ echo $1
a
$ echo $2
b
$ echo $3
c
The -- is the standard "don't treat anything following this as an option"
The "$@" are all the existing position paramters.
So the sequence
set -- haproxy "$@"
Will put the word haproxy in front of $1 $2 etc.
eg
$ echo $1,$2,$3
a,b,c
$ set -- haproxy "$@"
$ echo $1,$2,$3,$4
haproxy,a,b,c
set -- -z 2 3 4 and set -z 2 3 4. The -- stops the -a being interpreted as an option. In this case it's not necessary, but it's "good practice" to get into the habit of using -- when you're note sure what is following :-)
Commented
Sep 9, 2016 at 16:09
set -- $arg1 $arg2 will allow your sourced script to read the arguments, otherwise only Bash supports passing arguments directly to a sourced script (and that can be tricky to notice).
Commented
Aug 14, 2018 at 18:34
-- stops the -z being interpreted as an options", right?
#!/bin/bash set $1 $2 $3. Run that with -c and you'll get an error similar to ./x: line 2: set: -c: invalid option which shows the command is being interpreted as options (do help set in bash and see what set -a does). But if the line said set -- $1 $2 $3 then you don't get an error and it sets the positional paramters.
Commented
Sep 19, 2020 at 22:28
The set command supports options (including flags) and arguments
set [-abefhkmnptuvxBCEHPT] [-o option-name] [--] [-] [argument …]
options can be used to change how the command works (i.e. how it interprets filenames)arguments tell commands what to do: i.e. your user-inpute.g. the set command has an option
-a: Each variable or function that is created or modified is given the export attribute and marked for export to the environment of subsequent commands.
When we execute:
$ set -a 12; echo $1
12
you can see that $1 is set to 12, but -a was consumed by the set command
If we really want to set -a as $1 (and 12 as $2), we can use -- to tell the set command to stop processing options (i.e. everything that follows is an argument)
$ set -- -a 12; echo $1 $2
-a 12
docker run haproxy --helpwill pass my "--help" to haproxy inside the container.