How can I modify this bash script so that the port argument is optional?

Question

I often connect through ssh to Docker containers running on a remote server. Every time the container is stopped or removed, I need to copy my ssh key on the container again, in order to connect. For this reason, on my Mac laptop I put this simple bash script, whose goal is simply to add my ssh key to the Docker container to which I want to connect.

#!/bin/bash
# A POSIX variable
OPTIND=1 # Reset in case getopts has been used previously in the shell.
# Default parameter value:
PORT=...
PUBKEY=...
HOST_IP=...
# show_help function
show_help() {
 echo "Usage: $(basename "0ドル") [-h] [-p PORT] [-f PUBKEY] [-i HOST_IP]"
 echo
 echo " -p PORT add key to container running on port PORT (default: ...)"
 echo " -f PUBKEY add key file PUBKEY (default: ...)"
 echo " -i HOST_IP connect to host HOST_IP (default: ...)"
 echo
 return
}
while getopts ":h:p:f:i:" option; do
 case "$option" in
 \?)
 echo "invalid argument"
 show_help
 exit 1
 ;;
 h)
 show_help
 exit 0
 ;;
 p) PORT=$OPTARG
 ;;
 f) PUBKEY=$OPTARG
 ;;
 i) HOST_IP=$OPTARG
 ;;
 esac
done
shift $((OPTIND-1))
USER_AT_HOST="root@$HOST_IP"
PUBKEYPATH="$HOME/.ssh/$PUBKEY"
ssh-copy-id -i "$PUBKEYPATH" "$USER_AT_HOST" -p "$PORT"
exit 0

I have two problems:

1. (smaller) the arguments have non-intuitive letters (f for the PUBKEY, i for the HOST_IP) because if I'm not mistaken, getopts only support single-letter arguments. Is there any way to circumvent this limitation? If not, could you suggest some more creative/self-explanatory letters for the arguments? 🙂

2. (bigger) currently the -p argument (PORT) is mandatory, and the user used for the ssh connection is always root. I would like to modify the script so that, if the -p argument is not passed, then another argument -u (USER) must be passed, and the connection command becomes

    USER_AT_HOST="$USER@$HOST_IP"
    PUBKEYPATH="$HOME/.ssh/$PUBKEY"
    ssh-copy-id -i "$PUBKEYPATH" "$USER_AT_HOST"
   

   instead than

    USER_AT_HOST="root@$HOST_IP"
    PUBKEYPATH="$HOME/.ssh/$PUBKEY"
    ssh-copy-id -i "$PUBKEYPATH" "$USER_AT_HOST" -p "$PORT"
   

How could I modify the script in order to obtain this result?

Answer 1

With this implementation, you don't specify default values up front.

The arguments to ssh-copy-id are built up in an array. Note the use of += to append to the array.

Other notes:

• OPTIND=1 is only needed if you source this script. If you're running it as a script, then the variable is unset before you get to getopts

• I'd remove exit 0 at the bottom of the script: let it exit with ssh-copy-id's exit status.

• Your code forgot i in the opt string for getopts. Double check this.

• ?* is a shell pattern (aka "wildcard") that matches at least 1 character.

• Get out of the habit of using ALLCAPS variable names, leave those as reserved by the shell. One day you'll write PATH=something and then wonder why your script is broken.

• an alternate way to set a default value is to use the : command

  : ${var:="default value"}
  

  But that's a bit opaque.

---

#!/bin/bash
# show_help function
show_help() {
 echo "Usage: $(basename "0ドル") [-h] [-p PORT|-u] [-f PUBKEY] [-i HOST_IP]"
 echo
 echo " -p PORT add key to container running on port PORT (default: ...)"
 echo " -u USER help string here..."
 echo " -f PUBKEY add key file PUBKEY (default: ...)"
 echo " -i HOST_IP connect to host HOST_IP (default: ...)"
 echo
 return
}
while getopts ":hi:p:f:u:" option; do
 case "$option" in
 p) port=$OPTARG ;;
 f) pubkey=$OPTARG ;;
 i) host_ip=$OPTARG ;;
 u) user=$OPTARG ;;
 h) show_help; exit 0 ;;
 ?) echo "invalid argument"; show_help; exit 1 ;;
 esac
done
shift $((OPTIND-1)) # not needed if there are no positional parameters
[[ -z $host_ip ]] && host_ip="default value"
[[ -z $pubkey ]] && pubkey="default value"
cmd_args=( -i "$HOME/.ssh/$pubkey" )
case "$port,$user" in
 ,) # empty port and empty user
 echo "If you don't specify a port, you need to specify a user" >&2
 exit 1
 ;;
 ,?*) # empty port but user is specified
 cmd_args+=( "$user@$host_ip" )
 ;;
 ?*,*) # port has been given. user doesn't matter
 cmd_args+=( -p "$port" "root@$host_ip" )
 ;;
esac
ssh-copy-id "${cmd_args[@]}"

---

To address your comments:

• only options that take an argument use a colon. To demonstrate

  With a colon,

  $ set -- -h
  $ getopts :h: opt
  $ declare -p opt OPTARG OPTIND
  declare -- opt=":"
  declare -- OPTARG="h"
  declare -i OPTIND="2"
  

  As the manual for getopts says:

  If a required argument is not found [...] a colon (:) is placed in name and OPTARG is set to the option character found.

  Without a colon, option parsing proceeds as you expect

  $ unset opt OPTARG OPTIND
  $ set -- -h
  $ getopts :h opt
  $ declare -p opt OPTARG OPTIND
  declare -- opt="h"
  declare -- OPTARG
  declare -- OPTIND="2"
  

• Regarding default values, do the option parsing first. Then, if the variables are not set by options, provide the default value.

  [[ -z $host_ip ]] && host_ip="default value"
  

  This means: if the value of host_ip is empty (-z), set the value to a default value.

• DeltaIV: thanks for the answer! A few questions: while getopts ":hi:p:f:u:". Shouldn't there be a : between h and i, i.e., while getopts ":h:i:p:f:u:"? Also, you say that With this implementation, you don't specify default values up front.. However, I find default values useful because I often use the script with the same parameters. Could you explain a bit more in detail why I can't use default parameters? Finally, could you please explain the meaning of these two lines [[ -z $host_ip ]] && host_ip="default value" [[ -z $pubkey ]] && pubkey="default value"? Thanks again