4

I'm writing a simple bash script and I would like it to accept parameters from the command line in any order.

I've browsed around the web and wrote a simple function with a case statement in a while loop. Right now, the 'any order' part works - but it only picks up the first parameter I set. I'm certainly doing something wrong but scripting is quite new to me and I hadn't been able to figure it out - your help would be greatly appreciated. The flags part of the script is as follows:

#Parameters - source,destination,credentials,bandwidth,timeout,port,help flags () { while test $# -gt 0; do case "$1" in -s|--source) shift if test $# -gt 0; then export SOURCE=$1 else echo "No source directory specified!" exit 1 fi ;; -d|--destination) shift if test $# -gt 0; then export DESTINATION=$1 fi ;; -c|--credentials) shift if test $# -gt 0; then export CREDENTIALS=$1 fi ;; -b|--bandwidth) shift if test $# -gt 0; then export BANDWIDTH=$1 fi ;; -t|--timeout) shift if test $# -gt 0; then export TIMEOUT=$1 fi ;; -p|--port) shift if test $# -gt 0; then export PORT=$1 fi ;; -h|--help) shift if test $# -gt 0; then echo "Help goes here" fi ;; -l|--compression-level) shift if test $# -gt 0; then export COMPRESS_LEVEL=$1 fi ;; *) break ;; esac done } flags "$@" echo "source is $SOURCE, destination is $DESTINATION, credentials are $CREDENTIALS, bandwidth is $BANDWIDTH, timeout is $TIMEOUT, port is $PORT"

Ideally, some of those parameters would be mandatory, and others optional - but that's not a must.

How can I fix this script to accept any of those parameters (both long and short forms, ideally) in any order?

Improve this question
7
  • After you consume the argument (for example for credentials), you need another shift. You should be consistent in your error reporting for non-existent arguments. If you get -h or --help, you should simply print the help and exit; you should not test for more arguments. If help is requested, you give it and do nothing else. You should also echo errors to standard error: echo "message" >&2. Your messages should be prefixed with the script/program name: arg0=$(basename "$0" .sh) and echo "$arg0: message" >&2 etc. Commented Oct 8, 2020 at 7:01
  • 1
    Also check out the GNU getopt(1) command. It will probably make life easier for you; it handles long options. In most (all?) shells, the built-in getopts commands do not, especially if they're POSIX compliant. Commented Oct 8, 2020 at 7:08
  • Thanks! the extra shift solved it. I hadn't done any work on the rest as this is really early stages - all great tips and I'll keep them in mind. Thank you! I'll check out getopt as well. Commented Oct 8, 2020 at 7:09
  • owsiak.org/argument-parsing-in-bash Commented Oct 8, 2020 at 7:41
  • BashFAQ #35 Commented Oct 8, 2020 at 17:02

1 Answer 1

Reset to default
5

As noted in the comments, after you consume the argument (for example for credentials), you need another shift. You should be consistent in your error reporting for non-existent arguments. If you get -h or --help, you should simply print the help and exit; you should not test for more arguments. If help is requested, you give it and do nothing else. You should also echo errors to standard error: echo "message" >&2. Your messages should be prefixed with the script/program name: arg0=$(basename "$0" .sh) and echo "$arg0: message" >&2 etc.

Putting the changes together, you might come up with a script like this:

#!/bin/sh arg0=$(basename "$0" .sh) blnk=$(echo "$arg0" | sed 's/./ /g') usage_info() { echo "Usage: $arg0 [{-s|--source} source] [{-d|--destination} destination] \\" echo " $blnk [{-c|--credentials} credentials] [{-b|--bandwidth} bandwidth] \\" echo " $blnk [{-t|--timeout} timeout] [{-p|--port} port] \\" echo " $blnk [-h|--help] [{-l|--compression-level} level]" } usage() { exec 1>2 # Send standard output to standard error usage_info exit 1 } error() { echo "$arg0: $*" >&2 exit 1 } help() { usage_info echo echo " {-s|--source} source -- Set source directory (default: .)" echo " {-d|--destination} destination -- Set destination" echo " {-c|--credentials} credentials -- Set credentials" echo " {-b|--bandwidth} bandwidth -- Set maximum bandwidth" echo " {-t|--timeout} timeout -- Set timeout (default: 60s)" echo " {-p|--port} port -- Set port number (default: 1234)" echo " {-l|--compression-level} level -- Set compression level (default: 1)" echo " {-h|--help} -- Print this help message and exit" # echo " {-V|--version} -- Print version information and exit" exit 0 } flags() { while test $# -gt 0 do case "$1" in (-s|--source) shift [ $# = 0 ] && error "No source directory specified" export SOURCE="$1" shift;; (-d|--destination) shift [ $# = 0 ] && error "No destination specified" export DESTINATION="$1" shift;; (-c|--credentials) shift [ $# = 0 ] && error "No credentials specified" export CREDENTIALS="$1" shift;; (-b|--bandwidth) shift [ $# = 0 ] && error "No bandwidth specified" export BANDWIDTH="$1" shift;; (-t|--timeout) shift [ $# = 0 ] && error "No timeout specified" export TIMEOUT="$1" shift;; (-p|--port) shift [ $# = 0 ] && error "No port specified" export PORT="$1" shift;; (-l|--compression-level) shift [ $# = 0 ] && error "No compression level specified" export COMPRESS_LEVEL="$1" shift;; (-h|--help) help;; # (-V|--version) # version_info;; (*) usage;; esac done } flags "$@" echo "source is $SOURCE" echo "destination is $DESTINATION" echo "credentials are $CREDENTIALS" echo "bandwidth is $BANDWIDTH" echo "timeout is $TIMEOUT" echo "port is $PORT"

Sample run (script name: flags53.sh):

$ sh flags53.sh -c XYZ -d PQR -s 123 -l 4 -t 99 -b 12 -p 56789 source is 123 destination is PQR credentials are XYZ bandwidth is 12 timeout is 99 port is 56789 $ sh flags53.sh -c XYZ --destination PQR -s 123 -l 4 --timeout 99 -b 12 --port 56789 source is 123 destination is PQR credentials are XYZ bandwidth is 12 timeout is 99 port is 56789 $ sh flags53.sh -c XYZ -h Usage: flags53 [{-s|--source} source] [{-d|--destination} destination] \ [{-c|--credentials} credentials] [{-b|--bandwidth} bandwidth] \ [{-t|--timeout} timeout] [{-p|--port} port] \ [-h|--help] [{-l|--compression-level} level] {-s|--source} source -- Set source directory (default: .) {-d|--destination} destination -- Set destination {-c|--credentials} credentials -- Set credentials {-b|--bandwidth} bandwidth -- Set maximum bandwidth {-t|--timeout} timeout -- Set timeout (default: 60s) {-p|--port} port -- Set port number (default: 1234) {-l|--compression-level} level -- Set compression level (default: 1) {-h|--help} -- Print this help message and exit $

Note that requested help can go to standard output instead of standard error, though sending the help to standard error would not be an egregious crime. The help gets the usage message and extra information about the meaning of each option. Noting defaults (and setting them) is a good idea too. It may not be necessary to export the settings — you could simply set the variables without an explicit export. You should really set the variables to their defaults before calling the flags function, or at the start of the flags function. This avoids accidentally inheriting exported values (environment variables). Unless, of course, you want to accept environment variables, but then your names should probably be given a systematic prefix appropriate for the script name. Most programs should have a --version or -V option (use -v for 'verbose', not for version). If the command does not accept any non-option (file name) arguments, add a check after the parsing loop and complain about unwanted arguments. If the command must have at least one non-option argument, check that instead. Do not report an error on receiving -- as an argument; terminate the checking loop and treat any remaining arguments as non-option arguments.

One residual problem — the shifts in the function affect the function's argument list, not the global "$@". You'd have to work out how to deal with that from this skeleton. I think I'd probably create an analogue to $OPTIND that reports how many arguments to shift to get to the non-option arguments. The code in the flags function should keep track of how many arguments it shifts.

That leads to the revised code:

#!/bin/sh arg0=$(basename "$0" .sh) blnk=$(echo "$arg0" | sed 's/./ /g') usage_info() { echo "Usage: $arg0 [{-s|--source} source] [{-d|--destination} destination] \\" echo " $blnk [{-c|--credentials} credentials] [{-b|--bandwidth} bandwidth] \\" echo " $blnk [{-t|--timeout} timeout] [{-p|--port} port] \\" echo " $blnk [-h|--help] [{-l|--compression-level} level]" } usage() { exec 1>2 # Send standard output to standard error usage_info exit 1 } error() { echo "$arg0: $*" >&2 exit 1 } help() { usage_info echo echo " {-s|--source} source -- Set source directory (default: .)" echo " {-d|--destination} destination -- Set destination" echo " {-c|--credentials} credentials -- Set credentials" echo " {-b|--bandwidth} bandwidth -- Set maximum bandwidth" echo " {-t|--timeout} timeout -- Set timeout (default: 60s)" echo " {-p|--port} port -- Set port number (default: 1234)" echo " {-l|--compression-level} level -- Set compression level (default: 1)" echo " {-h|--help} -- Print this help message and exit" # echo " {-V|--version} -- Print version information and exit" exit 0 } flags() { OPTCOUNT=0 while test $# -gt 0 do case "$1" in (-s|--source) shift [ $# = 0 ] && error "No source directory specified" export SOURCE="$1" shift OPTCOUNT=$(($OPTCOUNT + 2));; (-d|--destination) shift [ $# = 0 ] && error "No destination specified" export DESTINATION=$1 shift OPTCOUNT=$(($OPTCOUNT + 2));; (-c|--credentials) shift [ $# = 0 ] && error "No credentials specified" export CREDENTIALS=$1 shift OPTCOUNT=$(($OPTCOUNT + 2));; (-b|--bandwidth) shift [ $# = 0 ] && error "No bandwidth specified" export BANDWIDTH=$1 shift OPTCOUNT=$(($OPTCOUNT + 2));; (-t|--timeout) shift [ $# = 0 ] && error "No timeout specified" export TIMEOUT="$1" shift OPTCOUNT=$(($OPTCOUNT + 2));; (-p|--port) shift [ $# = 0 ] && error "No port specified" export PORT=$1 shift OPTCOUNT=$(($OPTCOUNT + 2));; (-l|--compression-level) shift [ $# = 0 ] && error "No compression level specified" export COMPRESS_LEVEL="$1" shift OPTCOUNT=$(($OPTCOUNT + 2));; (-h|--help) help;; # (-V|--version) # version_info;; (--) shift OPTCOUNT=$(($OPTCOUNT + 1)) break;; (*) usage;; esac done echo "DEBUG-1: [$*]" >&2 echo "OPTCOUNT=$OPTCOUNT" >&2 } flags "$@" echo "DEBUG-2: [$*]" >&2 echo "OPTCOUNT=$OPTCOUNT" >&2 shift $OPTCOUNT echo "DEBUG-3: [$*]" >&2 echo "source is $SOURCE" echo "destination is $DESTINATION" echo "credentials are $CREDENTIALS" echo "bandwidth is $BANDWIDTH" echo "timeout is $TIMEOUT" echo "port is $PORT"

There are other ways of writing the arithmetic if you wish to experiment. Don't use expr though.

Improve this answer
Sign up to request clarification or add additional context in comments.

4 Comments

One residual problem — the shifts in the function affect the function's argument list, not the global "$@". You'd have to work out how to deal with that from this skeleton. I think I'd probably create an analogue to $OPTIND that reports how many arguments to shift to get to the non-option arguments. The code in the flags function should keep track of how many arguments it shifts.
A lot of work went into this answer -- maybe migrate it to the canonical instance of the question at stackoverflow.com/questions/192249/… if it contains content that isn't already present there?
Thanks, @CharlesDuffy. Almost all of what I've done is more or less in the top answers over there. I think that my error reporting and related code is better (more complete) than those, but there are 44 answers and some with around a thousand up-votes. I won't bother moving this. The OP's code was actually a pretty good basis, so it didn't take long to hack it into shape — it's something I've done many times over the years.
Hi, thank you so much for your detailed answer! There's a lot in there I didn't have in mind and will now take into consideration. I clearly have much to aspire to :)

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.