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I have 10 text files with me that I plan on displaying as an output simulating a manual page on how a user could use my script. However I'm looking for the most optimal way in going about displaying the 10 pages page per page.

In example: If a user press 1 on their keyboard they'll be taken to the first page, and if they press 2 on their keyboard they'll be taken to the second page. I originally planned to use a case structure here. If the user presses 3 it'll open a text file containing my contact info, and if they press 4 they'll go back to the main menu. However considering that I have 10 pages that are to be displayed, it would be very inefficient to use a case construct on this, I believe.

So to summarize, I would only like to use 1 for the user to go to the previous page, and 2 for the user to go to the next page.

I'm kinda bit of stuck in this part of my script for a while now. Thank you very much!

Gifter Villanueva
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  • The conventional way to do this in a bash script is to *make it someone else's problem*. Just use a pager -- ideally, honoring any `PAGER` environment variable set by the user -- to do the work for you. `less` and `more` exist to solve the exact problem you're dealing with. Use them. – Charles Duffy Sep 11 '17 at 15:51
  • `less` or its kin will do a much better job than anything you can build by hand -- it'll check the window size and display only that much content; it'll let the user configure line wrap to their preference; etc. – Charles Duffy Sep 11 '17 at 15:53
  • ...you'll note that `git log` and `systemctl show` and other upstream commands do it this way -- they don't try to implement paging themselves. You shouldn't either. – Charles Duffy Sep 11 '17 at 16:02

2 Answers2

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Every UNIX system has a piece of software that will do this for you -- it's called a pager. Actually most systems have several pagers -- more, the traditional UNIX one, and less, the GNU project's modern, featureful take.

By convention, a system's user can configure their own pager by setting the environment variable PAGER, so they can even decide to have text displayed in some other piece of software -- a window in their pre-opened editor, a GUI window, a text-to-speech system, or whatever else they use and prefer.

The following is an example of how to select a pager in a robust way.

# Function that picks the best available pager
pager() {
  # if stdout is not to a TTY, copy directly w/o paging
  [ -t 1 ] || { cat; return; }

  if [ -n "$PAGER" ]; then  ## honor the user's choice, if they have a pager configured
    "$PAGER"
  elif command -v less >/dev/null 2>&1; then
    less
  elif command -v more >/dev/null 2>&1; then
    more
  else
    echo "WARNING: No pager found; falling back to cat" >&2
    cat
  fi
}

pager <<'EOF'
here is our documentation
several pages of content here
etc, etc, etc
EOF
Charles Duffy
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  • There's a typo in `command- v more >/dev/null`. Also in bash double square brackets are preferred (less error-prone), – Hubert Grzeskowiak Nov 06 '17 at 00:56
  • Also the quotes around `EOF` are unnecessary. – Hubert Grzeskowiak Nov 06 '17 at 00:57
  • @HubertGrzeskowiak, the quotes on `EOF` are meaningful: They stop any expansions present in the heredoc from being processed. If your documentation includes an example that could be processed by the shell -- like `$(foo)` or `$bar` -- a regular heredoc will run the expansion; a quoted one won't. – Charles Duffy Nov 06 '17 at 01:28
  • @HubertGrzeskowiak, ...as for the decision between `[ ]` and `[[ ]]`, the function as currently written is POSIX-compliant, and so works with ash or dash; making that change would make it bash-only. I'm all for using bashisms when they improve correctness, but when there's no value, why pay the cost? – Charles Duffy Nov 06 '17 at 01:30
  • Ah I didn't know about that EOF-quoting feature. Thanks! Re brackets: the OP asked about bash (in the tags), but yeah it makes not much difference here. – Hubert Grzeskowiak Nov 06 '17 at 02:50
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I was able to find out a solution for this and it just so happened that I could stack the 10 manual pages into an array and scroll through them one by one. Here's my solution. 

 declare -a manual=("$pg0" "$pg1" "$pg2" "$pg3" "$pg4" "$pg5" "$pg6" "$pg7" "$pg8" "$pg9" "$pg10")

promptchoice()
{
echo -e "\nPress the following keys to browse through this section!
1) Previous Page
2) Next Page
3) Contact Details
4) Back to main Menu"
echo -n "Your selection: "
}

pagearray()
{
x=0
clear
#promptchoice
#read a
while [ $x -lt "11" ]

do
promptchoice
read a    
    if [ $a -eq 1 -a $x -gt 0 ];
    then echo -e "${manual[$"x-1"]}\n"
    x=$(($x-1))


    elif [ $a -eq 2 -a $x -lt 10 ];
    then echo -e "${manual[$"x+1"]}\n" 
    x=$(($x+1))


    elif [ $a -eq 3 ];
    then echo -e "$contact\n"

    elif [ $a -eq 4 ];
    then bash mainmenu.sh

    else [ $a -eq * ];
    echo "Error in your input! Did you press previous despite not being on any page first?"
    echo "Or perhaps you went over the last page and still pressed next?"
    echo "Please try again!"
    sleep 3
    pagearray

if ($x = "11");
    then
    echo "You have seen the last customer care page! You will now be taken back to the prev menu."
    sleep 2
    pagearray
    fi

    fi

done
}

Hopefully someone who might have the same problem as I do finds this helpful as well.

Gifter Villanueva
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