bash Cookbook

Starting Simple by Printing Dashes

Problem

To print a line of dashes with a simple command might sound easy—and it is. But as soon as you think you’ve got a simple script, it begins to grow. What about varying the length of the line of dashes? What about changing the character from a dash to a user-supplied character? Do you see how easily feature creep occurs? Can we write a simple script that takes those extensions into account without getting too complex?

Solution

Consider this script:

 1 #!/usr/bin/env bash
 2 # cookbook filename: dash
 3 # dash - print a line of dashes
 4 # options: # how many (default 72)
 5 #         -c X use char X instead of dashes
 6 #
 7 function usagexit ( )
 8 {
 9     printf "usage: %s [-c X] [#]\n" $(basename $0)
10     exit 2
11 } >&2

12 LEN=72
13 CHAR='-'
14 while (( $# > 0 ))
15 do
16     case $1 in
17     [0-9]*) LEN=$1;;
18     -c) shift
19         CHAR=$1;;
20     *) usagexit;;
21     esac
22     shift
23 done

24 if (( LEN > 4096 ))
25 then
26     echo "too large" >&2
27     exit 3
28 fi

29 # build the string to the exact length
30 DASHES=""
31 for ((i=0; i<LEN; i++))
32 do
33     DASHES="${DASHES}${CHAR}"
34 done
35 printf "%s\n" "$DASHES"

Discussion

The basic task is accomplished by building a string of the required number of dashes (or an alternate character) and then printing that string to standard output (STD-OUT). That takes only the six lines from 30–35. Lines 12 and 13 set the default values. All the other lines are spent on argument parsing, error checking, user messages, and comments.

You will find that ...