====== Print a horizontal line ======

---- dataentry snipplet ----
snipplet_tags: terminal, line
LastUpdate_dt: 2010-07-31
Contributors: Jan Schampera, prince_jammys, ccsalvesen, others
type: snipplet
----

The purpose of this small code collection is to show some code that draws a horizontal line using as less external tools as possible (it's not a big deal to do it with AWK or Perl, but with pure or nearly-pure Bash it gets more interesting).

In general, you should be able to use this code to repeat any character or character sequence.

===== The simple way: Just print it =====
Not a miracle, just to be complete here.

<code bash>
printf '%s\n' --------------------
</code>

===== The iterative way =====
This one simply loops 20 times, always draws a dash, finally a newline

<code bash>
for ((x = 0; x < 20; x++)); do
  printf %s -
done
echo
</code>

===== The simple printf way =====
This one uses the ''printf'' command to print an **empty** field with a **minimum field width** of 20 characters. The text is padded with spaces, since there is no text, you get 20 spaces. The spaces are then converted to ''-'' by the ''tr'' command.

<code bash>
printf '%20s\n' | tr ' ' -
</code>

whitout an external command, using the (non-POSIX) substitution expansion and ''-v'' option:
<code bash>
printf -v res %20s
printf '%s\n' "${res// /-}"
</code>

===== A line across the entire width of the terminal =====
This is a variant of the above that uses ''tput cols'' to find the width of the terminal and set that number as the minimum field witdh.

<code bash>
printf '%*s\n' "${COLUMNS:-$(tput cols)}" '' | tr ' ' -
</code>

===== The more advanced printf way =====
This one is a bit tricky. The format for the ''printf'' command is ''%.0s'', which specified a field with the **maximum** length of **zero**. After this field, ''printf'' is told to print a dash. You might remember that it's the nature of ''printf'' to repeat, if the number of conversion specifications is less than the number of given arguments. With brace expansion ''{1..20}'', 20 arguments are given (you could easily write ''1 2 3 4 ... 20'', of course!). Following happens: The **zero-length field** plus the dash is repeated 20 times. A zero length field is, naturally, invisible. What you see is the dash, repeated 20 times.

<code bash>
# Note: you might see that as ''%.s'', which is a (less documented) shorthand for ''%.0s''
printf '%.0s-' {1..20}; echo
</code>

If the 20 is variable, you can use [[commands:builtin:eval | eval]] to insert the expansion (take care that using ''eval'' is potentially dangerous if you evaluate external data):

<code bash>
eval printf %.0s- '{1..'"${COLUMNS:-$(tput cols)}"\}; echo
</code>

Or restrict the length to 1 and prefix the arguments with the desired character.

<code bash>
eval printf %.1s '-{1..'"${COLUMNS:-$(tput cols)}"\}; echo
</code>

You can also do it the crazy ormaaj way™ following basically the same principle as this [[commands:builtin:eval#expansion_side-effects | string reverse example]]. It completely depends on Bash due to its brace expansion evaluation order and array parameter parsing details. As above, the eval only inserts the COLUMNS expansion into the expression and isn't involved in the rest, other than to put the ''_'' value into the environment of the ''_[0]'' expansion. This works well since we're not creating one set of arguments and then editing or deleting them to create another as in the previous examples.

<code bash>
_=- command eval printf %s '"${_[0]"{0..'"${COLUMNS:-$(tput cols)}"'}"}"'; echo
</code>

===== The parameter expansion way =====

Preparing enough dashes in advance, we can then use a non-POSIX subscript expansion:

<code bash>
hr=---------------------------------------------------------------\
----------------------------------------------------------------
printf '%s\n' "${hr:0:${COLUMNS:-$(tput cols)}}"
</code>

A more flexible approach, and also using modal terminal line-drawing characters instead of hyphens:

<code bash>
hr() {
  local start=$'\e(0' end=$'\e(B' line='qqqqqqqqqqqqqqqq'
  local cols=${COLUMNS:-$(tput cols)}
  while ((${#line} < cols)); do line+="$line"; done
  printf '%s%s%s\n' "$start" "${line:0:cols}" "$end"
}
</code>

===== Related articles =====
  * [[commands:builtin:printf]]