Bash add trailing spaces to justify string

If those characters are all single-byte, that is if you're in a locale where the charset is single-byte (like iso8859-1) or if the locale's charset is UTF-8 but the text is ASCII only, you can do:

printf -v y %-40.40s "$y"

That will cover both truncating and padding.

If not, you can always add 40 spaces and use your ${y:0:40} approach.

printf -v pad %40s
y=$y$pad
y=${y:0:40}

zsh has dedicated operators for left and right padding:

y=${(r:40:)y}

(also does truncation). zsh's printf counts in characters instead of bytes, so wouldn't have bash's issue above. However note that you need zsh 5.3 or newer for the -v option.

See also this answer to a related question for more details if you're faced with characters that don't all have the same width.

You should try printf:

printf '%-40s' "$y"

Pure bash:

ten="          " 
forty="$ten$ten$ten$ten" 
y="very short text"
y="${y:0:40}${forty:0:$((40 - ${#y}))}"
echo "'${y}'"

The method is to add 0-40 spaces to every string after truncating it.

Output, (note the single quote positions):

'very short text                         '