How to trim whitespace from a Bash variable?

I have a shell script with this code:

var=`hg st -R "$path"`
if [ -n "$var" ]; then
    echo $var
fi

But the conditional code always executes, because hg st always prints at least one newline character.

• Is there a simple way to strip whitespace from $var (like trim() in PHP)?

or

• Is there a standard way of dealing with this issue?

I could use sed or AWK, but I'd like to think there is a more elegant solution to this problem.

Solution 1:

A simple answer is:

echo "   lol  " | xargs

Xargs will do the trimming for you. It's one command/program, no parameters, returns the trimmed string, easy as that!

Note: this doesn't remove all internal spaces so "foo bar" stays the same; it does NOT become "foobar". However, multiple spaces will be condensed to single spaces, so "foo bar" will become "foo bar". In addition it doesn't remove end of lines characters.

Solution 2:

Let's define a variable containing leading, trailing, and intermediate whitespace:

FOO=' test test test '
echo -e "FOO='${FOO}'"
# > FOO=' test test test '
echo -e "length(FOO)==${#FOO}"
# > length(FOO)==16

How to remove all whitespace (denoted by [:space:] in tr):

FOO=' test test test '
FOO_NO_WHITESPACE="$(echo -e "${FOO}" | tr -d '[:space:]')"
echo -e "FOO_NO_WHITESPACE='${FOO_NO_WHITESPACE}'"
# > FOO_NO_WHITESPACE='testtesttest'
echo -e "length(FOO_NO_WHITESPACE)==${#FOO_NO_WHITESPACE}"
# > length(FOO_NO_WHITESPACE)==12

How to remove leading whitespace only:

FOO=' test test test '
FOO_NO_LEAD_SPACE="$(echo -e "${FOO}" | sed -e 's/^[[:space:]]*//')"
echo -e "FOO_NO_LEAD_SPACE='${FOO_NO_LEAD_SPACE}'"
# > FOO_NO_LEAD_SPACE='test test test '
echo -e "length(FOO_NO_LEAD_SPACE)==${#FOO_NO_LEAD_SPACE}"
# > length(FOO_NO_LEAD_SPACE)==15

How to remove trailing whitespace only:

FOO=' test test test '
FOO_NO_TRAIL_SPACE="$(echo -e "${FOO}" | sed -e 's/[[:space:]]*$//')"
echo -e "FOO_NO_TRAIL_SPACE='${FOO_NO_TRAIL_SPACE}'"
# > FOO_NO_TRAIL_SPACE=' test test test'
echo -e "length(FOO_NO_TRAIL_SPACE)==${#FOO_NO_TRAIL_SPACE}"
# > length(FOO_NO_TRAIL_SPACE)==15

How to remove both leading and trailing spaces--chain the seds:

FOO=' test test test '
FOO_NO_EXTERNAL_SPACE="$(echo -e "${FOO}" | sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//')"
echo -e "FOO_NO_EXTERNAL_SPACE='${FOO_NO_EXTERNAL_SPACE}'"
# > FOO_NO_EXTERNAL_SPACE='test test test'
echo -e "length(FOO_NO_EXTERNAL_SPACE)==${#FOO_NO_EXTERNAL_SPACE}"
# > length(FOO_NO_EXTERNAL_SPACE)==14

Alternatively, if your bash supports it, you can replace echo -e "${FOO}" | sed ... with sed ... <<<${FOO}, like so (for trailing whitespace):

FOO_NO_TRAIL_SPACE="$(sed -e 's/[[:space:]]*$//' <<<${FOO})"