How to trim whitespace from a Bash variable?

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.

There is a solution which only uses Bash built-ins called wildcards:

var="    abc    "
# remove leading whitespace characters
var="${var#"${var%%[![:space:]]*}"}"
# remove trailing whitespace characters
var="${var%"${var##*[![:space:]]}"}"   
printf '%s' "===$var==="

Here's the same wrapped in a function:

trim() {
    local var="$*"
    # remove leading whitespace characters
    var="${var#"${var%%[![:space:]]*}"}"
    # remove trailing whitespace characters
    var="${var%"${var##*[![:space:]]}"}"   
    printf '%s' "$var"
}

You pass the string to be trimmed in quoted form. e.g.:

trim "   abc   "

A nice thing about this solution is that it will work with any POSIX-compliant shell.

Reference

• Remove leading & trailing whitespace from a Bash variable (original source)

Bash has a feature called parameter expansion, which, among other things, allows string replacement based on so-called patterns (patterns resemble regular expressions, but there are fundamental differences and limitations). [flussence's original line: Bash has regular expressions, but they're well-hidden:]

The following demonstrates how to remove all white space (even from the interior) from a variable value.

$ var='abc def'
$ echo "$var"
abc def
# Note: flussence's original expression was "${var/ /}", which only replaced the *first* space char., wherever it appeared.
$ echo -n "${var//[[:space:]]/}"
abcdef

In order to remove all the spaces from the beginning and the end of a string (including end of line characters):

echo $variable | xargs echo -n

This will remove duplicate spaces also:

echo "  this string has a lot       of spaces " | xargs echo -n

Produces: 'this string has a lot of spaces'

Strip one leading and one trailing space

trim()
{
    local trimmed="$1"

    # Strip leading space.
    trimmed="${trimmed## }"
    # Strip trailing space.
    trimmed="${trimmed%% }"

    echo "$trimmed"
}

For example:

test1="$(trim " one leading")"
test2="$(trim "one trailing ")"
test3="$(trim " one leading and one trailing ")"
echo "'$test1', '$test2', '$test3'"

Output:

'one leading', 'one trailing', 'one leading and one trailing'

Strip all leading and trailing spaces

trim()
{
    local trimmed="$1"

    # Strip leading spaces.
    while [[ $trimmed == ' '* ]]; do
       trimmed="${trimmed## }"
    done
    # Strip trailing spaces.
    while [[ $trimmed == *' ' ]]; do
        trimmed="${trimmed%% }"
    done

    echo "$trimmed"
}

For example:

test4="$(trim "  two leading")"
test5="$(trim "two trailing  ")"
test6="$(trim "  two leading and two trailing  ")"
echo "'$test4', '$test5', '$test6'"

Output:

'two leading', 'two trailing', 'two leading and two trailing'