Bash: How to trim whitespace before using cut

问题:

I have a line of output from a command like this:

[]$ <command> | grep "Memory Limit"
Memory Limit:           12345 KB

I'm trying to pull out the 12345. The first step - separating by the colon - works fine

[]$ <command> | grep "Memory Limit" | cut -d ':' -f2
           12345 KB

Trying to separate this is what's tricky. How can I trim the whitespace so that cut -d ' ' -f1 returns "12345" rather than a blank?

回答1:

Pipe your command to awk, print the 3rd column, a space and the 4th column like this

<command> | awk '{print $3, $4}'

This results in:

12345 KB

or

<command> | awk '{print $3}'

if you want the naked number.

回答2:

tr helps here.

$ echo "Memory Limit:           12345 KB" | tr -s " " | cut -d " " -f3
12345

• tr -s " " - squeeze all spaces into one
• cut -d " " -f3 - split into columns by space and select third

回答3:

You can use awk and avoid using any cut, sed or regex:

<command> | awk '/Memory Limit/{print $(NF-1)}'

12345

• /Memory Limit/ will make this print only a line with Memory Limit text
• $(NF-1) will get you last-1th field in input.

回答4:

do you have to use cut? cut is only for single character delimiter extraction. i don't think cut can be as easy as you would expect.

sed is straightforward:

$ echo "Memory Limit:           12345 KB" | sed 's/.*:\s*//'
12345 KB

explanation:

.*:\s* matches all letters before the last colon, then matches all the empty chars after that and delete them by substituting to an empty string.

---

it turns out that you were expecting a single number. then i would say just go ahead and match the numbers:

$ echo "Memory Limit:           12345 KB" | grep -o -P '\d+'
12345

回答5:

bash also has regular expression matching you can use.

result=$(<command> | grep "Memory Limit")
regex='Memory Limit:[[:space:]]+([[:digit:]]+)[[:space:]]KB'
if [[ $result =~ $regex ]]; then
  result=${BASH_REMATCH[0]}
else
  # deal with an unexpected result here
fi

The value of $regex can be adjusted as necessary.

回答6:

awk is perhaps the best for this task, but here is an unorthodox way

$ grep -oP "(?<=Memory Limit:)\s+[0-9]+" file | xargs

lookbehind for matching the label and output only matching, use xargs to eat up spaces