Nested command and pipe command in shell script

Question

I'm trying a few tests, in a shell script as below:

line="100:xx"
echo "$line" | grep -Po \\d+

result: 100

but,

line="100:xx"
echo `echo "$line" | grep -Po \\d+`

result is empty

Why?

is echo "$line" displaying the correct value? How about: line="100:xx" echo `echo "$line" | grep -Po \\d+` — filype
– filype, Commented Apr 27, 2015 at 9:25
Because you are using backticks instead of $() and they suck. — user4453924
– user4453924, Commented Apr 27, 2015 at 9:25
It's in the escaping of the backslash. Try echo `echo "$line" | grep -Po \\\\d+` or echo `echo "$line" | grep -Po '\\d+'` (But do switch to $(), backticks have been deprecated for a long time.) — Biffen
– Biffen, Commented Apr 27, 2015 at 9:25

Jonathan Leffler · Accepted Answer · 2015-05-01 23:31:55Z

5

Because backticks allow expansions like double quoted strings, one of your backslashes is being eaten too soon:

$ echo `echo "$line" | grep -Po \\d+ | cat`

$ echo `echo "$line" | grep -Po \\\d+`
100

That being said, just quote the regex

$ echo `echo "$line" | grep -Po '\d+'`
100

Jonathan Leffler

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answered Apr 27, 2015 at 10:21

glenn jackman

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1 Comment

Using single quotes around the regex is the way to go.

Jahid · Accepted Answer · 2015-04-27 12:36:01Z

2

You can do this too:

echo $(echo "$line" | grep -Po \\d+)

to avoid your backslash being get eaten.

answered Apr 27, 2015 at 12:36

Jahid

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Really? Backticks have better results than nested POSIX with pipes.

@Sandburg : Stop using backticks and start using $() instead. This is the new standard...