evaluate program output as an integer in bash

Question

I have an application which outputs a string with a number, like this:

output number is 20.

I have a code which parses output and cutting out only the number:

result=$(./my_script | awk 'print $4')
echo $result

the result output will be "20" as expected, but now, if I would try to use it as an integer, for example:

result=$((result+1))

then I will get an error:

13915: syntax error: operand expected (error token is "20")

Using it as a seq argument will also give an error

$(seq 0 $result) 
seq: invalid floating point argument: ‘\033[?25h\033[?0c20’

trying to print it with %q will give the same result:

printf '%q' $result
'\E[?25h\E[?0c20'

So, it looks like there are some unexpected characters in the string, but I'm not sure how to trim them?

Thank you!

The application has written ANSI escapes like "Make cursor visible" even though stdout is not a terminal. This is not canonical Unix behavior, and the application shouldn't be doing it. It's not supposed to be your responsibility to trim it. Which application is this? — that other guy
– that other guy, Commented May 10, 2018 at 3:36
Specifically, well-behaved programs will look at whether output is a TTY, and only put escape sequences in place if that check returns true (if the program is implemented in shell, this is accomplished with [ -t 1 ] for stdout, [ -t 2 ] for stderr; in C, the call is isatty()). This program is buggy, and you should report those bugs to its author. — Charles Duffy
– Charles Duffy, Commented May 10, 2018 at 4:38
@thatotherguy Actually, i run that script on other machine using:ssh user@host <<'ENDSSH'; result=$(./my_script | awk 'print $4'); echo $result; ENDSSH; Unfortunately, I can't make new lines in comment. — Tutankhamen
– Tutankhamen, Commented May 10, 2018 at 5:59
@CharlesDuffy As I mention in my previous comment - I run it in nested ssh session and try to capture all output, so, probably, I'm getting some garbage too. — Tutankhamen
– Tutankhamen, Commented May 10, 2018 at 6:04
Consider ssh -T to explicitly disable TTY emulation on SSH's part. BTW, echo $result introduces some bugs in and of itself, like replacing a * in the output with a list of files. Better to use echo "$result", or -- to remove everything but numbers -- echo "${result//[^[:digit:]]/}"... or just to avoid the command substitution altogether and let the pipeline write direct to stdout. — Charles Duffy
– Charles Duffy, Commented May 10, 2018 at 16:55

z3nth10n · Accepted Answer · 2018-05-10 02:20:57Z

3

You can try to get the number by using Regex.

It worked for me:

#!/bin/bash

result=$(bash output.sh | sed 's/[^0-9]//g')
r=$((result+1))
echo $r

Hope this helps.

answered May 10, 2018 at 2:20

z3nth10n

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1 Comment

Charles Duffy Over a year ago

When dealing with a single line, sed (or any other external tool) is much higher overhead than bash's built-in string manipulation; for example, result=${result//[^[:digit:]]/}

Diego Torres Milano · Accepted Answer · 2018-05-10 02:27:09Z

0

Or, simply, change the field separator

result=$(./my_script | awk -F '[. ]' '{print $4}')
echo $result

answered May 10, 2018 at 2:27

Diego Torres Milano

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