Bash: assigning output of command substitution to an integer variable

Question

In Bash, I need an integer variable, like this:

declare -i n_jobs

to be assigned as a value the number of current background jobs:

jobs | wc -l

If I assign it as so:

n_jobs=$(jobs | wc -l)

is seems like a working integer, e.g.:

echo $((++n_jobs))

but... printing it (without running the increment above) reminds me that it contains blanks:

$ echo "$n_jobs"
$ <space><space><space><space><space>4

so I resort to this construct:

n_jobs=$(( $(jobs | wc -l) ))

to force immediate "casting" to int.

Is there a better way to take the output of a command substitution list and assign it to a variable as an integer?

The integer flag in bash is almost never used. Why do you actually care that it's set here? — Charles Duffy
– Charles Duffy, Commented Mar 24, 2013 at 23:41
If the flag exists, it marks a variable as int, I know I want only int there, why would I not use what seems like a very convenient flag, designed exactly to express this condition? — Robottinosino
– Robottinosino, Commented Mar 24, 2013 at 23:42
Without specifying the integer flag, the spaces I mention appear.. it has its uses! — Robottinosino
– Robottinosino, Commented Mar 25, 2013 at 0:03
The only thing the integer attribute does is causes the RHS of an assignment to be evaluated as an arithmetic expression. It's not as useful as it sounds: x=7+9 works, but x=3*(7+9) still produces a syntax error and needs to be written as x=$(( 3*(7+9) )). A simple comment the first time you assign to a variable stating # n_jobs is an integer would work just as well. Also, I cannot reproduce the spaces you are observing. — chepner
– chepner, Commented Mar 25, 2013 at 0:08

chepner · Accepted Answer · 2013-03-25 00:50:44Z

4

Still bash-specific, but a cleaner way of getting a space-free integer might be:

job_pids=( $(jobs -p) )
n_jobs=${#job_pids[@]}

A POSIX-compliant version:

n_jobs=$( jobs -p | awk '{print NR}' )

(The -p isn't necessary, but you really don't need anything more than a unique line per job to feed to awk, so may as well make the lines as brief as possible.)

edited Mar 25, 2013 at 0:50

answered Mar 25, 2013 at 0:29

chepner

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1 Comment

Robottinosino Over a year ago

Whoa.. I like this one!

DigitalRoss · Accepted Answer · 2013-03-24 23:55:45Z

3

It's not really necessary, but you could do something like:

n_jobs=$(($n_jobs + 0))

Or

n_jobs=`expr $(jobs | wc -l)`

I would suggest that you actually avoid declare ... it exists in only one shell and it is not specified by Posix.

The shell is partly a macro processor and in its standardized form all variables are kept as strings. This is not a big handicap for jobs appropriate for the shell, and adding declare and integer variables makes bash only microscopically more suitable for complicated things.

Do those more complicated things in something like Ruby, Python, Perl, Go, or perhaps even something like node.js.

And by the way, I couldn't reproduce your original issue. When using declare -i, it does seem to evaluate your expression arithmetically and it does not seem to end up with any leading spaces.

declare -i qn_jobs
qn_jobs=$(jobs | wc -l)
echo "x$qn_jobs"
# => x0

edited Mar 24, 2013 at 23:55

answered Mar 24, 2013 at 23:43

DigitalRoss

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7 Comments

Robottinosino Over a year ago

Well, I am beginner, I forgot to mention that.

DigitalRoss Over a year ago

Sure, np. While I like bash for its nice interactive features and its incorporation of the csh ! history mechanism, it's way too complex to easily digest the manpage and learn shell programming. On Linux you may want to install dash, if for no other reason than to get a readable manpage. Here is another easy one with mostly-pure-Posix features: netbsd.gw.com/cgi-bin/man-cgi?sh++NetBSD-current (That's the shell dash is based on.)

Robottinosino Over a year ago

Seriously? I had only heard little about "dash".. do you recommend it over Bash, even?

Robottinosino Over a year ago

Yeah, if I do not put the -i in there.. I get the spaces! Try it and let me know if you see that too..

Robottinosino Over a year ago

So, even if I got criticism in the comments for using -i it does actually serve that microscopic purpose, doesn't it? (I liked your use of "microscopic", as you can see) ;)

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