On: Interesting commands for the Linux shell

[stiangrindvoll]

Doesn't matter how long you've been using a GNU/Linux shell, you'll always learn something new every day. Thanks for this.

[hnlmorg → stiangrindvoll]

Funny enough I was saying this to a work colleague yesterday when I was looking at a shell script and scratching me head wondering how it was working when a variable named $RANDOM wasn't assigned a value. After a little investigation it turned out Bash (and possibly other $SHELL's?) returns a random number with each call of the $RANDOM variable. Handy little trick.

[Tepix → hnlmorg]

Indeed! Also if $RANDOM is too small for your needs, you can use $RANDOM$RANDOM (and it will have different values)

[jwilk → Tepix]

$RANDOM$RANDOM doesn't have uniform distribution.

Use $((RANDOM + (RANDOM << 15))) instead.

[vacri → hnlmorg]

There is also the inbuilt variable $SECONDS, which seconds since the instance of bash was started. Makes it really easy to do 'time elapsed' at the end of a bash script.

    $ echo $SECONDS
    83783
    $ echo $SECONDS
    83784
    $ echo $SECONDS
    83787

[monsieurbanana → vacri]

That's nifty.

    $ echo "$SECONDS / 60 / 60" | bc
    362.38500000000000000000

[1024core → monsieurbanana]

"bc", without the "-l" option, will give you integers only. So the response of the command above will be "362", and not "362.38500000000000000000"

[mikegerwitz → monsieurbanana]

echo "$(( SECONDS / 60 / 60 ))"

[vram22 → monsieurbanana]

echo "scale=2; $SECONDS / 60 / 60" | bc

# will give 2 digits after the decimal point, instead of bc -l which gives more digits of precision after the decimal point, than we may usually want for this particular calculation.

[lozf → vram22]

Recently I needed results that included nothing after the decimal point, so resorted to awk to do the math:

   awk "BEGIN { print int($SECONDS /60 /60)}"

[vram22 → lozf]

Interesting. I suppose you had to type Ctrl-D or Ctrl-Z to signal no input to awk, since it expects some in the above command? Never tried running awk without either a filename as input or stdin coming from a file or a pipe. Will check it and see what happens.

Modifying my example from above:

echo "scale=0; $SECONDS / 60 / 60" | bc

might also work, need to check.

[otherflavors → vram22]

printf "%s 60 60 *2k/p" "$SECONDS" | dc

[alexanderdmitri → vacri]

Yeah, SECONDS is great. I just discovered this a few days ago to get a general idea of execution time for each process a script handles:

# At top of script

SECONDS = 0

# script commands

...

# At bottom of script

duration=$SECONDS

echo "$(($duration / 60)) minutes and $(($duration % 60)) seconds elapsed."

[samueloph → vacri]

I prefer to use time when i want to know how much time some script took:

$ time sleep 2

real 0m2.003s

user 0m0.002s

sys 0m0.001s

[Aissen → stiangrindvoll]

Indeed, I was about to say this. I clicked thinking "No way I'll learn something here". Turns out there's 2 or 3 commands that I didn't know and might prove useful!

[kps → stiangrindvoll]

  > Doesn't matter how long you've been using a GNU/Linux shell

Or a Unix shell, even.