How can I append values to an empty array in bash shell script?

Question

I am a beginner in bash script programming. I wrote a small script in Python to calculate the values of a list from an initial value up to the last value increasing with 0.5. The Python script is:

# A script for grid corner calculation. 
#The goal of the script is to  figure out the longitude values which increase with 0.5

# The first and the last longitude values according to CDO sinfo
L1 = -179.85
L2 = 179.979
# The list of the longitude values
n = []             

while (L1 < L2):
    L1 = L1 + 0.5
    n.append(L1)

print "The longitude values are:", n 
print "The number of longitude values:", len(n)

I would like to create a same script by bash shell. I tried the following:

!#/bin/bash
L1=-180
L2=180
field=()

while [ $L1 -lt $L2 ]
do
  scale=2
  L1=$L1+0.5 | bc
  field+=("$L1")

done

echo ${#field[@]}

but it does not work. Could someone inform me what I did wrong? I would appreciate if someone helped me.

doesn't work? not good enough. Maybe because floating point comparison isn't working in bash? — Jean-François Fabre
– Jean-François Fabre ♦, Commented Jan 19, 2017 at 12:51
Your she-bang line is wrong, should have been #!/bin/bash — Inian
– Inian, Commented Jan 19, 2017 at 12:52
The array part is fine; it's the assignment to L1 that is wrong. — chepner
– chepner, Commented Jan 19, 2017 at 12:55

chepner · Accepted Answer · 2017-01-19 13:01:59Z

2

You aren't correctly assigning the value to L1. Also, -lt expects integers, so the comparison will fail after the first iteration.

while [ "$(echo "$L1 < $L2" | bc)" = 1 ]; do
do
  L1=$(echo "scale=2; $L1+0.5" | bc)
  field+=("$L1") 
done

If you have seq available, you can use that instead, e.g.,

field=( $(seq -f %.2f -180 0.5 179.5) )

edited Jan 19, 2017 at 13:01

answered Jan 19, 2017 at 12:56

chepner

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1 Comment

Beata Over a year ago

Thank you very much for your quick answer. Both version works correctly.

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