getting last three lines of python output assigned to 3 variables in bash-CodePudding

I have a python script that outputs a long list of files.

The script is ran from bash, and at the end i need to assign the last three files each to a specific variables.

I'm able to capture the output into a bash Variable, but then when I try to pipe it to 'tail' command, it seems to be one line only?

I've tried tips i've seen around, but can't seem to figure out how to work on an individual line basis with output.

it seems that any python output will be interpreted as one line?


echo "TESTING sample multi-line python output"
OUTPUT=$(python -c "for i in range(5): print(i)")

# check output
echo "$OUTPUT"

variable3=$(echo $OUTPUT | tail -1)

echo "VARIABLE CAPTURED"
echo $variable3

here is what i get:

But I actually need to capture variable1 as 3rd line from end, variable2 as 2nd line from end and variable3 as 3rd line from the end

So that in the above example at the end the desired result is:

variable1 = 2
variable2 = 3
variable3 = 4

in order to pass these variables to next stage of the script...

CodePudding user response：

You have get quoting right, this will assign 4 to variable3:

variable3="$(echo "$OUTPUT" | tail -1)"
echo "$variable3"

Similarly, to get variable2:

variable2="$(echo "$OUTPUT" | tail -2 | head -1)"

And variable1:

variable1="$(echo "$OUTPUT" | tail -3 | head -1)"

To sum up, your script should be:

#!/usr/bin/env bash

echo "TESTING sample multi-line python output"
OUTPUT="$(python -c "for i in range(5): print(i)")"

# check output
echo "$OUTPUT"

variable3="$(echo "$OUTPUT" | tail -1)"
variable2="$(echo "$OUTPUT" | tail -2 | head -1)"
variable1="$(echo "$OUTPUT" | tail -3 | head -1)"

echo variable1: "$variable1"
echo variable2: "$variable2"
echo variable3: "$variable3"

CodePudding user response：

So I can't explain why this goes that way, but it seems that pythons print separates lines by '\n' and tail separates lines by \n\r. So I got this working by changing some lines

echo "TESTING sample multi-line python output"
OUTPUT=$(python -c "for i in range(5): print(str(i)   '\n\r', end='')")

# check output
echo "$OUTPUT"

variable3=$(echo $OUTPUT | tail -1)

echo "VARIABLE CAPTURED"
echo $variable3

This will result in

TESTING sample multi-line python output
0
1
2
3
4

VARIABLE CAPTURED
4

CodePudding user response：

One option could be to tail -3 and read those last lines into an array:

#!/bin/bash

echo "TESTING sample multi-line python output"
readarray -t arr <<< "$(python -c "for i in range(5): print(i)" | tail -3)"

echo "VARIABLE CAPTURED"
echo "${arr[0]}"
echo "${arr[1]}"
echo "${arr[2]}"

echo "VIA LOOP"
for var in "${arr[@]}"; do
    echo "$var"
done

Output

TESTING sample multi-line python output
VARIABLE CAPTURED
2
3
4
VIA LOOP
2
3
4

If you really need the full output saved before extracting the three last lines:

OUTPUT="$(python -c "for i in range(5): print(i)")"
readarray -t arr <<< "$(echo "$OUTPUT" | tail -3)"

CodePudding user response：

With bash version >= 4.2:

# disable job control and enable lastpipe to run while loop in current environment    
set  m; shopt -s lastpipe

python ... | while read -r line; do
  variable1="$variable2";
  variable2="$variable3";
  variable3="$line";
done

echo "variable1: $variable1"
echo "variable2: $variable2"
echo "variable3: $variable3"

Output:

variable1: 2
variable2: 3
variable3: 4