How do I iterate through each line of a command's output in bash?

Question

I have a script that reads from `/proc/stat` and calculates CPU usage. There are three relevant lines in `/proc/stat`: ``` cpu 1312092 24 395204 12582958 77712 456 3890 0 0 0 cpu0 617029 12 204802 8341965 62291 443 2718 0 0 0 cpu1 695063 12 190402 4240992 15420 12 1172 0 0 0 ``` Currently, my script only reads the first line and calculates usage from that: ``` cpu=($( cat /proc/stat | grep '^cpu[^0-9] ' )) unset cpu[0] idle=${cpu[4]} total=0 for value in "${cpu[@]}"; do let total=$(( total+value )) done let usage=$(( (1000*(total-idle)/total+5)/10 )) echo "$usage%" ``` This works as expected, because the script only parses this line: ``` cpu 1312092 24 395204 12582958 77712 456 3890 0 0 0 ``` It's easy enough to get only the lines starting with `cpu0` and `cpu1` ``` cpu=$( cat /proc/stat | grep '^cpu[0-9] ' ) ``` but I don't know how to iterate over each line and apply this same process. Ive tried resetting the internal field separator inside a subshell, like this: ``` cpus=$( cat /pr…

Accepted Answer

Change this line in the middle of your loop:

```
IFS=' ' cpu=($cpu)
```

You need this because outside of your loop you're setting `IFS=$'
', but with that setting`cpu($cpu)` won't do what you expect.

Btw, I would write your script like this:

```
#!/bin/bash -e

grep ^cpu /proc/stat | while IFS=$'
' read cpu; do
    cpu=($cpu)
    name=${cpu[0]}
    unset cpu[0]
    idle=${cpu[4]}
    total=0
    for value in "${cpu[@]}"; do
        ((total+=value))
    done
    ((usage=(1000 * (total - idle) / total + 5) / 10))
    echo "$name $usage%"
done
```

The equivalent using `awk`:

```
awk '/^cpu/ { total=0; idle=$5; for (i=2; i<=NF; ++i) { total += $i }; print $1, int((1000 * (total - idle) / total + 5) / 10) }' < /proc/stat
```

How do I iterate through each line of a command's output in bash?

Solution

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