Looping over comma-separated values in Bash

Author: simonw

bash/loop-over-csv.md

@@ -0,0 +1,59 @@
+# Looping over comma-separated values in Bash
+
+Given a file (or a process) that produces comma separated values, here's how to split those into separate variables and use them in a bash script.
+
+The trick is to set the Bash `IFS` to a delimiter, then use `my_array=($my_string)` to split on that delimiter.
+
+Create a text file called `data.txt` containing this:
+```
+first,1
+second,2
+```
+You can create that by doing:
+```bash
+echo 'first,1
+second,2' > /tmp/data.txt
+```
+To loop over that file and print each line:
+```bash
+for line in $(cat /tmp/data.txt);
+do
+  echo $line
+done
+```
+To split each line into two separate variables in the loop, do this:
+```bash
+for line in $(cat /tmp/data.txt);
+do
+  IFS=$','; split=($line); unset IFS;
+  # $split is now a bash array
+  echo "Column 1: ${split[0]}"
+  echo "Column 2: ${split[1]}"
+done
+```
+Outputs:
+```
+Column 1: first
+Column 2: 1
+Column 1: second
+Column 2: 2
+```
+Here's a script I wrote using this technique for the TIL [Use labels on Cloud Run services for a billing breakdown](https://til.simonwillison.net/til/til/cloudrun_use-labels-for-billing-breakdown.md):
+```bash
+#!/bin/bash
+for line in $(
+  gcloud run services list --platform=managed \
+    --format="csv(SERVICE,REGION)" \
+    --filter "NOT metadata.labels.service:*" \
+  | tail -n +2)
+do
+  IFS=$','; service_and_region=($line); unset IFS;
+  service=${service_and_region[0]}
+  region=${service_and_region[1]}
+  echo "service: $service    region: $region"
+  gcloud run services update $service \
+    --region=$region --platform=managed \
+    --update-labels service=$service
+  echo
+done
+```