Using awk to add a prefix

Author: simonw

bash/use-awk-to-add-a-prefix.md

@@ -0,0 +1,37 @@
+# Using awk to add a prefix
+
+I wanted to dynamically run the following command against all files in a directory:
+
+```bash
+pypi-to-sqlite content.db -f /tmp/pypi-datasette-packages/packages/airtable-export.json \
+-f /tmp/pypi-datasette-packages/packages/csv-diff.json \
+--prefix pypi_
+```
+
+I can't use `/tmp/pypi-datasette-packages/packages/*.json` here because I need each file to be processed using the `-f` option.
+
+I found a solution using `awk`. The `awk` program `'{print "-f "$0}'` adds a prefix to the input, for example:
+```
+% echo "blah" | awk '{print "-f "$0}'      
+-f blah
+```
+I wanted that trailing backslash too, so I used this:
+
+```awk
+{print "-f "$0 " \\"}
+```
+Piping to `awk` works, so I combined that with `ls ../*.json` like so:
+
+```
+% ls /tmp/pypi-datasette-packages/packages/*.json | awk '{print "-f "$0 " \\"}' 
+-f /tmp/pypi-datasette-packages/packages/airtable-export.json \
+-f /tmp/pypi-datasette-packages/packages/csv-diff.json \
+-f /tmp/pypi-datasette-packages/packages/csvs-to-sqlite.json \
+```
+Then I used `eval` to execute the command. The full recipe looks like this:
+```bash
+args=$(ls /tmp/pypi-datasette-packages/packages/*.json | awk '{print "-f "$0 " \\"}')
+eval "pypi-to-sqlite content.db $args
+--prefix pypi_"
+```
+Full details in [datasette.io issue 98](https://github.com/simonw/datasette.io/issues/98).