Using Variables in SQLite CLI .read Scripts for Dynamic CSV Imports

Issue Overview: Dynamic File Paths in SQLite CLI .read Scripts

The core challenge revolves around dynamically specifying file paths within SQLite Command-Line Interface (CLI) scripts executed via the .read command. Users often need to parameterize portions of file paths (e.g., month/year identifiers in filenames) to automate repetitive tasks like importing CSV files with time-stamped names. For example, a script may need to import /home/ryan/www/default.website/reports/Jun2022.csv into a table, but the "Jun2022" portion must change programmatically each month without manual editing. SQLite’s CLI lacks native support for variable substitution in dot-commands (e.g., .import), leading to attempts to work around this limitation through shell variables, date/time functions, or filesystem hacks.

The discussion highlights three critical pain points:

1. Variable Substitution Limitations: SQLite CLI does not expand environment variables or shell-style variables (e.g., $monthyear) within .read scripts. Attempts to inject variables directly into dot-commands result in syntax errors or literal interpretation of the variable name as part of the file path.
2. Date/Time Handling Complexity: Generating the correct month/year suffix using SQLite’s strftime requires precise handling of edge cases, such as date arithmetic around month boundaries (e.g., subtracting one month from March 31 may inadvertently yield March 3 instead of February 1 if not adjusted properly).
3. Workflow Integration Challenges: Users seek solutions that integrate cleanly with automation workflows, avoiding manual file renaming or script modification. Proposals include symbolic links, temporary files, or leveraging external scripting languages to preprocess paths before invoking SQLite.

Possible Causes: Why Variables Fail in .read Scripts and Date Arithmetic Misbehaves

SQLite CLI’s Dot-Command Parsing Limitations

SQLite’s CLI processes dot-commands (e.g., .import, .read) using a simple parser that does not support variable interpolation or shell-like expansion. When a script contains $monthyear, the CLI treats it as a literal string rather than substituting it with an environment variable or script-defined value. This is intentional: the CLI is designed for interactive use and basic scripting, not as a full-fledged shell. For example:

.import --csv /path/to/$monthyear.csv Mytable  

This command attempts to import a file named $monthyear.csv (literally), which does not exist, resulting in an error.

Inadequate Date Arithmetic for Month Boundary Conditions

SQLite’s date/time functions can generate incorrect month values when subtracting months from dates near month-end. Consider:

SELECT strftime('%Y-%m', '2022-03-31', '-1 month');  
-- Returns '2022-03' (incorrectly)  

This occurs because SQLite adjusts dates by decrementing the month number without validating day values. March 31 minus one month becomes March 3 (since February has 28 or 29 days), but the month remains March. To avoid this, the start of month modifier must reset the day to 1 before arithmetic:

SELECT strftime('%Y-%m', '2022-03-31', 'start of month', '-1 month');  
-- Returns '2022-02' (correct)  

Without this adjustment, scripts targeting the previous month’s data will fail when run on days exceeding the previous month’s maximum day count.

Overreliance on SQLite for Filesystem Operations

The initial workaround proposed in the discussion uses fsdir() and WriteFile() to dynamically locate and copy CSV files. While creative, this approach is brittle:

• fsdir() is part of SQLite’s fileio extension, which must be explicitly loaded.
• The LIKE clause matching filenames assumes a specific naming convention (e.g., "Jun2022.csv") and may fail if multiple files match the pattern.
• Writing to /tmp/tmp.csv introduces concurrency issues if multiple instances of the script run simultaneously.

Troubleshooting Steps, Solutions & Fixes: Parameterization Strategies for SQLite CLI Scripts

Strategy 1: Shell Script Wrapper with Variable Preprocessing

Step 1: Generate the Dynamic File Path in the Shell
Use a shell script to compute the month/year suffix and pass it to SQLite via command-line arguments or temporary files. Example:

#!/bin/bash  
monthyear=$(date +"%b%Y" --date="last month")  
sqlite3 database.db ".read import_script.sql" "$monthyear"  

Step 2: Modify the SQLite Script to Use Placeholders
In import_script.sql, use ? placeholders for parameters passed via the shell:

.import --csv /home/ryan/www/default.website/reports/?.csv Mytable  

Caveat: This approach works only if the CLI is invoked with parameters (e.g., sqlite3 database.db ".read script.sql" "Jun2022"). However, .import does not accept parameters directly. To resolve this, generate the entire .import command dynamically in the shell:

#!/bin/bash  
monthyear=$(date +"%b%Y" --date="last month")  
echo ".import --csv /home/ryan/www/default.website/reports/${monthyear}.csv Mytable" > import_script.tmp  
sqlite3 database.db ".read import_script.tmp"  
rm import_script.tmp  

Strategy 2: Symbolic Links to Static Filenames

Step 1: Create a Symbolic Link to the Latest CSV
Use a cron job or script to update a symlink pointing to the current month’s CSV:

#!/bin/bash  
monthyear=$(date +"%b%Y" --date="last month")  
ln -sf "/home/ryan/www/default.website/reports/${monthyear}.csv" "/home/ryan/www/default.website/reports/latest.csv"  

Step 2: Reference the Symlink in the SQLite Script
In import_script.sql, import the static symlink path:

.import --csv /home/ryan/www/default.website/reports/latest.csv Mytable  

This decouples the SQLite script from dynamic filename generation, simplifying maintenance.

Strategy 3: In-SQLite Date Arithmetic with Boundary Handling

Step 1: Compute the Correct Month/Year in SQLite
Use strftime with start of month to avoid day-related arithmetic errors:

WITH params AS (
  SELECT 
    strftime('%m', 'now', 'localtime', 'start of month', '-1 month') AS month,
    strftime('%Y', 'now', 'localtime', 'start of month', '-1 month') AS year
)
SELECT 
  CASE month
    WHEN '01' THEN 'Jan' WHEN '02' THEN 'Feb' WHEN '03' THEN 'Mar'
    WHEN '04' THEN 'Apr' WHEN '05' THEN 'May' WHEN '06' THEN 'Jun'
    WHEN '07' THEN 'Jul' WHEN '08' THEN 'Aug' WHEN '09' THEN 'Sep'
    WHEN '10' THEN 'Oct' WHEN '11' THEN 'Nov' WHEN '12' THEN 'Dec'
  END || year AS monthyear
FROM params;  

Step 2: Use the Generated monthyear in File Operations
If the fileio extension is available, export the CSV path to a temporary file:

WITH ... (as above)
SELECT WriteFile('/tmp/monthyear.txt', monthyear) FROM params;  

Step 3: Read the Temporary File in the Shell
After running the SQLite script, read /tmp/monthyear.txt to construct the .import command. This hybrid approach requires splitting the workflow into SQL and shell components.

Strategy 4: Custom Builds with Tcl Extension Support

Step 1: Build SQLite with Tcl Extension
Compile SQLite with the tcl_shell_extension to enable Tcl variable substitution:

git clone https://github.com/sqlite/sqlite  
cd sqlite  
./configure --enable-tcl --with-tcl=/usr/lib/tcl8.6  
make tcl_shell_extension sqlite3x  

Step 2: Use Tcl Variables in Dot-Commands
In the SQLite CLI, set and use Tcl variables:

.. set monthyear [clock format [clock add [clock seconds] -1 month] -format "%b%Y"]  
.import --csv /home/ryan/www/default.website/reports/$monthyear.csv Mytable  

Note: This requires familiarity with Tcl syntax and custom SQLite builds, limiting portability.

Strategy 5: Parameterized Script Generation with sed/awk

Step 1: Generate the SQLite Script with sed
Use sed to replace placeholders in a template script:

monthyear=$(date +"%b%Y" --date="last month")  
sed "s/MONTHYEAR_PLACEHOLDER/$monthyear/g" template.sql > import_script.sql  

Step 2: Execute the Generated Script

sqlite3 database.db ".read import_script.sql"  

Template.sql:

.import --csv /home/ryan/www/default.website/reports/MONTHYEAR_PLACEHOLDER.csv Mytable  

This approach avoids modifying the original script and integrates seamlessly into CI/CD pipelines.

Strategy 6: Leverage SQLite’s .parameter Command

Step 1: Bind Parameters Interactively
While SQLite’s .parameter command does not support dot-commands, it can store values for use in SQL queries:

.parameter init  
.parameter set @monthyear "Jun2022"  

Step 2: Use the Parameter in SQL Queries

INSERT INTO Mytable SELECT * FROM csv_read('/home/ryan/www/default.website/reports/' || @monthyear || '.csv');  

Limitation: Requires the csv_read function (from the csv or fileio extension), and parameters cannot be used directly in .import.

Final Recommendation: Hybrid Shell-SQLite Workflow

For robustness and portability, combine shell-based preprocessing with SQLite’s date functions:

1. Shell Script: Compute the month/year suffix and pass it to SQLite via a temporary script.
2. SQLite Script: Use strftime with start of month to validate the date.
3. Error Handling: Check if the CSV file exists before importing, using fsdir() or shell conditional statements.

Example Shell Script:

monthyear=$(date +"%b%Y" --date="last month")  
csv_path="/home/ryan/www/default.website/reports/${monthyear}.csv"  
if [ ! -f "$csv_path" ]; then  
  echo "Error: $csv_path does not exist" >&2  
  exit 1  
fi  
echo ".import --csv $csv_path Mytable" > import.tmp  
sqlite3 database.db ".read import.tmp"  
rm import.tmp  

This approach minimizes dependencies, handles edge cases, and ensures the script fails gracefully when expected files are missing.