Tag: command line

Bulletproof Bash : Stop script on error

The other day I’ve been puzzled by the results of a cron job script.  The bash script in question was written in a hurry a while back, and I was under the assumption that if any of its steps fail, the whole script will fail.  I was wrong.  Some commands were failing, but the script execution continued.  It was especially difficult to notice, due to a number of unset variables, piped commands, and redirected error output.

Once I realized the problem, I got even more puzzled as to what was the best solution.  Sure, you can check an exit code after each command in the script, but that didn’t seem elegant of efficient.

A quick couple of Google searches brought me to this StackOverflow thread (no surprise there), which opened my eyes on a few bash options that can be set at the beginning of the script to stop execution when an error or warning occurs (similar to use strict; use warnings; in Perl).  Here’s the test script for you with some test commands, pipes, error redirects, and options to control all that.

#!/bin/bash

# Stop on error
set -e
# Stop on unitialized variables
set -u
# Stop on failed pipes
set -o pipefail

# Good command
echo "We start here ..."

# Use of non-initialized variable
echo "$FOOBAR"
echo "Still going after uninitialized variable ..."

# Bad command with no STDERR
cd /foobar 2> /dev/null
echo "Still going after a bad command ..."

# Good command into a bad pipe with no STDERR
echo "Good" | /some/bad/script 2> /dev/null
echo "Still going after a bad pipe ..."

# Benchmark
echo "We should never get here!"

Save it to test.sh, make executable (chmod +x test.sh), and run like so:

$ ./test.sh || echo Something went wrong

Then try to comment out some options and some commands to see what happens in different scenarios.

I think, from now on, those three options will be the standard way I start all of my bash scripts.

 

CakePHP 3 : Remove Shell Welcome Header

CakePHP 3 has an excellent support for command line Shells, Tasks, and Console Tools.  There are a few that are bundled with the framework itself, and that come from a variety of plugins.  And, of course, you can have your own commands, specific to your application.

$ ./bin/cake

Welcome to CakePHP v3.4.3 Console
---------------------------------------------------------------
App : src
Path: /home/leonid/Work/cakephp_test/src/
PHP : 7.0.16
---------------------------------------------------------------
Current Paths:

* app:  src
* root: /home/leonid/Work/cakephp_test
* core: /home/leonid/Work/cakephp_test/vendor/cakephp/cakephp

Available Shells:

[Bake] bake

[DebugKit] benchmark, whitespace

[Migrations] migrations

[CORE] cache, i18n, orm_cache, plugin, routes, server

[app] console

To run an app or core command, type `cake shell_name [args]`
To run a plugin command, type `cake Plugin.shell_name [args]`
To get help on a specific command, type `cake shell_name --help`

There is one tiny little annoyance though.  Sometimes, it’s useful to get an output of the CakePHP Shell and use it in another script.  For example, you might need to get a list of all loaded plugins and loop over them, performing another action, outside of CakePHP.  Say, in a bash script.  Getting a list of loaded plugins is easy with the bundled shell like so:

$ ./bin/cake plugin loaded

Welcome to CakePHP v3.4.3 Console
---------------------------------------------------------------
App : src
Path: /home/leonid/Work/cakephp_test/src/
PHP : 7.0.16
---------------------------------------------------------------
Bake
DebugKit
Migrations

But, as you can see, the output is not very useful for machine processing. The welcome header is in the way.  Sure, you can parse it out with regular expressions, or even a simple line count.  But that lacks elegance.  Is there a better way?  I thought there was.

My first approach was to use the –quiet option, which, I thought, would leave me with just the needed output.  It turns out, that’s not what it does.  It strips out all the output, and there is no list of plugins at all.

The second approach worked out better.  I learned about it from this thread.  The solution is to extend the needed CakePHP shell and overwrite the protected _welcome() method.  Here’s the content of the newly created application level shell in src/Shell/PluginShell.php:

<?php
namespace App\Shell;

use Cake\Shell\PluginShell as Shell;

class PluginShell extends Shell
{
    /**
     * Silence the welcome message
     *
     * @return void
     */
    protected function _welcome()
    {
    }
}

And now running the same command as before produces a cleaner output:

$ ./bin/cake plugin loaded
Bake
DebugKit
Migrations

This now can be easily used in other scripts without any need for regular expressions and other trimming techniques.

awless – a Mighty CLI for AWS

awless is a command line interface to the Amazon AWS.  While Amazon AWS already has its own set of tools for command line interface, awless makes things even simpler, with the following features:

  • run frequent actions by using simple commands
  • easily explore your infrastructure and cloud resources inter relations via CLI
  • ensure smart defaults & security best practices
  • manage resources through robust runnable & scriptable templates (see awless templates)
  • explore, analyse and query your infrastructure offline
  • explore, analyse and query your infrastructure through time

How To Use Git to Manage your User Configuration Files

There is probably a gadzillion different ways that you can manage and synchronize you configuration files (aka dotfiles) between different Linux/UNIX boxes – anything from custom symlink scripts, all the way to configuration management tools like Puppet and Ansible.  Here are a few options to look at if you are not doing it already.

Personally, I’m using Ansible and I’m quite happy with it, as it allows me to have multiple playbooks (base configuration, desktop configuration, development setup, etc), and do more things than just manage my configuration files (install packages and tools that I often need, setup correct permissions, and more).

Recently, I came across this tutorial from Digital Ocean on how to manage your configuration files with git.  Again, there are a few options discussed in there, as even with git, there’s more than one way to do it (TMTOWTDI).

The one that I’ve heard about a long time ago, but completely forgot, and which I think is quite elegant is the approach of separating the working directory from the git repository:

Now, we do things a bit differently. We will start by specifying a different working directory using the core.worktree git configuration option:

git config core.worktree "../../"

What this does is establish the working directory relative to the path of the .git directory. The first ../refers to the ~/configs directory, and the second one points us one step beyond that to our home directory.

Basically, we’ve told git “keep the repository here, but the files you are managing are two levels above the repo”.

I guess, if you stick purely to git, you can offload some of the additional processing, such as permission changes and package installation, into one of the git hooks.  Something like post-checkout or post-merge.

Terminals Are Sexy

Terminals are sexy is a curated list of Terminal frameworks, plugins & resources for CLI lovers.  There is plenty of links to applications, plugins and configurations.  For me personally, the most useful one was the link to sensible Bash configuration.