Skip to main content

sudo, pipelines, and complex commands with quotes

We've all run into problems like this:

$ echo 12000 > /proc/sys/vm/dirty_writeback_centisecs
-bash: /proc/sys/vm/dirty_writeback_centisecs: Permission denied

The command fails because the target file is only writeable by root. The fix seems obvious and easy:

$ sudo echo 12000 > /proc/sys/vm/dirty_writeback_centisecs
 -bash: /proc/sys/vm/dirty_writeback_centisecs: Permission denied

Huh? It still fails. What gives? The reason it fails is that it is the shell that sets up the re-direction before running the command under sudo. The solution is to run the whole pipeline under sudo. There are several ways to do this:

echo 'echo 12000 > /proc/sys/vm/dirty_writeback_centisecs' | sudo sh
sudo sh -c 'echo 12000 > /proc/sys/vm/dirty_writeback_centisecs'
echo 12000 | sudo tee /proc/sys/vm/dirty_writeback_centisecs

This is fine for simple commands, but what if you have a complex command that already includes quotes and shell meta-characters?

Here's what I use for that:

sudo su <<\EOF
echo 12000 > /proc/sys/vm/dirty_writeback_centisecs
EOF

Note that the backslash before EOF is important to ensure meta-characters are not expanded.

Finally, here's an example of a command for which I needed to use this technique:

sudo sh  << \EOF
perl -n -e '
use strict;
use warnings;
if (/^([^=]*=)([^\$]*)(.*)/) {
  my $pre = $1;
  my $path = $2;
  my $post = $3;
  (my $newpath = $path) =~ s/usr/usr\/local/;
  $newpath =~ s/://g;
  print "$pre$newpath:$path$post\n"
}
else {
  print
}' < /opt/rh/ruby193/enable > /opt/rh/ruby193/enable.new
EOF
Labels:

Comments

Post a Comment

Popular posts from this blog

Python logging with rich - writing to stderr - plain output when writing to file

Rich is a Python library for writing rich text (with color and style) to the terminal, and for displaying advanced content such as tables, markdown, and syntax highlighted code. Rich provides RichHandler , a logging handler for python's logging module which will format and colorize text written by the module. However, RichHandler writes to stdout by default. More specifically, it writes to a rich Console object which, by default, writes to stdout. To make RichHandler write to stderr by default, you must pass in a Console object which has been configured to write to stderr: import logging from rich.console import Console from rich.logging import RichHandler DATEFMT = "%Y-%m- %d T%H:%M:%SZ" FORMAT = " %(message)s " logging . basicConfig( level = "NOTSET" , format = FORMAT, datefmt = DATEFMT, handlers = [RichHandler(console = Console(stderr = True ))], ) logger = logging . getLogger(__name__) logger . i...
Post a Comment
Read more

Fix python import order on save in vim with ruff and ale

My IDE of choice is vim. I use various tools to perform linting and code formatting, and configure them all with ALE  (the Asynchronous Lint Engine). After using several discrete tools ( black , isort , flake8 , etc) I have settled on using Ruff to do my python code formatting and linting. Here's the relevant fragment of my ALE config in my .vimrc: " ALE config let g :ale_fixers = { \ 'python' : [ 'ruff' , 'ruff_format' ], \} let g :ale_linters = { \ 'python' : [ 'ruff' ], \} let g :ale_python_ruff_use_global = 1 One of the last remaining wrinkles I had was getting Ruff to automatically sort import statements. Sorting imports is performed by the Ruff linter, not the formatter, which is documented here . The fix on the command line is to add an option, like this: ruff check --select I --fix The difficulty I had was getting this to happen in the editor when the file was saved. It turns out, all I needed to do was ...
Post a Comment
Read more

Escaping special characters in wget username or password

I recently offered to help out with the hosting of a WordPress  site. It’s currently hosted somewhere with no shell access – just ftp – and there are a lot of images to transfer. I quickly figured out I could use wget to mirror the site, using something like: wget -m ftp://username:password@example.com However, this broke in this case because the username for the site contained an @ character (the username was user@example.com ). Turns out the solution was to encode the special chars using HTML notation. This is the command that did the trick: wget -m ftp://user%40example.com:password@example.com
Post a Comment
Read more