Occasionally useful Linux tricks

List the contents of all files in a folder

# Lists contents of all files in current directory, with filename and line-number prepended to each line
grep -n . *

# Recursively list contents of all files in current directory and subdirectories, with filename and line-number prepended to each line
grep -Rn .

You’ve been added to more groups, but don’t want to log off and back on again to use the new privileges:

sudo sudo -u mark bash

The first sudo gives us root access which is necessary for the second sudo, which logs us back in as ourself and starts a bash shell. This shell has the privileges of the new groups you were added to.

Transferring data over a slow network:

# Both of these are too slow due to our crappy internet connection
ssh user@server 'producer' | consumer
producer | ssh user@server 'consumer'

# If our CPU is sitting idle while waiting for the data to transfer, let's give it some work to do!
ssh user@server 'producer | pbzip2 -c9' | pbzip2 -d | consumer
producer | pbzip2 -c9 | ssh user@server 'pbzip2 -d | consumer'

These use the multithreaded implementation of bzip2 to achieve fairly fast and powerful compression to squeeze information along the wire faster.

If pbzip2 leaves you CPU-bottlenecked, you can reduce the compression level (e.g. -c5 instead of -c9) or use gzip which is faster but won’t compress as well. To use parallel gzip, you’ll want to replace pbzip2 with pigz.

If you still have plenty of CPU and also RAM to spare when using pbzip2 and the transfer is taking too long, try parallel lzma instead with pxz in place of pbzip2.

Monitoring the progress of transfers / measuring speed of devices

# Test sequential speed of disk sda by reading first 4GB of the disk (sudo needed for raw disk access)
sudo pv -bartpSs 4G /dev/sda > /dev/null

# File archiving over SSH (with pbzip2 as shown previously)
size="$(ssh user@server 'du -csB1 /path/to/files')"
ssh -T user@server "tar -c /path/to/files | pv -cbarteps ${size} --force | pbzip2 -c9" > /path/to/archives/name.tar.bz2

Running the above operations without making the machines grind to a halt

# CPU-heavy workloads can be told to play nicely by prepending them with "nice"
... | nice pbzip2 -c9 | ...

# IO-heavy workloads can be told to play nicely by giving them idle priority with "ionice"
ionice -c3 tar -c /path/to/files | ... | ionice -c3 > /path/to/archives/name.tar.bz2

# Example from (3) with progress:
size="$(ssh user@server 'du -csB1 /path/to/files')"
ssh -T user@server "ionice -c3 tar -c /path/to/files | pv -cbarteps ${size} --force | nice pbzip2 -c9" | ionice -c3 cat > /path/to/archives/name.tar.bz2

Firing up Eclipse CDT in a temporary Ubuntu environment with Docker

# Download an Ubuntu image
docker pull ubuntu

# Install eclipse-cdt
docker run -i ubuntu apt-get -y install eclipse-cdt

# Get ID of that container (which is now dead)
docker ps -a

# Snapshot that container
docker commit [ID] eclipse-cdt

# Run eclipse with workspace directory mapped to the host (select /root/workspace when Eclipse asks for workspace path)
docker run -v /tmp/.X11-unix:/tmp/.X11-unix -e DISPLAY=unix$DISPLAY -v ~/eclipse-workspace:/root/workspace eclipse-cdt eclipse

Speed & ping tests from shell

Having *finally* got internet access in our new London flat, a month after we ordered it, I wanted to test the quality of the connection.

Downstream speed test

Use a compressed Linux kernel source for our speed test, taking advantage of how cURL annoyingly violates the UNIX “rule of silence”:

$ curl 'https://cdn.kernel.org/pub/linux/kernel/v4.x/linux-4.3.tar.xz' > /dev/null

As the file is highly compressed, we are measuring raw bandwidth regardless of whether some part of our link has transparent compression. When the speed has stabilised, Ctrl+C to end the test.

Ping test

Run as root, required for ping flooding. Google probably won’t feel/notice/mind you ping-flooding their DNS servers:

$ ping -f -c 1000 -i 0.03 -W 1 -s 1350 -M do

Adjust 1350 to be slightly less than your MTU (1350 is a safe guess if you’re unsure). When the flood is complete (~10 seconds), you should receive your mean ping time, jitter, packet loss, etc.

Easy sandboxing for improved privacy and security

I have created a script to allow easy sandboxing of applications, by running them in a separate temporary single-use user account. By controlling which groups the temporary user is added to, one can control hardware access (e.g. audio, webcam, networking, etc). With permissions set correctly, one can protect their home folder from the prying eyes of dodgy closed-source software.

This script is not intended to be a *perfect* security solution – it is far from perfect, but will suffice in most situations where a little extra isolation is required.

There are two scripts:

The first script (sandbox) is used to launch a program in a sandbox. The second script (sandchrome) demonstrates this by launching an incognito chrome session in a sandbox, with access to audio explicitly granted.