Command Line Magic

Last year, I published Vim Magic, a compilation of useful commands for the Vim editor. I’ve been updating it ever since whenever I come across new tips and tricks. It has been a handy reference for me and hopefully for others as well. This post aims to do the same, but for the Linux/Unix command line.

This is not a beginner’s tutorial on how to use the command line. Instead, it’s a reference for those who are already familiar with the terminal and want to learn some new tricks or revisit older ones.

Scripts

#!/bin/bash Use this as the first line of a bash script to specify the interpreter.

Files

for i in *.png; do convert "$i" "${i%.*}.jpg"; done Convert all .png files to .jpg.
find . -type f -exec grep -El "someWord|someOtherWord" {} \; Find all files in the current directory (and subdirectories) containing someWord or someOtherWord. Use the escaped semicolon to indicate the end of the exec command.
find . -type f | xargs grep -El "something|appstore" Similar to the previous command but uses xargs instead of -exec.
find /path/to/files* -mtime +5 -exec rm {} \; Delete files older than five days.
find . -name '*.zip' -exec unzip {} \; Unzip all zip files in the current directory.
find . -type d | egrep -o '.*src$' Find directories named src but exclude their subdirectories.
find . 2>/dev/null Run find while suppressing permission denied messages.
wdiff one.txt two.txt | colordiff Highlight differences between one.txt and two.txt in color.
diff -rq(w) dir1 dir2 Compare two directory structures.
SOME_FILE=$(<some-file.txt) Load the contents of some-file.txt into the variable SOME_FILE.
cat oldfile.txt | tr -d '\n' > newfile.txt Create newfile.txt as a copy of oldfile.txt but with all newlines removed.
convert file.{jpg,png} Equivalent to convert file.jpg file.png.

With xargs, you can avoid loops by passing the output of one command as input to another:

find . -name "*.png" | xargs rm -rf Recursively delete all .png files.
git branch --merged | grep -v \* | xargs git branch -D Delete all local Git branches that have already been merged.

Users and Groups

cat /etc/passwd | cut -d: -f1 List all users.
cat /etc/group | cut -d: -f1 List all groups.
chown user file Change the owner of a file.
chgrp group file Change the group of a file.
chown user:group file Change both the owner and group of a file.

Networking and SSH

lsof -nP | grep 9092 Find the process using port 9092.
rsync -avz --remove-source-files -e ssh /local/dir remoteuser@remotehost:/remote/dir Transfer files via SSH and delete them from the source after copying.
ssh user@your-server.com -L 2000:25 Open an SSH session with port tunneling.
ssh -Y user@your-server.com Start an X11 session.
sudo route delete default gw 10.0.2.2 eth0 Remove the default gateway on eth0.
sudo route add default gw 192.168.1.254 eth0 Add a default gateway on eth0.
wget -c -t 0 --timeout=60 --waitretry=60 --read-timeout=10 http://example.com/file.mkv Resume a partially downloaded file (-c), retry indefinitely (-t 0), with specific timeouts and retries.
wget -r -np -nH -R index.html http://hostname/aaa/bbb/ccc/ddd/ Download all files and subdirectories from the ddd directory, without ascending (-np), skipping index.html files (-R index.html), and omitting the hostname directory (-nH).

Random Magic

<Ctrl>+r Reverse search: search for a previously used command.
cal Display the current month’s calendar.
sudo apt-get install build-essential checkinstall Install essential tools for building packages (for Debian-based systems).
openssl base64 -in input.txt -out encoded.b64 Encode input.txt in Base64 and save it as encoded.b64.
sudo !! Re-run the last command with sudo.
+command: Prevent the command from being saved in the history.
$( command ) Capture the output of a command.
service --status-all Display the status of all services.
echo "hello world" | sed -e "s/e/o/g" | sed -e "s/lo/a/g" | sed -e "s/world/mundo/g" Translate “hello world” into “hoa mundo” using multiple sed substitutions.
docker ps | awk '{print $8}' Display only the names of current Docker containers.

Remote SSH Sessions Without a Password

ssh-keygen -t rsa -b 2048 Generate an RSA key pair.
ssh-copy-id youruser@yourserver Authorize the key on the remote server.
ssh youruser@yourserver Log in without entering a password.

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