Useful Command Line Tips

I have been compiling noteworthy command line tools in a Note on my Mac, and thought it would be nice to share them. Example usage is provided for each. I continuously update this page as I find more useful commands.

Basic Movement / Text Entry

Scheduling Tasks (Linux)

Scheduled tasks (cron jobs) run at a certain recurring time. To edit them:

$ crontab -e

The format is space-delimited: minute hour day month day-of-week command. A * means every value of the field.

# Every day at 3am
0 3 * * * [command]

# April 9 at noon
0 12 9 4 * [command]

# 17:00 on the 1st and 15th of February and September
0 17 1,15 2,9 * [command]

# Every other hour
0 */2 * * * [command]

Tip: crontab.guru translates cron schedule expressions into plain English.

Symlinks

Symlinks create an alias to a local file or directory from another location.

$ ln -s /destination /link-name

To rename a symlink, use the -f (force) flag to remove the existing link before creating the new one.

Aliases

Add frequently-used commands as aliases in ~/.bashrc:

alias [alias name]="[command]"

Then reload:

$ source ~/.bashrc

Some useful aliases:

alias ins="sudo apt-get install"
alias h="history"
alias c="clear"
alias grep="grep --color -n"
alias df="df -h"
alias du="du -sh"
alias sc="source $HOME/.bashrc"

To run a command without its alias, prepend \:

$ \df

Install .deb Packages

$ sudo dpkg -i [package.deb]

Git — Rewrite Commit Author

To fix commits made with the wrong email address across history:

$ git filter-branch --env-filter 'if [ "$GIT_AUTHOR_EMAIL" = "old@email.com" ]; then
GIT_AUTHOR_EMAIL=new@email.com;
GIT_AUTHOR_NAME="Your Name";
GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL;
GIT_COMMITTER_NAME="$GIT_AUTHOR_NAME"; fi' -- --all

Then force-push:

$ git push --force

Transfer Files via SCP

# Download
$ scp user@host:/remote/file /local/file

# Upload a file
$ scp /local/file user@host:/remote/file

# Upload a directory
$ scp -r /local/directory user@host:/remote/directory

SSH Tunneling

Access an application on a remote port through an SSH tunnel:

$ ssh -N [user]@[IP] -L [local port]:[local address]:[remote port]

Example — access port 1234 on my server via localhost:4321:

$ ssh -N clrung@christopherrung.com -L 4321:localhost:1234

Show 10 Largest Files in a Directory

$ du -h * | sort -hr | head

Batch Rename Files

$ rename s/"IMG_"/"Ski Trip "/ *

Delete Files Older Than N Days

$ find /path/to/directory/* -mtime +N -exec rm {} \;

Re-run Previous Command as Root

!! is an alias for the last command you ran:

$ sudo !!

List Disk Names and Mount Points

$ lsblk
...
sdf      8:80   0 698.7G  0 disk
└─sdf1   8:81   0 698.7G  0 part /media/timemachine
...

Long-Running Commands with screen

The screen command lets you detach from a long-running process and reattach later.

# Start a screen session
$ screen

# Detach: Ctrl-A then d
# Reattach
$ screen -r

# List open screens
$ screen -ls

# Reattach to a specific screen
$ screen -r 5037

Check if Directory Contains Files

#!/bin/sh
if find /path/to/directory -mindepth 1 -print -quit | grep -q .; then
    echo "not empty"
else
    echo "empty"
fi

Determine Which Process is Using a Port

$ lsof -i tcp:[port]

# Example
$ lsof -i tcp:8000
COMMAND  PID   USER  FD  TYPE  DEVICE  SIZE/OFF  NODE NAME
java    2965  chris   5u  IPv4 ...           0t0   TCP *:irdmi (LISTEN)

Get WAN IP via Terminal

$ curl -s http://whatismyip.akamai.com/

That's it for now! Please let me know if you have any questions or corrections.