Linux Commands

This is simply a list of interesting, useful, or fun Linux commands.

Utility

General

• cd: Go to home
• cd -: Go to previous directory stored in OLDPWD env.
• ALT + .: Grabs the last argument of the last command. You can keep using this.
• CTL + ALT + F#: Changes TTY terminal you are viewing.
• ll: sometimes systems alias ll to ls -l
• lvs: List logical volumes.
• lsblk: List block devices.
• df: Display disk usage.
  • -H: Human readable.
• CTL+Z: Puts a process in the background.
• fg puts the most recent job into the foreground.
• jobs: lists suspended jobs.
• fg #: brings that process to the foreground.
• tac: Prints out the text but backwards.
• bat: Cat but syntax highlighting.
• dd: Used to copy data from one device to another (blocks). We can use this to format the device too.
  • if=<filename/device>: Input
  • of=<filename/device>: Output
• sudo -i: Run multiple sudo commands one after another (looks like sudo su).

Git

• git remote update: This will update all of the branches that we are currently set to track, unlike git fetch which will only update the information for the branch we are currently on. This does not merge the changes into our current files!
• git push --mirror <URL>: Push changes to the specified UR. This will also include any refs and things used by the remote branch to track changes. So we would use this if we are uploading our entire repository to another hosting provider. You should only use this once on the initial push!
• git status: Display the current state of the git repository. Are there untracked modifications, what has and has not been staged? This helps us track the state of the system!
• git checkout: Updates files to match the given tree or commit ID (index).
  • -b: This causes a new branch to be created.
• git switch: Allows us to switch the branch we are currently on.
• git branch: Allows us to list the current branch we are on with no arguments, or if arguments are provided it will create those branches.
• git format-patch -1 --cover-letter -v1 --rfc:
  • Remember to edit hte file to add a subject and a blurb.
• git send-email: Sends email based on the git configurations of the user.
  • (Optional) --smtp-debug=1: Displays debugging information.
  • --to=setup@COURSE_DOMAIN: Location to send.
  • v1*.patch: Files to send (If you create different versions change this!)
• git grep: Grep search within the bounds of the git repository ignoring git files and those ignored by .gitignore
  • -n: Tells us the line number.

GDB

• r: Run
• bt: Backtrace
• l: Show current command if compiled with -g
• disass: disassemble instructions

GCC

• gcc: GNU C Compiler
  • -o: Output file name. Traditionally will be a.out due to being assembler output.
  • -ffreestanding: No main function.
  • -nostdlib: Do not link to c-standard library.

Assemblers/Objects

• as: Assembler used by gcc most of the time.
• gas: GNU assembler
• nm: List symbols from a specified object file.
• readelf: Parse the else file
• objdump: Dump the contents of an executable or object file.

FTRACE

Important

This will take all your memory and ram if you leave it on.

• /sys/kernel/tracing/*: Directory containing files to control, and configure the ftrace utilities.
• /sys/kernel/tracing/set_ftrace_pid: Filter based on the PID of a process
• /sys/kernel/tracing/tracing_on: Put 0 in to stop tracing and 1 to start tracing.
• /sys/kernel/tracing/trace: Contains results of the trace.

Note

You can use echo to modify the contents of the files, you can also use dd to copy the trace file to a place to analyze.

SSH

• ssh -L <listen-port>:<target:port> user@hostname: Setup a local listener that will forward traffic over the ssh tunnel to the specified target through the remote machine you connected to.
• ssh -R <listen-port>:<target:port> user@hostname: Setup a remote listener that will forward traffic over the ssh tunnel to the specified target from the remote machine through the ssh tunnel to yours.

Note

Generally we pair the ssh tunnels (forwarding) with a -N and -f to not request a shell and put it in the background.

Making

• make -j $(nproc): The -j flag allows us to use multiple threads in-parallel to build the project. The nproc command returns the number of CPU cores on your systems.
• patch: Patch command used to apply the .patch files onto the current directory. As we are sending .patch files to one another this is how we will be testing them (really wrapped around git am).
• diff: Program that displays the differences between two files. This is what makes up the majority of a .patch file.

Tar

• tar -tf: allows us to view the files within a tar file without extracting them.
  • Add a -v for more file information, similar to the output we would see from ls -l

Logs

• dmsg: View kernel code.
  • -h: Puts the output in human readable form.

Mutt

• /: Like less this allows us to search for something (the name of our targets!).

Packages Fedora

• dnf -C rq --provides <PATH TO BIN>: Query what package provides this thing

Fun

• write: Command used to write messages to the other terminal. Simply injects the messages onto the screen.
  • writeall: This will send to all users.
  • That is wall
• ls /proc/PROCNUM/fd lists the file descriptors used by the process.
• time: This will time how long a command takes (user and system time).