Linux

Misc

  • Notes from

  • Resources

  • man <command> displays documentation for command

  • Special Characters

    • “>” redirects the output from a program to a file.
      • “>>” does the same thing, but it’s appending to an existing file instead of overwriting it, if it already exists.

  • Make a project directory and go to that directory

    mkdir code/project
cd $_

  • Get the dependency tree for a binary

    ldd path/to/binary

  • .bashrc is a shell script that Bash runs whenever it is started interactively. It initializes an interactive shell session. You can put any command in that file that you could type at the command prompt.

    • You put commands here to set up the shell for use in your particular environment, or to customize things to your preferences.
    • A common thing to put in .bashrc are aliases that you want to always be available.

  • The /opt directory is a standard directory used for installing optional or add-on software packages. It’s short for “optional” and is part of the Filesystem Hierarchy Standard (FHS).

  • Create symlinks

    • Also see
    • Example: Personal R Environment for User1

      • Create symlinks for R version and package directory

        su - user1
ln -s /opt/R/4.0.5/ ~/R-current

mkdir -p ~/R-lib
ln -s ~/R-lib ~/R-library

ls -l ~/R-current

        • A symlink called R-current in User1’s home directory, pointing to the R 4.0.5 installation.

        • -s stands for “symbolic” link

      • Add symlinks to .Renviron

        • Open .Renviron

          nano ~/.Renviron

        • Add symlinks to environment variables

          R_HOME=~/R-current
R_LIBS_USER=~/R-library

  • Debian vs. Ubuntu

    • Stability vs. Freshness:

      • Debian: Debian is known for its stability and reliability. It has a rigorous testing process and a conservative approach to updates, which makes it suitable for servers and systems where stability is crucial.
        • New versions released approximately every 2-3 years
        • Receives regular security and maintenance updates but not feature updates.
      • Ubuntu: Ubuntu is based on Debian but tends to be more up-to-date with software packages.
        • New versions released every six months (April and October). Every two years, an LTS (Long-Term Support) version is released, which is supported for five years.
        • Regular (non-LTS) releases receive updates for nine months, while LTS releases receive security and maintenance updates for five years

    • Package Management:

      • Debian: Debian uses the Debian Package Management System (dpkg) and Advanced Package Tool (APT) for package management. It has a vast repository of software packages.
        • Categorized into main, contrib, and non-free, with strict adherence to free software principles in the main repo
      • Ubuntu: Ubuntu also uses dpkg and APT but adds its own software management tools like Snap and Ubuntu Software Center. This can make software installation more user-friendly.
        • Categorized into main, universe, restricted, and multiverse, allowing more flexibility with proprietary and non-free software.

    • Community and Support:

      • Debian:
        • Community: It’s community-driven, so support may be slower and more focused on experienced users.
        • Support: Primarily community-based through forums, mailing lists, and IRC channels
      • Ubuntu:
        • Community: A vibrant community and benefits from Canonical’s commercial backing, which provides professional support options.
        • Support: In addition to community support, Ubuntu offers commercial support through Canonical, making it attractive for businesses.

    • Licensing:

      • Debian:
        • If you are committed to using entirely free and open-source software, Debian is the better choice. Debian’s strict licensing policy ensures that the software in its main repository adheres to the highest standards of software freedom.
        • Users who need non-free software must enable the “contrib” or “non-free” repositories manually, making it clear when non-free software is being used.
      • Ubuntu: While Ubuntu also includes mostly free and open-source software, it may include some proprietary drivers and software by default, which can be a concern for users who prioritize a completely open-source system

    • Performance

      • Debian
        • Server Performance: Debian is often preferred for servers due to its stability, minimal resource usage, and flexibility. It can be tuned for performance with fewer running services, making it ideal for high-performance server applications
        • Lightweight Desktops: Debian can be configured to run very efficiently on desktops, especially with lightweight environments or minimal installations.
      • Ubuntu
        • General Desktop Use: Ubuntu is optimized for a good balance of performance and user experience. It’s a solid choice for general desktop use, especially on modern hardware.
        • Cloud and Container Performance: Ubuntu is widely used in cloud and container environments (e.g., Ubuntu Server, Ubuntu Core), where its performance is optimized for these specific use cases

    • Hardware

      • Debian works well on older hardware. Debian still offers a 32-bit version of the distro, while Ubuntu no longer offers a 32-bit version.

Commands

Basic Commands

  • echo $SHELL - prints the type of shell you’re using
  • echo $PATH - prints all stored pathes
  • export PATH="my_new_path:$PATH" - store a new path
  • Command Syntax: command -options arguments
  • Piping Commands: cat user_names.txt|sort|uniq
  • du - Disk Usage; Useful for getting the size of directories (link)

    • Flags

      • -h - Human readable output (i.e. instead of bytes, you get kilobytes, megabytes, etc.)
      • -s - Summarize; size of a particular directory in bytes
      • -a - All files including directories
      • -c - Calculates the total size of the printed output
      • -d - Specify how levels deep into a directory you want stats for (e.g. -d 2)
      • –time - Time of last modification

    • Example: Sort directories by size

      du -sh /* | sort -h

    • Example: Calculate size of directory

      du -c -h /home/my/directory

Aliases

  • Custom commands that you can define in order to avoid typing lengthy commands over and over again

  • Examples

    alias ll="ls -lah"
alias gs="git status"
alias gp="git push origin master"

  • Create safeguards for yourself

    alias mv="mv -i"
    • mv will automatically use the i flag, so the terminal will warn you if the file you’re about to move does already exist under the new directory,
      • This way you don’t accidentally overwrite files that you didn’t mean to overwrite.

Files/Directories

  • List

    • List 10 most recently modified files: ls -lt | head
    • List files sorted by file size: ls -l -S
    • List multiple directories: ls ./docs ./text ./data

  • Look at first 3 rows: head -n3 students.csv

  • Create/Delete Directories

    mkdir <dir_name>
rmdir <dir_name>
    • -v means “verbose” so it tells us if it was successful

  • Output to file: echo “This is an example for redirect” > file1.txt

  • Append line to file: echo “This is the second line of the file” >> file1.txt

  • Create/Delete file(s):

    # Create files
touch file1.txt
touch file1.txt file2.tx

# Delete files
rm file1.txt
rm file1.txt file2.txt

  • Move files/dir; Rename

    # Move single file
mv my_file.txt /tmp
# Move multiple files
mv file1 file2 file3 /tmp
# move only .csv files to data directory and be verbose
mv -v *.csv ./data/
# Move a directory or multiple directories
mv d1 d2 d3 /tmp
# Rename the file using move command
mv my_file1.txt my_file_newname.txt
    • File(s) and directories being moved to “tmp” directory

  • Search

    • Find

      # syntax find <path> <expression>
# Find by name
find . -name “my_file.csv"
#Wildcard search
find . -name "*.jpg"
# Find all the files in a folder
find /temp
# Search only files
find /temp -type f
# Search only directories
find /temp -type d
# Find file modified in last 3 hours
find . -mmin -180
# Find files modified in last 2 days
find . -mtime -2
# Find files not modified in last 2 days
find . -mtime +2
# Find the file by size
find -type f -size +10M

    • Search inside files

      • zgrep - Search the compressed file or files content just providing the search term.

        • Default: Prints the matched file name and the complete line

        • Flags

          • -i: Ignore case
          • -n: Print only matched lines
          • -v: Print only unmatched lines (i.e. not pattern)
          • -o: Print only the matched part
          • -l: Print only file names
          • -h: Print only file lines
          • -c: Count matched lines
          • -e: Multiple search terms

        • Example: Search multiple files

          zgrep ismail auth.log.*.gz
          • Searches for the term “ismail” all files beginning with “auth.log.” in their names.
          • *Could also provide each file’s name separated by a space*

        • Example: Search for multiple terms

          zgrep -e "ismail" -e "ahmet" auth.log.2.gz

  • Locate (faster)

    • Docs

    • Install

      bash sudo apt install mlocate # Debian

    • Usage

      sudo updatedb # update before using
locate .csv

  • Unzip: unzip ./foia.zip

  • Split files

    # default: 1000 lines per file, names of new files: xaa, xab, xac, etc.
split my_file

# add a prefix to new file names
split my_file my_prefix

# specify split threshold (e.g. 5000) by number of lines
split --lines=5000 my_file

# specify split threshold by size (e.g. 10MB)
split --bytes=10 MB my_file

Permissions

  • Notes from Linux Permissions Explained: A Beginner’s Guide to File Security Command

  • Definitions

    • User (u): The file’s owner
    • Group (g): Members of the file’s assigned group
      • Check users and groups: id
    • Others (o): Everyone else on the system

  • Levels

    • Read (r): Value of 4
    • Write (w): Value of 2
    • Execute (x): Value of 1

  • Notation: The - seems to be a placeholder

    • 7 (rwx) = 4 + 2 + 1
    • 6 (rw-) = 4 + 2
    • 5 (r-x) = 4 + 1
    • 4 (r–) = 4

  • Examples

    # Symbolic mode
chmod u+x script.sh    # Add execute permission for user
chmod g-w file.txt     # Remove write permission for group
chmod o=r document.pdf # Set others to read-only

# Numeric mode
chmod 755 script.sh    # rwxr-xr-x
chmod 644 file.txt     # rw-r--r--

# Standard web directory permissions
chmod 755 /var/www/html
chmod 644 /var/www/html/*.html

# Create a shared directory
mkdir /shared
chmod 775 /shared
chown :developers /shared

# Make script executable
chmod +x script.sh
    • chmod - edit permissions
    • -rwxrwxrwx - sytax of permissions for a folder or directory
      • “rwx” stand for read, write, and execute rights, respectively
      • The 3 “rwx” blocks are for (1) user, (2) user group, and (3) everyone else.
        • In the given example, all 3 of these entities have read, write, as well as execute permissions.
      • The dash indicates that this is a file. Instead of the dash, you can also see a “d” for directory or “l” for a symbolic link.

  • ls -l problematic-file.txt See list of files and the permissions

  • sudo - “super user” - using this prefix gives you all the permissions to all the files

    • sudo su - opens a stand alone super user shell

    • Examples

      # Switch to root user
su -

# Execute single command as root
sudo apt update

# Edit system file with sudo
sudo nano /etc/hosts

  • chown - Change ownership

    • Examples

      # Change owner
chown user1 file.txt

# Change owner and group
chown user1:group1 file.txt

# Recursive ownership change
chown -R user1:group1 directory/

  • unmask - sets default permissions for new files and directories

    # Check current umask
umask

# Set new umask
umask 022  # Results in 755 for directories, 644 for files

Print

  • Print file content

    cat < my_file.txt
# or
cat my_file.txt

  • Print 1 pg at a time: less my_file.txt

  • Print specific number of lines: head -n<num_lines> <file.csv>

  • Print file content from bottom to top: tac my_file.txt

  • cat -b log.txt | grep error : shows all lines in log.txt that contain the string ‘error’, along with the line number (-b)

Logicals and Conditionals

  • Logicals

    • ; : command1 ; command2

      • command 1 and command 2 run independently of each other

    • & : command1 & command2

      • command 1 runs in the background and command 2 runs in the background

    • && : command1 && command2

      • If the first command errors out then the second command is not executed

    • || : command1 || command2

      • The second commmand is only execute if the first command errors

    • Example

      cd my_dir && pwd || echo “No such directory exist.Check”
      • If the my_dir exists, then the current working directory is printed. If the my_dir doesn’t exist, then the message “No such directory exists. check” message is printed.
  • Conditionals
    • Use [[ ]] for conditions in if / while statements, instead of [ ] or test.
      • [[ ]] is a bash builtin, and is more powerful than [ ] or test.
      • Example: if [[ -n "${TRACE-}" ]]; then set -o xtrace; fi

String Matching

  • Example: Search for “error” and write to file

    #output to a file again
cat file1 file2 file3 | grep error | cat > error_file.txt
#Append to the end
cat file1 file2 file3 | grep error | cat >> error_file.txt
    • Prints lines into grep which searches for “error” in each line. Lines with “error” get written to “error_file.txt”

  • Filter lines

    grep -i “Doctorate” adult_t.csv |grep -i “Husband”|grep -i “Black”|csvlook
# -i, --ignore-case-Ignore  case  distinctions,  so that characters that differ only in case match each other.
    • Select all the candidates who have doctorates and a husband and race are Black
    • csvlook is pretty printing from csvkit package (see Big Data >> Larger Than Memory >> csvkit)

  • Count how many rows fit the criteria

    grep -i “Doctorate” adult_t.csv | wc -l
    • Counts how many rows have “Doctorate”
      • -wc is “word count”

Variables

  • Misc

    • autoenv - If a directory contains an .env file, it will automatically be executed when you cd into it

  • Local Variable:

    • Declared at the command prompt
    • Use lower case for name
    • Available only in the current shell
    • Not accessible by child processes or programs
    • All user-defined variables are local variables

  • Environment (global) variables:

    • Create with export command
    • Use upper case for name
    • Available to child processes

  • Declare local and environment variables then access via “$”

    # local
ev_car=’Tesla’
echo 'The ev car I like is' $ev_car

# environment
export EV_CAR=’Tesla’
echo 'The ev car I like is' $EV_CAR
    • No spaces in variable assignment

  • Calling variables

    • ${var} or $var vs ${var?}

      mv file1 file2 $subdir # oops, I overwrote file2
mv file1 file2 ${subdir?} # error message instead of disaster
      • Using ${var?} throws an error when var has not been defined

  • Always quote variable accesses with double-quotes.

    • One place where it’s okay not to is on the left-hand-side of an [[ ]] condition. But even there I’d recommend quoting.
    • When you need the unquoted behaviour, using bash arrays will likely serve you much better.

  • Functions

    • Use local variables in functions.
    • Accept multiple ways that users can ask for help and respond in kind.
      • Check if the first arg is -h or –help or help or just h or even -help, and in all these cases, print help text and exit.
    • When printing error messages, please redirect to stderr.
      • Use echo 'Something unexpected happened' >&2 for this

Functions

  • Basic

    say_hello() {
  echo "hello"
}
say_hello

  • Using Return

    failing_func () {
  return 1
}
    • return cannot take strings — only numbers 1 to 255

  • With arguments

    say_hello() {
  echo "Hello $1 and $2"
}
say_hello "Ahmed" "Layla"

  • Declaring local and global variables

    say_hello() {
  local x
  x=$(date)
  y=$(date)
}
    • local is a keyword
    • x is local and y is global

  • Suppress errors

    local x=$(moose)
    • When local is used in the same line as the variable declaration, then the variable never errors. e.g. Even if moose doesn’t exist, this line won’t trigger an error

Loops

  • For

    • Create Multiple Files

      #!/bin/bash

# Create a directory for the output files
mkdir output

# Loop through numbers 0 to 9 and create empty files
for i in {0..9}; do
    touch "output/sample_${i:0:3}.txt"
done

echo "Files created in the output directory."

      • The ${i:0:3} syntax ensures that the number i is padded with leading zeros to a width of 3 digits.

      • Files that get created

        output/sample_000.txt
output/sample_001.txt
output/sample_002.txt
...

Scripting

  • Use the .sh (or .bash) extension for your script

  • Use long options, where possible (like –silent instead of -s). These serve to document your commands explicitly.

  • If appropriate, change to the script’s directory close to the start of the script.

    • And it’s usually always appropriate.
    • Use cd "$(dirname "$0")", which works in most cases.

  • Use shellcheck — analysis too for shell scripts. Heed its warnings. (link)

  • Shebang line

    • Contains the absolute path of the bash interpreter
      • List paths to all shells: cat/etc/shells
    • Use as the first line even if you don’t give executable permission to the script file.
    • Starts with “#!” the states the path of the interpreter
    • Example: #!/bin/bash
      • Interpreter installed in directory “/bin”
    • Example: #!/usr/bin/env bash

  • Commands that should start your script

    • Use set -o errexit
      • So that when a command fails, bash exits instead of continuing with the rest of the script.
    • Use set -o nounset
      • This will make the script fail, when accessing an unset variable. Saves from horrible unintended consequences, with typos in variable names.
      • When you want to access a variable that may or may not have been set, use "${VARNAME-}" instead of "$VARNAME", and you’re good.
    • Use set -o pipefail
      • This will ensure that a pipeline command is treated as failed, even if one command in the pipeline fails.
    • Use set -o xtrace, with a check on $TRACE env variable.
      • For copy-paste: if [[ -n "${TRACE-}" ]]; then set -o xtrace; fi.
      • This helps in debugging your scripts, a lot.
      • People can now enable debug mode, by running your script as TRACE=1 ./script.sh instead of ./script.sh .

  • Example: Basic Execution a Bash Script

    • Create a directory bash_script: mkdir bash_script

    • Create a hello_world.sh file: touch hello_script.sh

    • Open hello_script.sh (text editor?)

    • Add code, save, and close

      #!/bin/bash
echo ‘Hello World’

    • Make file executable: chmod +x hello_world.sh

    • Execute file: ./hello_world.sh

  • Example: Create symlinks for mulitple accounts

    #!/bin/bash
for user in user1 user2 user3; do
    sudo -u $user ln -s /opt/R/4.0.5/ /home/$user/R-current
    sudo -u $user mkdir -p /home/$user/R-lib
    sudo -u $user ln -s /home/$user/R-lib /home/$user/R-library
    echo -e "R_HOME=/home/$user/R-current\nR_LIBS_USER=/home/$user/R-library" | sudo tee -a /home/$user/.Renviron
done

  • Setting and Executing Scripts with Arguments

  • Template

    #!/usr/bin/env bash
set -o errexit
set -o nounset
set -o pipefail
if [[ -n "${TRACE-}" ]]; then
    set -o xtrace
fi
if [[ "$1" =~ ^-*h(elp)?$ ]]; then
    echo 'Usage: ./script.sh arg-one arg-two
This is an awesome bash script to make your life better.
'
    exit
fi
cd "$(dirname "$0")"
main() {
    echo do awesome stuff
}
main "$@"

Debugging

  • Also see set -o xtrace in Scripting >> Commands that should start your script

Job Management

  • Programs/Scripts will by default run in the foreground, and prevent you from doing anything else until the program is done.
  • While program is running:
    • control+c - Will send a SIGINT (signal interrupt) signal to the program, which instructs the machine to interrupt the program immediately (unless the program has a way to handle these signals internally).
    • control+z - Will pause the program.
      • After pausing the program can be continued either by bringing it to the foreground (fg), or by sending it to the backgroud (bg).
  • Execute script to run in the background: python run.py &
  • jobs - shows all running jobs and process ids (PIDS)
  • kill - sends signals to jobs running in the background
    • kill -STOP %1 sends a STOP signal, pausing program 1.
    • kill -KILL %1 sends a KILL signal, terminating program 1 permanently.

tmux

  • Terminal Multiplexer
  • Enables you to easily create new terminal sessions and navigate between them. This can be extremely useful, for example you can use one terminal to navigate your file system and another terminal to execute jobs.
  • Installation (if necessary): sudo apt install tmux
    • Typically comes with the linux installation
  • Sessions
    • tmux - starts an unnamed session
    • tmux new -s moose creates new terminal session with name ‘moose’
    • tmux ls - lists all running sessions
    • tmux kill-session -t moose - kills session named “moose”
    • exit - stops and quits the current session
    • Kill all sessions (various opinions on how to do this)
      • tmux kill-session
      • tmux kill-server
      • tmux ls | grep : | cut -d. -f1 | awk '{print substr($1, 0, length($1)-1)}' | xargs kill
  • Attach/Detach
    • When you log out of a remote machine (either on purpose or accidentally), all of the programs that were actively running inside your shell are automatically terminated. On the other hand, if you run your programs inside a tmux shell, you can come simply detach the tmux window, log out, close your computer, and come back to that shell later as if you’ve never been logged out.
    • tmux detach - detach current session
    • control+bthen pressd`: When you have multiple sesssions running, this will allow you to select the session to detach
    • From inside bash and not inside a session
      • tmux a : attach to latest created session
      • tmux a -t moose : attach to session called ‘moose’
  • Pane Creation and Navigation
    • control+b then press ” (i.e. shift+’): add another terminal pane below
    • control+b then press % (i.e. shift+5) : add another terminal pane to the right
    • control+b then press → : move to the terminal pane on the right (similar for left, up, down)

SSH

  • Typically uses a key pair to log into remote machines
    • Key pair consists of a public key (which both machines have access to) and a private key (which only your own machine has access to)
    • “ssh-keygen” is a program for generating such a key pair.
      • If you run ssh-keygen, it will by default create a public key named “id_rsa.pub” and a private key named “id_rsa”, and place both into your “~/.ssh” directory
      • You’ll need to add the public key to the remote machine by piping together cat, ssh, and a streaming operator
        • cat .ssh/id_rsa.pub | ssh user@remote 'cat >> ~/.ssh/authorized_keys'
  • Connect to the remote machine: ssh remote -i ~/.ssh/id_rsa
  • Create a config file instead

    • Location: “~/.ssh/config”

    • Contents

      Host dev
  HostName remote
  IdentityFile ~/.ssh/id_rsa
  • Connect using config: ssh dev
  • For Windows and using Putty, see

Packages

  • Common package managers: apt, Pacman, yum, and portage
  • APT (Advanced Package Tool)

    • The apt command is a smaller section of the apt-get and apt-cache options. The apt command gives the end user just enough tools to install, remove, search and update APT packages. The apt-get command has a lot more options that are useful for writing low-level scripts and tools.

    • Install Packages

      # one pkg
sudo apt-get install <package_name>
# multiple
sudo apt-get install <pkg_name1> <pkg_name2>
      • Install but no upgrade: sudo apt-get install <pkg_name> --no-upgrade

    • Search for an installed package: apt-cache search <pkg_name>

    • Update package information prior to “upgrading” the packages

      sudo apt-get update
      • Downloads the package lists from the repositories and “updates” them to get information on the newest versions of packages and their dependencies.

    • Upgrade

      # all installed packages
sudo apt-get upgrade

# To upgrade only a specific program
sudo apt-get upgrade <package_name>

# Upgrades and handles dependencies; delete obsolete, add new
apt-get dist-upgrade

# together
sudo apt-get update && sudo apt-get dist-upgrade

Expressions

  • Sort data, filter only unique lines, and write to file: cat adult_t.csv | sort | uniq -c > sorted_list.csv