leejunkim

Bash commands

7 min read

One Bash command: command/program option(s) argument(s)

Location / Navigation

  • A path on the shell = delimited list of directories
    • / - Linux/Mac
      • the path / is the “root” of the file system, under which all directories and files lie
      • A path that starts with / is called an absolute path. Any other path is a relative path.
    • \ - Windows
      • there is one root for each disk partition (e.g., C:\)
  • . and ..
    • . = current
    • .. = parent
  • pwd
    • print working directory → gives us current directory
    • certain shells might be configured in a way that they show us the current directory we’re at
      • leejun@leejun-VirtualBox:~$ → the ~ tells us that we’re rn in our home directory (our user folder) . Can be configured.
  • cd [directory_name]
    • change directory
    • just ~ → user home directory (like `/home/leejun)
      • cd ~/Desktop
      • same as just typing cd
    • cd /
      • going into the absolute root folder path where everything starts
    • cd ..
      • going up to parent
    • cd ../..
      • going up 2 parents
    • cd ~ or cd
      • always goes to user’s home directory (no matter where you are)
    • cd -
      • directory before

Basic Commands

  • Executing multiple commands
    • Just add a semicolon
    • command1; command2;
      • can optionally omit the last ;
    • echo -n 'Hello'; echo 'World'
  • echo
    • allows us to output text in the terminal
    • string wrapped in single quotes, or escape just the relevant character with \ (My\ Photos)
    • echo -n 'I love Bash'
      • echo → command
      • -n → option(s)
      • I love Bash → argument(s)
    • options
      • -n
        • by default, it will output a line break at the end
        • we can disable this with the option -n
      • -e
        • enables backlash escapes
        • echo -e 'Line \nLine2'
    • (you can combine options
      • echo -en 'I love Bash!\n' (order x matter mostly, depends on the command)
      • echo -e -n 'I love Bash!\n'
  • >
  • ls
    • prints the contents of working directory
    • ls [options..] [path]
    • options
      • -a: lists all entries including hidden files
      • -r: reverse order while sorting
      • -t: sort by modification time, newest first
      • -l: long format (includes timestamp)
      • --color={always,never,auto}: enables colorful output (enabled in ubuntu by default)
    • Can also specify a path we want to list
      • ls -ta --color ~/Desktop
      • ls ready/
  • ls -l
    • Example:
      • drwxr-xr-x 1 Leejun 197121 0 Feb 1 2025 Web_dev/
        • d - file type is directory
        • Permissions
          • rwx - Owner Permissions (Leejun) - Read, Write, Execute
          • r-x - Group Permissions (197121) - Read, Execute
          • r-x - Permission for others - Read, Execute
        • 1 - number of hard links (how many hard links point to this directory)
        • Leejun - Owner
        • 197121 - Group
        • 0 - Size of the item/folder in bytes (often misleading)
        • Feb 1 2025 - last modification date
        • Web_dev/ - name of the directory
      • -rw-r--r-- 1 Leejun 197121 28077 Jan 17 2025 증명사진.jpg
        • - - file type is file
  • date
    • tells you the date
    • Sat, Aug 23, 2025 6:37:56 PM
  • clear
    • clears terminal
    • Ctrl + L

Getting help

  • --help/ -h
    • a built-in “usage” of the command, and not all commands implement it, or at least not the same way
      • shows possible list of args
    • ls -h
      • just add this after the command
    • Inside: Usage: ls [OPTION]... [FILE]...
      • ... → 1 or more
      • [] → means optional
      • option → anything that takes a value
      • flag → anything that DOESN’T take a value
  • man
    • a command by itself which is a pager program that reads manual
    • man <COMMAND>
    • man ls
    • man pages (documentation) must be installed on your system!
    • (it seems like ppl usually prefer man than --help)

File Management

  • touch
    • create 1 or more empty file(s)
    • touch test1.txt test2.txt
    • actual use case
      • it modifies the timestamp of a file!
      • if file alr exists, only its timestamp will be modified. Otherwise, a new (and empty) file is created!
    • use . to create hidden files
  • mkdir
    • creates a new directory
  • mv <file> <destination>/
    • moves an existing file to another location
      • mv test.txt folder/
    • can also be used to rename an existing file
      • mv test.txt test2.txt
    • both at once
      • mv text.txt folder/test2.txt
    • like “cut and paste”
  • cp
    • copy existing file(s)
    • cp -r : copies a whole folder
      • -r stands for recursive
    • cp text1.txt text2.txt
      • will copy the contents of text1.txt into a new file called text2.txt.
    • cp text1.txt newFolder
    • cp text1.txt newFolder/text2.txt
    • like “copy and paste”
  • rm
    • removes a file(s)
    • for deleting a directory, use the option -r
      • works for empty and non-empty directories
    • for skipping confirmation, use -f (NO UNDO)
    • deletes it PERMANENTLY
    • rm <file>
    • rm -r <directory>
    • rm -f <file>
    • rm -rf <directory>
  • rmdir
    • removes empty directory (way safer than rm -r)
  • tree
    • shows tree structure
  • find
    • provide path as first parameter
    • we get a list of all the files & folders in the path/folder (ex. find .)
    • filter: type
      • find . -type f (type file - f, type directory - d)
    • filter: modification within the last 7 days
      • find . -type f -mtime-7
    • filter: files larger than 10MB
      • find . -type f -size +10M
    • delete all empty files in a directory
      • find . -empty -delete

File Management - text

  • cat
    • allows to print the contents of a file
    • can actually kinda print binary data (but it might not work properly or break the terminal)
    • might cut the top parts of the file if file too big
  • head
    • shows the start of a text file
    • we can specify the Parameter -n for the number of lines
    • head -n 5 data.txt
  • tail
    • read file from the end
    • same options as head
    • tail -n 5 data.txt
  • less
    • we can use this to read large files
    • arrow keys - navigate through the tile
    • f (forward) and b (backward) - navigate a whole page ahead or back
    • 50p - navigating to percentage of our content (ex. to 50%)
    • = - shows our current progress
    • -N - shows rows
    • search
      • /search term - forward search
      • ?search term - backward search
    • q - quit
    • waaay more options but these are the most important
  • wc
    • usually used to check if file is big or not (with the -l flag)
    • wc file.txt → same as wc -lwc file.txt
    • number of lines, words, bytes
    • can use -l, -w, -c flags separately (most important flags)
  • du (disk usage)
    • calculates the size of all items in the folder (or we can pass in a file name as parameter)
    • du -s - to get summary
    • du romeo.txt
    • the unit depends (-b for bytes, -k for kilobytes, -h for human readable format)
      • ubuntu usually tells the side in kb w/o the flags

Editing files w/ nano

  • nano
    • no built in text editor for bash, so we have to install additional software for that
    • 4 popular options
      • pico / nano (upgraded ver of pico) - simple editor for text files in bash
      • vi / vim (upgraded ver of vi) - more advanced text editor
    • Install process depends on the system you’re using
  • nano text.txt
    • creates new file if it doesn’t exist
  • Ctrl + O - write out/save

Commands used with pipe |

  • tee
    • With | and the tee command, you can create a stdout and write it to a file at the SAME time!
    • Example
      • echo 'Hello Bash' | tee hello.txt
        • You want Hello Bash to be printed to stdout and also written into hello.txt
      • echo 'Hello Bash' | tee hello.txt | wc -c
        • multiple pipes
        • counting the text
    • Appending
      • echo 'Hello Bash' | tee -a hello.txt
  • ping
    • Allows us to send a ping packet to another server (ex. a remote server).
    • We can use to test if our connection is reachable or disrupted
    • Example
      • ping google.com
        • sends a ping packet to google.com
      • ping non_existing.com 2>&1 | tee ping.txt
        • Redirects stderr (2) to stdout (1), so both error messages and normal output are now part of stdout.
        • The tee command takes stdin and writes it to both terminal (stdout) and ping.txt

Permission

  • chmod
    • change mode
    • chmod +x semester
      • gives execute command permission to semester

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