Terminal Commands

6 minute read

These are commands that I have found useful doing challenges (like those on overthewire or Hack the Box), or through personal projects. These are ordered by when I came across them, not by category.

For additional explanations, go to https://explainshell.com/

ssh into a system

ssh username@location -p n -i file_name

ssh == secure shell, this is the keyword for accessing a remote system
username == if you need a username to access a system
- @ == connects the username and location
location == what system you’re accessing
-p == optional, shows we’re specifying a port
- n == the port number
-i == optional, shows we’re using a key to access the system
- file_name == file that holds the key

Open files with dash (-) for a name

vi ./-

vi == opens with vim
. == current directory
/ == path separator

Open files with spaces in their name

vi file\ name

vi == opens with vim
\ == signifies the following space is part of the filename

Show hidden files

ls -a

ls == list files/directories
-a == all… doesn’t ignore entries starting with a period (.)

Determine file type of those in this directory and defined amount of subdirectories

file */*

file == lists file type
*/* == goes through current directory and the ones directly below
- Can go deeper (*/*/* etc.)

Find files of a certain size in current directory and all subdirectories

find . -size n[cwbkMG]

find == search that goes through file hierarchy (will look at subdirectories)
. == current working directory
n == numerical size of file
Suffixes used to notate what file size is being used:
- c == bytes
- w == two-byte words
- b == 512-byte blocks (default)
- k == kilobytes
- M == Megabytes
- G == Gigabytes

Lists long format of files in this directory and defined amount of subdirectories

ls -l */*

ls == lists files/directories
-l == long listing format
- Permissions (read, write, execute)
- Owner of file and group owner (root, specific user, etc.)
- File size
- Modification or creation time (whichever is most recent)
- File/directory name
*/* == goes through current directory and the ones directly below
- Can go deeper (*/*/* etc.)
- Not having anything will just go through current directory

Find something that exists somewhere in the entire server

find /

find == search that will go through entire file hierarchy
/ == root directory
Do whatever search criteria

Suppress error output (permission denied, etc.)

2>/dev/null

2 == redirects stderr
- 1 == redirects stdout (i.e. everything but the errors)
> == redirects output to a file
/dev/null == suppresses the output… throw-away bin

Print line in a textfile that contains a phrase/word/etc.

grep "word" file.txt

grep == searches file for lines that match given pattern. Default returns entire line
“word” == pattern you want grep to search for
file.txt == file you want to search

Output text that is never repeated in a file

sort file.txt | uniq -u

sort == puts elements in file in ascending order
| == pipes the output of the first section to the second section
uniq == removes repeats so only one remains (i.e. aaabcc becomes abc)
- Only if the repeat occurs directly after (why you sort first)
-u == removes elements that had repeats

Find specific printable characters in a file

strings file.txt | grep 'x'

strings == only returns printable characters
file.txt == file name
| == pipes output of first part to the second part
grep == searches given file for lines that match patter
- ‘^xx’ == searches at the beginning of the line (with xx representing the pattern you’re searching for)
- ‘xx’ searches anywhere in the line (with xx representing the pattern you’re searching for)
- ‘xx$’ == searches at the end of the line (with xx representing the pattern you’re searching for)

Decode base64 encryption inside text file

base64 --decode file.txt

base64 == keyword to encode/decode with base64 encryption
--decode == selects the decode functionality
file.txt == file name

Decode rot13 encryption inside text file for upper and lowercase letters

cat data.txt | tr '[n-za-m]' '[a-z]' | tr '[N-ZA-M]' '[A-Z]'

cat == outputs contents of the file
| == pipes output of the left into the right
tr == translates characters (case sensitive)
- ’[ ]’ == first set of characters; those to be translated
- ’[ ]’ == second set of characters; those being translated
- So, n would translate to a and N would translate to A

Undo a hexdump

xxd -r original_file new_file

xxd keyword to create a hexdump or reverse
- Reverse converts from hexadecimal to binary
-r == reverses to binary
original_file == file containing the hexdump
new_file == file receiving the converted binary

Get data from file that has been compressed many times

This is multistep. We will first need to determine the type of compression used.

file file_name

file == gets information about the file; here it will tell us what type of compression was used
file_name == the compressed file

The following steps depend on what type of compression was used

If it was gzip:

zcat file_name | file -

zcat == reads compressed data and writes decompressed data to standard output
file_name == the compressed file
| == pipes decompressed data to the right side
file == will tell you if the file has another compression or is ASCII characters
- == tells file to read standard input as a file name

If it was bzip2

bzcat file_name | file -

bzcat == reads compressed data and writes decompressed data to standard output
file_name == the compressed file
| == pipes decompressed data to the right side
file == will tell you if the file has another compression or is ASCII characters
- == tells file to read standard input as a file name

If it was tar archive (GNU)

tar -x -O file_name | file -

tar == Undoes layer of tar archive
-x == extracts file from archive
-O == outputs the extract to stdout (allows for piping)
file_name == the compressed file
| == pipes decompressed data to the right side
file == will tell you if the file has another compression or is ASCII characters
- == tells file to read standard input as a file name

Pipe the results together for multiple compressions

zcat file_name | bzcat | zcat | tar -x -O | file -

Add a new addition to piping based on the output from file -. Continue to do this until ASCII is returned

Running the same command without file - at the end will output the file content

Send a message to port

echo 'message' | nc address port

echo == displays line of text
‘message’ == text you want displayed
| == pipes displayed text to the right side
nc == reads and writes tcp and udp
destination == localhost, etc.
port == port number you’re sending stuff to

Make a file executable

chmod +x filename

chmod == Changes access permissions of a file
+x == makes file an executable

Create an alias for a command

alias your_command="default_command"

alias == Keyword that allows you to make a shortcut command
your_command == the alias/shortcut command you’ve come up with
default_command == the original command. Can be very long
Example would be alias dls=”cd ~/Downloads”

Run an executable

./filename

. == Current directory (ensures you are executing the program you mean to, not one of the same name in a different directory)
/ == Path separator

Alicia Kincaid