• 1. Introduction
  • 1.1. Scope
  • 1.2. Background
  • 1.3. Purpose
  • 1.4. Next steps
• 2. Basics
  • 2.1. Common commands
  • 2.2. Shortcuts
    • 2.2.1. Navigation
    • 2.2.2. Editing
    • 2.2.3. Recall from history
• 3. Streams, Pipes and Redirects
  • 3.1. Streams
  • 3.2. Redirections
    • 3.2.1. Types
    • 3.2.2. Additional Examples
      • 3.2.2.1. Send standard output to sout.txt and standard error to serr.txt
      • 3.2.2.2. Send standard output and standard error streams to the same file sone.txt
      • 3.2.2.3. Check if a string is present in a shell variable
      • 3.2.2.4. Ignore both standard input and output
  • 3.3. Pipe
  • 3.4. xargs
  • 3.5. tee
• 4. Filename expansion
• 5. Brace expansion
  • 5.1. Examples
• 6. Job control
• 7. Process handling
  • 7.1. difference between ps and jobs
• 8. Quoting
• 9. Basic file management
  • 9.1. list directory ls
  • 9.2. show file contents
  • 9.3. file handling
    • 9.3.1. copy
    • 9.3.2. move or rename
    • 9.3.3. delete
    • 9.3.4. file linking
    • 9.3.5. change directory
    • 9.3.6. display disk usage
    • 9.3.7. file ownership and permissions
• 10. Special shell variables
• 11. Exit codes
• 12. grep
  • 12.1. Useful grep options
    • 12.1.1. Examples
  • 12.2. Regular expression in grep
  • 12.3. Examples
• 13. find
  • 13.1. Examples
• 14. sed filter and transform text
  • 14.1. Overview
  • 14.2. Examples
  • 14.3. Grouping
    • 14.3.1. Grouping Examples:
  • 14.4. Hold Buffer
    • 14.4.1. Example
• 15. awk
  • 15.1. Actions
  • 15.2. Special variables
  • 15.3. Examples
• 16. Arithmetic expansion
  • 16.1. Example
• 17. Command substitution
  • 17.1. Examples
• 18. Process substitution
  • 18.1. Examples:
• 19. Subshell
• 20. Command grouping
  • 20.1. using parentheses
  • 20.2. using curly braces
• 21. Text editing with cut, paste and join
  • 21.1. cut
    • 21.1.1. Examples
  • 21.2. paste
    • 21.2.1. Examples
  • 21.3. join
    • 21.3.1. Examples
• 22. Aliases
  • 22.1. useful aliases
• 23. Functions
  • 23.1. useful functions
• 24. sort
• 25. uniq
• 26. Conditions
  • 26.1. If else
  • 26.2. Short circuiting
    • 26.2.1. example
• 27. Loops
  • 27.1. while loops
    • 27.1.1. example
  • 27.2. for loops
    • 27.2.1. example
• 28. ssh
• 29. curl
  • 29.1. Examples
  • 29.2. handling REST calls
    • 29.2.1. GET
    • 29.2.2. POST
    • 29.2.3. PUT
    • 29.2.4. DELETE
• 30. wget
  • 30.1. Examples
• 31. One liners
• 32. Further reading
• 33. Change History

• This document will mainly focus on things that are usually done in the command line
• The focus is on tools and techniques for one shot adhoc tasks.
• Most of the things in the document can be applied to shell scripting. However, shell scripting is not the focus of this document
• This document is for Linux/Unix

• I am documenting things I learnt and am still learning
• I had multiple files with random snippets.
• I decided to organize the mess into a single piece, so that I can efficiently look up things
• I cannot claim to be an expert in command line.

• The document will not help anyone become an expert.
• However, it may help people become effective.
• I have kept the document concise. I skipped over details that I never used much and can be looked up later in the internet.
• I have focused on concrete examples of common or interesting use cases.

• rephrase unclear material
• correct incorrect material
• add missing material
• add further reading material
• Contributions are welcome

A linux shell's inputs and outputs are sequence of characters called streams. There are three standard I/O streams:

Input and output redirections are done using angular brackets (<>)

command1 > sout.txt 2> serr.txt

command1 > sone.txt 2>&1

OR

command1 &> sone.txt

grep "ABC" <<< $var

command1 &> /dev/null

/dev/null is a null device file. This will discard anything written to it, and will return EOF on reading.

Piping can redirect the standard output of one command to the input of another command.

command1 | command2 paramater1 | command3 parameter1 parameter2 | command4

Examples:

• Print file contents only once. remove duplicate records:
  sort file1 | uniq

• print 5 most frequently used commands
  history | awk '{a[$2]++}END{for(i in a){print a[i] " " i}}' | sort -rn | head -5

• print the file types and their frequencies
  ls | rev | cut -f1 -d'.' | rev | sort | uniq -c | sort -n

• short for extended arguments
• some commands can take arguments from both standard input and as command-line arguments.
• however, there are some commands that cannot take input from standard input. They accept inputs only from arguments. For these commands we need to use args.
• xargs converts input from standard input into arguments to a command.
• divides the arguments to a permitted number and runs the command repeatedly over each greoup of arguments.
• using -n option, the number of arguments per command can be specified. find . | xargs -n1 basename
• in order to assign the std input to a placeholder , use -I{}. the placeholder is usually used when we want to place the std input in the middle of a command
  ls | xargs -I{} echo {} file is found
• xargs, by default, divides the input into arguments using whitespace. When -0 option is used, xargs uses null termination to identify arguments.

Examples:

• print the number lines/words/characters in the files in a directory
  ls | wc
• print the file types and their frequencies
  find . -type f | xargs basename -a | grep "\." | rev | cut -f1 -d'.' | rev | sort | uniq -c | sort -n
• rename all files in a directory
  ls | xargs -I{} mv {} {}.bkp
• copy all .c files to a specific directory
  find . -name "*.c" -print0 | xargs -0 -n1 -I{} cp {} some/folder/
  The print0 option of the find command prints the file names with the null character as termination character. By using the -0 option with xargs, it starts looking for the null character for reading filenames from the stdin.

• the command reads from standard input and writes it to a standard output and to one or more files
• this is useful when we want to see the output of a command both on the screen and also want to save the output in a file for later analysis.

Examples:

• ls | tee fileList.txt

• a wildcard character is a character that is used to represent one or more characters in a filename or foldername.
• file globbing is the operation that recognizes these wildcard characters and does the expansion.

• it is generation of a range of strings to be used on the command line
• useful for generating a list of sequential data or running any command on sequence of data.
• a range or comma-separated list of data can be used for expansion. They are enclosed within curly braces.
• example:
  • {0..4} generates a sequence from 0 to 4
  • {alice, bob, charlie} generates alice, bob and charlie
• the list can be preceded by an optional string (called preamble or prefix). this is prefixed to each string in the braces
• the list can be followed by an optional string (called postscript or postfix). this is appended to each string in the braces
• syntax -
  • string list
    {String1, String2}
  • range list
    {<start>..<end>}
  • range list with custom increment
    {<start>..<end>..<increment>}
  • <prefix>{ string or range }<postfix>

• create files for each day of month January
  touch {1..31}
• print all even numbers from 1 to 100
  echo {2..100..2}
• create input, output, backup and error folders
  mkdir /some/path/{input, output, backup, error}
• print the alphabets in reverse order
  echo {z..a}
• create a backup of a file
  cp file.txt{,.bkp}
• restore from .bkp file
  mv file.txt{.bkp,}
• create folder for each day of January, February and March
  mkdir -p ~/logs/{January/{1..31},February/{1..28},March/{1..31}}

• command line provides the ability to stop/suspend the execution of a process and resume a suspended process at a later point in time
• each running program is called job
• unique id is assigned to every job

• to start a job in the background, we need to add an ampersand(&) at the end of the command

Examples:

• start a job in the background
  sleep 1000 &
• bring the job to the foreground
  fg %2 # where 2 is the job number
• suspend a job
  stop %2 # where 2 is the job number
• resume a job in the background
  bg %2 # where 2 is the job number
• terminate a job
  kill %2 # where 2 is the job number

• Get a snapshot of processes running with ps command
• Get the process id of a command by ps -ef | grep command
• wait for a process to finish by using bash wait <process id>
• kill a process by using kill <process id>
• wait for completion of all child processes
  wait
• wait for completion of specific process
  wait 1234 # where 1234 is the process id

• jobs tells the list of jobs the current shell is managing
• ps tells the list of all the processes running in the system

• Quoting is used to remove special meanings from characters or words
• single quotes - when the single quotes are used, every character within the quotes is preserved and is not evaluated
• double quotes - when the double qoutes are used, the dollar sign, back quotes and blackslashes are evaluated and interpreted.
• escape character - \ is used to preserve the literal value of the following character.
• ANSI C quoting - backslash escaped characters are treated according to ANSI C standard. Format:- $'string

Examples:

• a symbolic link is a file that points to another file or directory
• there are 2 types of links
  • Hard link -> it is an additional name for the existing file. Each file is associated with an unique number. This unique number is called inode. Hardlink associate 2 or more filenames to the same inode and in turn the same file. If the original file is removed, the contents are still available via the hardlink
  • Soft link -> it is an indirect pointer to a file or directory. It has only the path of the original file and not its contents

• there are special variables that are set internally
• these variables are available to the user

• also known as return code
• it is the code that is returned by command or process after it finishes
• following are few exit codes and their meaning

• searches for pattern in files and prints each line that matches the input pattern grep -<options> <pattern> <filenames>
• grep can be used to search in a single file or in multiple files

• lets say a demo file(demo.txt) has following content

THIS IS UPPER CASE LINE
this is lower case line
This is regular line
This line is also regular
Line number four
Line #5
last line

• Search for a string in a file
  grep "this" demo.txt

• Search for a string in a file, without matching case
  grep -i "this" demo.txt

• Search for a string in a file and get both the line number and output
  grep -n "this" demo.txt

• Get the number of lines matching the searched string
  grep -c "this" demo.txt

• Get the filename in which the searched string is found
  grep -l "this" demo.txt

• Get 2 lines after the matching line
  grep -A2 "This" demo.txt

• Get 2 line before the matching line
  grep -B2 "This" demo.txt

• Get 2 lines before and after the matching line
  grep -C2 "This" demo.txt

• Search recursively in all files in subdirectories
  grep -inrI 'some text' /path/to/dir

• a regular expression is a sequence of characters that specifies the search pattern in text.
• different characters have special meaning in regular expressions

• Match any one character
  grep "[Tt]his" demo.txt

• Search for line starting with the search pattern
  grep "^last" demo.txt

• Search for line ending with the search patter
  grep "regular$" demo.txt

• Search for line with a character in specified range
  grep "[0-9]" demo.txt

• Search for line without a character in specified range
  grep "[^0-9]" demo.txt

• Search for a line where the middle characters are not known
  grep "line.*regular" demo.txt

Note: grep can have regular expressions in the search pattern part, and can have wildcards in the files to search section.

• the find command is used to search and locate the list of files and directories
• the list returned will satisfy the conditions used in find command.
• syntax is find [starting point] [expression]
• -exec [command] can be used to run command on the files located by find
• by default all the results are newline terminated. -print0 option can be used to make the results null terminated

• find files with specific name
  find . -name demo.txt
• find files with a specific pattern
  find ./Codes -name *.cpp
• find directories with specific name
  find . -name Codes -type d
• find files with permission as 777 and change permissions to 644
  find . -type f -perm 0777 -print -exec chmod 644 {} \;
• find and remove files
  find . -type f -name "*.bkp" -exec rm -f {};

• looks for pattern and edits them
• works on both files and stdin
• original files is not updated
• results are put into standard output
• syntax is sed 'instructions' file
• instruction is of format '[address]command/regex/replace/modifier'
  • example - '5,15s/abc/ABC/g' -> this will substitute abc with ABC in lines 5 to 15.
    • modifier g specifies that all occurences of abc in line will be replaced with ABC
• when address is used, the lines belonging to the address are examined/modified. -> sed '1,100 s/A/a/' file.txt. this restricts substition to first 100 lines.
• By default all the lines are printed. When '-n' flag is used, this behavior is suppressed. Nothing will be printed unless an explicit request to print is found.
• A pattern can be used as a address -> sed -n '/start/,/stop/ p' file.txt
• When '!' is used, the command is run outside of the address -> sed -n '/match/ !p' file.txt

• remove blank lines
  sed '/^$/d' file.txt
• remove all lines with search string
  sed '/Search/d' file.txt
• remove all instances of search string
  sed 's/Search//g' file.txt
• print lines with the search string
  sed '/Search/ p' file.txt
• substitue a string with another
  sed 's/oldString/newString/g' file.txt
• remove trailing spaces
  sed 's/ *$//' file.txt
• remove leading spaces
  sed `s/^ *//' file.txt
• add spaces to start of everyline
  sed 's/^/ /' file.txt
• print the first 10 line
  sed '10 q' file.txt
sed -n '1,10 p' file.txt
• append line after a pattern
  sed '/pattern/ a add line here' file.txt
• insert line before a pattern
  sed '/pattern/ i add line here' file.txt
• change a line with a pattern
  sed '/pattern/ c line changed here' file.txt
• change a->p, b->q, c->r
  sed 'y/abc/pqr/' file.txt

• sed allows capturing specific parts of text into groups.
• these groups can be manipulated
• group is enclosed within parentheses expression "(" and ")" in the search string
• each group is assigned a number. The first group is assigned \1 and so on.
• \1 can be both in pattern string and replacement string

• switch first and second columns
  sed 's/\([a-z]*\) \([a-z]*\)/\2 \1/' file.txt
• print lines which have consecutive duplicate words
  sed -n '/\([a-z][a-z]*\) \1/p' file.txt
• remove consecutive duplicate words in a line
  sed 's/\([a-z][a-z]*\) \1/\1/' file.txt

• When sed read text, each line is placed into a temporary space.
• When a new line is read, the old text is replaced by the new line in the temporary space.
• This temporary space is called pattern space.
• Hold buffer is like a long term storage. text can be copied to and from pattern space.

• print one line after and before the pattern match
  sed -n '/999/ !{x;d};/999/ {x;p;x;p;n;p}' file.txt
• add space after every line
  sed 'G' file.txt
• insert blank line above every line which matches pattern
  sed '/start/ {x;p;x}' file.txt
• Insert blank line after every line which matches pattern
  sed '/start/ {G}' file.txt
• Insert blank line before and after every line which matches pattern
  sed '/start/ {x;p;x;G}' file.txt

• command line utility to find, process and transform text files
• the basic syntax is pattern { action }
  • the pattern is compared with every input line. pattern can be any regular expression
  • when the pattern matches, the action is performed
  • when no pattern is provided, the action is applied to every line
  • example -> /^HTTP/ {print}
    • every line starting with HTTP is printed
  • there are 2 other important patterns
    • BEGIN - specifies actions to be performed, before any line is read
    • END - specifies actions to be performed, after all lines are read
    • Example -> awk 'BEGIN{print "start"} {print} END{print "end"}' file.txt
      • first start is printed, then all lines of file.txt are printed and then end is printed
• awk interprets each line as a record of fields
• one or more consecutive spaces or tabs are considered as a single delimiter between fields
• $1, $2, etc. represents the first field, second field, and so on.
• $0 represents the entire input line
• awk has 2 data types - strings and integers
• awk internally converts variable according to context
• awk supports associative arrays. example var[key] = value
• on integers, basic arithmatic operations (+-*/%) are supported. autoincrement(++) and decrement(--) is also supported.

• following are few actions that can be performed

Note: -F option can be used to update the input field separator -> awk -F":"'{ print $1 }' file.txt

• split up “,” (comma) separated fields and print the third field ($3)
  awk -F"," '{print $2}' file.txt
• print the 3rd field of a csv if the second field ($2) exists and is not empty
  awk -F"," '{if ($2)print $3}' file.txt
• print the last field in each line
  awk -F"," '{ print $NF }' file.txt
• print the line after the line matching search pattern
  awk '/pattern/ { i=1; next; } {if(i) {i--; print;}}' file.txt
• print the line and the 2 lines after the line matching search pattern
  awk '/regexp/ {i=3;} { if(i) {i--; print;}}' file.txt
• print the lines from a file starting at the line matching start until the line matching stop
  awk '/start/,/stop/' file.txt
• count lines (wc -l)
  awk 'END{print NR}' file.txt
• print matching lines (grep)
  awk '/pattern/'
• print non matching lines (grep -v)
  awk '!/pattern/'
• remove duplicate consecutive lines (uniq)
  awk 'a !~ $0 {print}; {a=$0}' file.txt
• print first 10 lines of file (head)
  awk 'NR < 11' file.txt
• print last 10 lines of file (tail)
  awk '{vect[NR]=$0;} END{for(i=NR-9;i<=NR;i++) {print vect[i];}}' file.txt
• print the total number of bytes used by files
  ls -l | awk '{ x += $5 } END { print "Total bytes: " x }'
• read a csv(comma separated) file and print the first and third field in semicolon separated format
  awk 'BEGIN{FS=",";OFS=";"}{print $1, $3}' file.txt

• arithmetic expression can be evaluated and the result can then be used.
• syntax - $(( expression ))

• print sum of 2 numbers
  echo $((1+3))

• In command substitution, the output of a command replaces the command
• the output of a command can be used an argument to another command
• syntax is `command` and $(command)
• using backticks (`) is discouraged and has been deprecated
• $(commmand) supports nesting i.e. $(command1 $(command2))

• assign the output of a command to a variable
  thedate=`date`
• use the output of command as a parameter of another command
  vi $(grep -l 123 *)
• assign contents of file to a variable
  filedata=$(cat file.txt)
filedata=$(<file.txt)
  second option is faster

• the input or output of a command can appear as a file. This is known as process substitution
• this technique is useful when we want to use the output of multiple commands as the input to a command
• process substitution can also be used to capture output and redirect it to the input of a process
• template - <(command) and >(command)

• sort and compare two files
  diff <(sort file1) <(sort file2)

• compare 2 folders
  diff <(ls $first_directory) <(ls $second_directory)

• a subshell is a child process launched by a shell
• whenever a shell script is run, a subshell is created and the script is run in the subshell
• variables defined in parent shell can be accessed if export is used while defining the variable
• subshells can also be created using parentheses
  (command1; command2; command3)
• subshells are a convenient way to group commands.
• can be used to temporarily move to a different directory

#do something in current directory
(cd some/other/directory; other-command)
#back in the original directory

• to run a command or script in the current shell, without creating a subshell, use '.' as in . script.sh

• sometimes we need to run multiple commands and redirect all the output to a single file. Command grouping helps in this.
• without command grouping we need to redirect each command output individually to a file

• Grouping list of commands can be done using parentheses ()
• a subshell is created
• example - (date; uptime) > file.txt

• Grouping commands can be done using curly braces {}
• causes the list to be executed in the current shell context
• no subshell is created
• example - {date; uptime;} > file.txt

• cut command cuts out sections from each line and writes result to standard output
• syntax is cut OPTION [FILE]

when -c option is used, ranges can be specified. Each range can be one of:

• print the first and third columns of a csv file
  cut -f1,3 -d"," file.txt
• print the first 3 characters of each line
  cut -c -3 file.txt

• merges lines of files
• by default, the lines from each files are delimited by tab
• when '-' is used instead of filename, the command reads from standard input
• syntax paste [OPTION] [FILE] [FILE]

lets take 2 files - number.txt and name.txt

cat number.txt 1
2
3
4

cat name.txt Alice
Bob
Charlie
David

• merge 2 files, first file will give the first column and second file will give the second column
  paste number.txt name.txt
• merge 2 files, delimited by ','
  paste -d"," number.txt name.txt
• merge 2 files, sequentially, i.e first only first file is printed and then only the second file
  paste -s number.txt name.txt

• join lines of two files on a common field
• syntax join [OPTIONS] FILE1 FILE2

lets take 2 files - number.txt and name.txt

cat number.txt 1 100
2 101
3 102
4 103
5 104

cat name.txt 1 Alice
2 Bob
3 Charlie
4 David

• join 2 files based on the first column
  join number.txt name.txt

• aliases are short names for long commands

• when we need to execute long commands multiple times, it is advisable to create aliases

• syntax - alias [-p] [name[=value]]

• we can create permanent alias by storing them in configuration files

• to temporarily bypass an alias, use \ -> \ll

• creating a alias
  alias name='values'

• removing alias
  unalias name

• print all defined alias
  alias -p

alias gh='history|grep'
alias c=clear
alias cx='chmod +x'
alias ..='cd ..'
alias sl=ls
alias left='ls -t -1'
alias count='find . -type f | wc -l'
alias f='find . |grep '

• set of commands that accomplish a specific task
• can be used numerous times
• helps avoid writing the same code repeatedly
• can use loops and conditions within them
• arguments can be passed to functions
• by using export -f functionname, we can make the functions available to shell scripts
  • export can be added to configuration files like .bashrc
  • To keep things modular, create a new file called ~/.bash_functions and then have your .bashrc load it
• syntax
  

function_name () {
  commands
}

function_name () { commands; }

mcd() { mkdir -p "$1"; cd "$1";}
cdl() { cd "$1"; ls;}

• sort lines text files

• report or omit repeated lines
• the input file must be sorted

• if-then-else is supported in command line
• Syntax

if [ condition1 ]; then command1;
elif [ condition2 ]; then command2;
else command3; fi

if [ condition1 ]; then command1; elif [ condition2 ]; then command2; else command3; fi

• there are different types of conditions
  1. file based condiitons

2. string based conditions

3. number based conditions

• 0 is considered true and numbers greater than 0 are considered false.
  • this is because in Unix/Linix, when a process ends successfuly, it returns 0

• an alternative way of using conditions is by using logical AND (&&) and logical OR(||)
• evaluation of a logical expression is stopped, as soon as the outcome has been determined. This is known as short-circuiting.
• in case of Logical AND, as soon as sub-expression becomes false, the whole expression evaluates to false
  • in case of expr1 && expr2, if expr1 evaluates to false, then the whole expression will evaluate to false. So, expr2 is not evaluated at all.
  • && can be used to ensure that command2 is run only if command1 ends successfully. example ->command1 && command2
• in case of Logical OR, as soon as sub-expression becomes true, the whole expression evaluates to true
  • in case of expr1 || expr2, if expr1 evaluates to true, then the whole expression will evaluate to true. So, expr2 is not evaluated at all.
  • || can be used to ensure that command2 is run only if command1 fails. example -> command1 || command2

• create folder if it does not exist

  [ -d ./some/path/folder ] || mkdir /some/path/folder

• create create file only if folder exists

  cd /some/path/folder && touch file.txt

• the loop runs as long as the given condition is true
• syntax

  while [ condition ]; do commands; done

• print all the folders with .c files
  

find . -name *.c | {while read -r filename; do dirname $filename; done;} | sort | uniq # dirname returns the directory name

• the loop iterates over a list of values or preset number of times
• syntax

  for <variable name> in <a list of items>;do <some command> $<variable name>;done;

• copy files from one folder to another

for file in ./code/*.txt; do cp $file /home/code/backup; done

• ssh (SSH client) is a program for logging into a remote machine and for executing commands on a remote machine
• syntax ssh user@host
• running a single command on remote server
  ssh user@host command_to_run
• logging into server with different port
  ssh -p portnum user@host
• ssh connection using host in the middlessh -t reachable_host ssh unreachable_host

• used to transfer data from one server to another
• syntax - curl [options] [URL]

• retrieve a webpage
  curl example.com
• save a webpage
  curl example.com -o example.html
• resume a download
  curl -C - -O https://releases.ubuntu.com/21.10/ubuntu-21.10-desktop-amd64.iso
• fetch headers only
  curl -I example.com
• fetch weather
  curl wttr.in/london

• GET is used to fetch resource
• the GET method is the default method
• example curl https://reqres.in/api/users/2

• POST is used to create resource in a server
• To send a curl POST request we use the option -X POST
• example curl -X POST -H "Content-Type: application/json" -d '{"email": "eve.holt@reqres.in","password": "pistol"}' https://reqres.in/api/register

• PUT is used to update resource
• To send a curl PUT request we use the option -X PUT
• example curl -X PUT -H "Content-Type: application/json" -d '{"name": "morpheus","job": "zion resident"}' https://reqres.in/api/users/2

• DELETE is used to remove resource
• To send a curl DELETE request we use the option -X DELETE
• example curl -X DELETE https://reqres.in/api/users/2

• command line utility to download file

• download a file
  wget https://releases.ubuntu.com/21.10/ubuntu-21.10-desktop-amd64.iso
• resume a download
  wget -c https://releases.ubuntu.com/21.10/ubuntu-21.10-desktop-amd64.iso

• print the files and directories in tree structure
  find . | sed -e "s/[^-][^\/]*\// |/g" -e "s/|\([^ ]\)/|-\1/"
• print the directories in tree structure
  find . -type d | sed -e "s/[^-][^\/]*\// |/g" -e "s/|\([^ ]\)/| - \1/"
• quick access to ascii table
  man ascii
• find common lines in 2 files
  cat file1 file2 | uniq -d
• find lines in file1 that is not present in file2
  cat file1 file2 file2 | uniq -u
• find most frequently used commands
  history | cut -c8- | sort | uniq -c | sort -rn | head
• recursively remove only directories with no files
  find . -depth -type d -exec rmdir {} \;
• count the number of total active network interfaces
  wc -w <<<$(netstat -i | cut -d" " -f1 | egrep -v "^Kernel|Iface|lo")
• display all the files/directories sorted by size
  du -ah . 2>/dev/null | sort -hr | less
• make multiple copies of file
  tee filecopy{1..10}.txt < file.txt

• presented in reversed chronological order i.e. the latest change is at the top