## Getting started

### Introduction

This is a quick reference to getting started with Bash scripting.

-   [Learn bash in y minutes](https://learnxinyminutes.com/docs/bash/) _(learnxinyminutes.com)_
-   [Bash Guide](http://mywiki.wooledge.org/BashGuide) _(mywiki.wooledge.org)_
-   [Bash Hackers Wiki](https://web.archive.org/web/20230406205817/https://wiki.bash-hackers.org/) _(wiki.bash-hackers.org)_

### Example

```
#!/usr/bin/env bash

name="John"
echo "Hello $name!"
```

### Variables

```
name="John"
echo $name  # see below
echo "$name"
echo "${name}!"
```

Generally quote your variables unless they contain wildcards to expand or command fragments.

```
wildcard="*.txt"
options="iv"
cp -$options $wildcard /tmp
```

### String quotes

```
name="John"
echo "Hi $name"  #=> Hi John
echo 'Hi $name'  #=> Hi $name
```

### Shell execution

```
echo "I'm in $(pwd)"
echo "I'm in `pwd`"  # obsolescent
# Same
```

See [Command substitution](https://web.archive.org/web/20230326081741/https://wiki.bash-hackers.org/syntax/expansion/cmdsubst)

### Conditional execution

```
git commit && git push
git commit || echo "Commit failed"
```

### Functions

```
get_name() {
  echo "John"
}

echo "You are $(get_name)"
```

See: [Functions](https://devhints.io/bash#functions)

### Conditionals

```
if [[ -z "$string" ]]; then
  echo "String is empty"
elif [[ -n "$string" ]]; then
  echo "String is not empty"
fi
```

See: [Conditionals](https://devhints.io/bash#conditionals)

### Strict mode

```
set -euo pipefail
IFS=$'\n\t'
```

See: [Unofficial bash strict mode](http://redsymbol.net/articles/unofficial-bash-strict-mode/)

### Brace expansion

```
echo {A,B}.js
```

| Expression | Description |
| --- | --- |
| `{A,B}` | Same as `A B` |
| `{A,B}.js` | Same as `A.js B.js` |
| `{1..5}` | Same as `1 2 3 4 5` |
| `{{1..3},{7..9}}` | Same as `1 2 3 7 8 9` |

See: [Brace expansion](https://web.archive.org/web/20230207192110/https://wiki.bash-hackers.org/syntax/expansion/brace)

## Parameter expansions

### Basics

```
name="John"
echo "${name}"
echo "${name/J/j}"    #=> "john" (substitution)
echo "${name:0:2}"    #=> "Jo" (slicing)
echo "${name::2}"     #=> "Jo" (slicing)
echo "${name::-1}"    #=> "Joh" (slicing)
echo "${name:(-1)}"   #=> "n" (slicing from right)
echo "${name:(-2):1}" #=> "h" (slicing from right)
echo "${food:-Cake}"  #=> $food or "Cake"
```

```
length=2
echo "${name:0:length}"  #=> "Jo"
```

See: [Parameter expansion](https://web.archive.org/web/20230408142504/https://wiki.bash-hackers.org/syntax/pe)

```
str="/path/to/foo.cpp"
echo "${str%.cpp}"    # /path/to/foo
echo "${str%.cpp}.o"  # /path/to/foo.o
echo "${str%/*}"      # /path/to

echo "${str##*.}"     # cpp (extension)
echo "${str##*/}"     # foo.cpp (basepath)

echo "${str#*/}"      # path/to/foo.cpp
echo "${str##*/}"     # foo.cpp

echo "${str/foo/bar}" # /path/to/bar.cpp
```

```
str="Hello world"
echo "${str:6:5}"    # "world"
echo "${str: -5:5}"  # "world"
```

```
src="/path/to/foo.cpp"
base=${src##*/}   #=> "foo.cpp" (basepath)
dir=${src%$base}  #=> "/path/to/" (dirpath)
```

### Prefix name expansion

```
prefix_a=one
prefix_b=two
echo ${!prefix_*}  # all variables names starting with `prefix_`
prefix_a prefix_b
```

### Indirection

```
name=joe
pointer=name
echo ${!pointer}
joe
```

### Substitution

| Code | Description |
| --- | --- |
| `${foo%suffix}` | Remove suffix |
| `${foo#prefix}` | Remove prefix |
| `${foo%%suffix}` | Remove long suffix |
| `${foo/%suffix}` | Remove long suffix |
| `${foo##prefix}` | Remove long prefix |
| `${foo/#prefix}` | Remove long prefix |
| `${foo/from/to}` | Replace first match |
| `${foo//from/to}` | Replace all |
| `${foo/%from/to}` | Replace suffix |
| `${foo/#from/to}` | Replace prefix |

```
# Single line comment
```

```
: '
This is a
multi line
comment
'
```

### Substrings

| Expression | Description |
| --- | --- |
| `${foo:0:3}` | Substring _(position, length)_ |
| `${foo:(-3):3}` | Substring from the right |

### Length

| Expression | Description |
| --- | --- |
| `${#foo}` | Length of `$foo` |

### Manipulation

```
str="HELLO WORLD!"
echo "${str,}"   #=> "hELLO WORLD!" (lowercase 1st letter)
echo "${str,,}"  #=> "hello world!" (all lowercase)

str="hello world!"
echo "${str^}"   #=> "Hello world!" (uppercase 1st letter)
echo "${str^^}"  #=> "HELLO WORLD!" (all uppercase)
```

### Default values

| Expression | Description |
| --- | --- |
| `${foo:-val}` | `$foo`, or `val` if unset (or null) |
| `${foo:=val}` | Set `$foo` to `val` if unset (or null) |
| `${foo:+val}` | `val` if `$foo` is set (and not null) |
| `${foo:?message}` | Show error message and exit if `$foo` is unset (or null) |

Omitting the `:` removes the (non)nullity checks, e.g. `${foo-val}` expands to `val` if unset otherwise `$foo`.

## Loops

### Basic for loop

```
for i in /etc/rc.*; do
  echo "$i"
done
```

### C-like for loop

```
for ((i = 0 ; i < 100 ; i++)); do
  echo "$i"
done
```

### Ranges

```
for i in {1..5}; do
    echo "Welcome $i"
done
```

#### With step size

```
for i in {5..50..5}; do
    echo "Welcome $i"
done
```

### Reading lines

```
while read -r line; do
  echo "$line"
done <file.txt
```

### Forever

## Functions

### Defining functions

```
myfunc() {
    echo "hello $1"
}
```

```
# Same as above (alternate syntax)
function myfunc {
    echo "hello $1"
}
```

```
myfunc "John"
```

### Returning values

```
myfunc() {
    local myresult='some value'
    echo "$myresult"
}
```

```
result=$(myfunc)
```

### Raising errors

```
myfunc() {
  return 1
}
```

```
if myfunc; then
  echo "success"
else
  echo "failure"
fi
```

### Arguments

| Expression | Description |
| --- | --- |
| `$#` | Number of arguments |
| `$*` | All positional arguments (as a single word) |
| `$@` | All positional arguments (as separate strings) |
| `$1` | First argument |
| `$_` | Last argument of the previous command |

**Note**: `$@` and `$*` must be quoted in order to perform as described. Otherwise, they do exactly the same thing (arguments as separate strings).

See [Special parameters](https://web.archive.org/web/20230318164746/https://wiki.bash-hackers.org/syntax/shellvars#special_parameters_and_shell_variables).

## Conditionals

### Conditions

Note that `[[` is actually a command/program that returns either `0` (true) or `1` (false). Any program that obeys the same logic (like all base utils, such as `grep(1)` or `ping(1)`) can be used as condition, see examples.

| Condition | Description |
| --- | --- |
| `[[ -z STRING ]]` | Empty string |
| `[[ -n STRING ]]` | Not empty string |
| `[[ STRING == STRING ]]` | Equal |
| `[[ STRING != STRING ]]` | Not Equal |
| `[[ NUM -eq NUM ]]` | Equal |
| `[[ NUM -ne NUM ]]` | Not equal |
| `[[ NUM -lt NUM ]]` | Less than |
| `[[ NUM -le NUM ]]` | Less than or equal |
| `[[ NUM -gt NUM ]]` | Greater than |
| `[[ NUM -ge NUM ]]` | Greater than or equal |
| `[[ STRING =~ STRING ]]` | Regexp |
| `(( NUM < NUM ))` | Numeric conditions |

#### More conditions

| Condition | Description |
| --- | --- |
| `[[ -o noclobber ]]` | If OPTIONNAME is enabled |
| `[[ ! EXPR ]]` | Not |
| `[[ X && Y ]]` | And |
| `[[ X || Y ]]` | Or |

### File conditions

| Condition | Description |
| --- | --- |
| `[[ -e FILE ]]` | Exists |
| `[[ -r FILE ]]` | Readable |
| `[[ -h FILE ]]` | Symlink |
| `[[ -d FILE ]]` | Directory |
| `[[ -w FILE ]]` | Writable |
| `[[ -s FILE ]]` | Size is > 0 bytes |
| `[[ -f FILE ]]` | File |
| `[[ -x FILE ]]` | Executable |
| `[[ FILE1 -nt FILE2 ]]` | 1 is more recent than 2 |
| `[[ FILE1 -ot FILE2 ]]` | 2 is more recent than 1 |
| `[[ FILE1 -ef FILE2 ]]` | Same files |

### Example

```
# String
if [[ -z "$string" ]]; then
  echo "String is empty"
elif [[ -n "$string" ]]; then
  echo "String is not empty"
else
  echo "This never happens"
fi
```

```
# Combinations
if [[ X && Y ]]; then
  ...
fi
```

```
# Equal
if [[ "$A" == "$B" ]]
```

```
# Regex
if [[ "A" =~ . ]]
```

```
if (( $a < $b )); then
   echo "$a is smaller than $b"
fi
```

```
if [[ -e "file.txt" ]]; then
  echo "file exists"
fi
```

## Arrays

### Defining arrays

```
Fruits=('Apple' 'Banana' 'Orange')
```

```
Fruits[0]="Apple"
Fruits[1]="Banana"
Fruits[2]="Orange"
```

### Working with arrays

```
echo "${Fruits[0]}"           # Element #0
echo "${Fruits[-1]}"          # Last element
echo "${Fruits[@]}"           # All elements, space-separated
echo "${#Fruits[@]}"          # Number of elements
echo "${#Fruits}"             # String length of the 1st element
echo "${#Fruits[3]}"          # String length of the Nth element
echo "${Fruits[@]:3:2}"       # Range (from position 3, length 2)
echo "${!Fruits[@]}"          # Keys of all elements, space-separated
```

### Operations

```
Fruits=("${Fruits[@]}" "Watermelon")    # Push
Fruits+=('Watermelon')                  # Also Push
Fruits=( "${Fruits[@]/Ap*/}" )          # Remove by regex match
unset Fruits[2]                         # Remove one item
Fruits=("${Fruits[@]}")                 # Duplicate
Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate
lines=(`cat "logfile"`)                 # Read from file
```

### Iteration

```
for i in "${arrayName[@]}"; do
  echo "$i"
done
```

## Dictionaries

### Defining

```
declare -A sounds
```

```
sounds[dog]="bark"
sounds[cow]="moo"
sounds[bird]="tweet"
sounds[wolf]="howl"
```

Declares `sound` as a Dictionary object (aka associative array).

### Working with dictionaries

```
echo "${sounds[dog]}" # Dog's sound
echo "${sounds[@]}"   # All values
echo "${!sounds[@]}"  # All keys
echo "${#sounds[@]}"  # Number of elements
unset sounds[dog]     # Delete dog
```

### Iteration

#### Iterate over values

```
for val in "${sounds[@]}"; do
  echo "$val"
done
```

#### Iterate over keys

```
for key in "${!sounds[@]}"; do
  echo "$key"
done
```

## Options

### Options

```
set -o noclobber  # Avoid overlay files (echo "hi" > foo)
set -o errexit    # Used to exit upon error, avoiding cascading errors
set -o pipefail   # Unveils hidden failures
set -o nounset    # Exposes unset variables
```

### Glob options

```
shopt -s nullglob    # Non-matching globs are removed  ('*.foo' => '')
shopt -s failglob    # Non-matching globs throw errors
shopt -s nocaseglob  # Case insensitive globs
shopt -s dotglob     # Wildcards match dotfiles ("*.sh" => ".foo.sh")
shopt -s globstar    # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')
```

Set `GLOBIGNORE` as a colon-separated list of patterns to be removed from glob matches.

## History

### Commands

| Command | Description |
| --- | --- |
| `history` | Show history |
| `shopt -s histverify` | Don’t execute expanded result immediately |

### Expansions

| Expression | Description |
| --- | --- |
| `!$` | Expand last parameter of most recent command |
| `!*` | Expand all parameters of most recent command |
| `!-n` | Expand `n`th most recent command |
| `!n` | Expand `n`th command in history |
| `!<command>` | Expand most recent invocation of command `<command>` |

### Operations

| Code | Description |
| --- | --- |
| `!!` | Execute last command again |
| `!!:s/<FROM>/<TO>/` | Replace first occurrence of `<FROM>` to `<TO>` in most recent command |
| `!!:gs/<FROM>/<TO>/` | Replace all occurrences of `<FROM>` to `<TO>` in most recent command |
| `!$:t` | Expand only basename from last parameter of most recent command |
| `!$:h` | Expand only directory from last parameter of most recent command |

`!!` and `!$` can be replaced with any valid expansion.

### Slices

| Code | Description |
| --- | --- |
| `!!:n` | Expand only `n`th token from most recent command (command is `0`; first argument is `1`) |
| `!^` | Expand first argument from most recent command |
| `!$` | Expand last token from most recent command |
| `!!:n-m` | Expand range of tokens from most recent command |
| `!!:n-$` | Expand `n`th token to last from most recent command |

`!!` can be replaced with any valid expansion i.e. `!cat`, `!-2`, `!42`, etc.

## Miscellaneous

### Numeric calculations

```
$((a + 200))      # Add 200 to $a
```

```
$(($RANDOM%200))  # Random number 0..199
```

```
declare -i count  # Declare as type integer
count+=1          # Increment
```

### Subshells

```
(cd somedir; echo "I'm now in $PWD")
pwd # still in first directory
```

### Redirection

```
python hello.py > output.txt            # stdout to (file)
python hello.py >> output.txt           # stdout to (file), append
python hello.py 2> error.log            # stderr to (file)
python hello.py 2>&1                    # stderr to stdout
python hello.py 2>/dev/null             # stderr to (null)
python hello.py >output.txt 2>&1        # stdout and stderr to (file), equivalent to &>
python hello.py &>/dev/null             # stdout and stderr to (null)
echo "$0: warning: too many users" >&2  # print diagnostic message to stderr
```

```
python hello.py < foo.txt      # feed foo.txt to stdin for python
diff <(ls -r) <(ls)            # Compare two stdout without files
```

### Inspecting commands

```
command -V cd
#=> "cd is a function/alias/whatever"
```

### Trap errors

```
trap 'echo Error at about $LINENO' ERR
```

or

```
traperr() {
  echo "ERROR: ${BASH_SOURCE[1]} at about ${BASH_LINENO[0]}"
}

set -o errtrace
trap traperr ERR
```

### Case/switch

```
case "$1" in
  start | up)
    vagrant up
    ;;

  *)
    echo "Usage: $0 {start|stop|ssh}"
    ;;
esac
```

### Source relative

```
source "${0%/*}/../share/foo.sh"
```

### printf

```
printf "Hello %s, I'm %s" Sven Olga
#=> "Hello Sven, I'm Olga

printf "1 + 1 = %d" 2
#=> "1 + 1 = 2"

printf "This is how you print a float: %f" 2
#=> "This is how you print a float: 2.000000"

printf '%s\n' '#!/bin/bash' 'echo hello' >file
# format string is applied to each group of arguments
printf '%i+%i=%i\n' 1 2 3  4 5 9
```

### Transform strings

| Command option | Description |
| --- | --- |
| `-c` | Operations apply to characters not in the given set |
| `-d` | Delete characters |
| `-s` | Replaces repeated characters with single occurrence |
| `-t` | Truncates |
| `[:upper:]` | All upper case letters |
| `[:lower:]` | All lower case letters |
| `[:digit:]` | All digits |
| `[:space:]` | All whitespace |
| `[:alpha:]` | All letters |
| `[:alnum:]` | All letters and digits |

#### Example

```
echo "Welcome To Devhints" | tr '[:lower:]' '[:upper:]'
WELCOME TO DEVHINTS
```

### Directory of script

### Getting options

```
while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in
  -V | --version )
    echo "$version"
    exit
    ;;
  -s | --string )
    shift; string=$1
    ;;
  -f | --flag )
    flag=1
    ;;
esac; shift; done
if [[ "$1" == '--' ]]; then shift; fi
```

### Heredoc

```
cat <<END
hello world
END
```

### Reading input

```
echo -n "Proceed? [y/n]: "
read -r ans
echo "$ans"
```

The `-r` option disables a peculiar legacy behavior with backslashes.

```
read -n 1 ans    # Just one character
```

### Special variables

| Expression | Description |
| --- | --- |
| `$?` | Exit status of last task |
| `$!` | PID of last background task |
| `$$` | PID of shell |
| `$0` | Filename of the shell script |
| `$_` | Last argument of the previous command |
| `${PIPESTATUS[n]}` | return value of piped commands (array) |

See [Special parameters](https://web.archive.org/web/20230318164746/https://wiki.bash-hackers.org/syntax/shellvars#special_parameters_and_shell_variables).

### Go to previous directory

```
pwd # /home/user/foo
cd bar/
pwd # /home/user/foo/bar
cd -
pwd # /home/user/foo
```

### Check for command’s result

```
if ping -c 1 google.com; then
  echo "It appears you have a working internet connection"
fi
```

### Grep check

```
if grep -q 'foo' ~/.bash_history; then
  echo "You appear to have typed 'foo' in the past"
fi
```

## Also see

-   [Bash-hackers wiki](https://web.archive.org/web/20230406205817/https://wiki.bash-hackers.org/) _(bash-hackers.org)_
-   [Shell vars](https://web.archive.org/web/20230318164746/https://wiki.bash-hackers.org/syntax/shellvars) _(bash-hackers.org)_
-   [Learn bash in y minutes](https://learnxinyminutes.com/docs/bash/) _(learnxinyminutes.com)_
-   [Bash Guide](http://mywiki.wooledge.org/BashGuide) _(mywiki.wooledge.org)_
-   [ShellCheck](https://www.shellcheck.net/) _(shellcheck.net)_#!/bin/bash


### SHORTCUTS and HISTORY

`CTRL+A`    move to beginning of line<br>
`CTRL+B`    moves backward one character<br>
`CTRL+C`    halts the current command<br>
`CTRL+D`    deletes one character backward or logs out of current session, similar to exit<br>
`CTRL+E`    moves to end of line<br>
`CTRL+F`    moves forward one character<br>
`CTRL+G`    aborts the current editing command and ring the terminal bell<br>
`CTRL+H`    deletes one character under cursor (same as DELETE)<br>
`CTRL+J`    same as RETURN<br>
`CTRL+K`    deletes (kill) forward to end of line<br>
`CTRL+L`    clears screen and redisplay the line<br>
`CTRL+M`    same as RETURN<br>
`CTRL+N`    next line in command history<br>
`CTRL+O`    same as RETURN, then displays next line in history file<br>
`CTRL+P`    previous line in command history<br>
`CTRL+Q`    resumes suspended shell output<br>
`CTRL+R`    searches backward<br>
`CTRL+S`    searches forward or suspends shell output<br>
`CTRL+T`    transposes two characters<br>
`CTRL+U`    kills backward from point to the beginning of line<br>
`CTRL+V`    makes the next character typed verbatim<br>
`CTRL+W`    kills the word behind the cursor<br>
`CTRL+X`    lists the possible filename completions of the current word<br>
`CTRL+Y`    retrieves (yank) last item killed<br>
`CTRL+Z`    stops the current command, resume with fg in the foreground or bg in the background<br>

`ALT+B`   moves backward one word<br>
`ALT+D`   deletes next word<br>
`ALT+F`   moves forward one word<br>
`ALT+H`   deletes one character backward<br>
`ALT+T`   transposes two words<br>
`ALT+.`   pastes last word from the last command. Pressing it repeatedly traverses through command history.<br>
`ALT+U`   capitalizes every character from the current cursor position to the end of the word<br>
`ALT+L`   uncapitalizes every character from the current cursor position to the end of the word<br>
`ALT+C`   capitalizes the letter under the cursor. The cursor then moves to the end of the word.<br>
`ALT+R`   reverts any changes to a command you’ve pulled from your history if you’ve edited it.<br>
`ALT+?`   list possible completions to what is typed<br>
`ALT+^`   expand line to most recent match from history<br>

`CTRL+X` then `(`     start recording a keyboard macro<br>
`CTRL+X` then `)`     finish recording keyboard macro<br>
`CTRL+X` then `E`     recall last recorded keyboard macro<br>
`CTRL+X` then `CTRL+E`invoke text editor (specified by $EDITOR) on current command line then execute resultes as shell commands<br>
`CTRL+A` then `D`   logout from screen but don't kill it, if any command exist, it will continue<br>

BACKSPACE  # deletes one character backward
DELETE     # deletes one character under cursor

history   # shows command line history
!!        # repeats the last command
!<n>      # refers to command line 'n'
!<string> # refers to command starting with 'string'
esc :wq   # exits and saves script

exit      # logs out of current session


### BASH BASICS

env                 # displays all environment variables

echo $SHELL         # displays the shell you're using
echo $BASH_VERSION  # displays bash version

bash                # if you want to use bash (type exit to go back to your previously opened shell)
whereis bash        # locates the binary, source and manual-page for a command
which bash          # finds out which program is executed as 'bash' (default: /bin/bash, can change across environments)

clear               # clears content on window (hide displayed lines)


### FILE COMMANDS


ls                            # lists your files in current directory, ls <dir> to print files in a specific directory
ls -l                         # lists your files in 'long format', which contains the exact size of the file, who owns the file and who has the right to look at it, and when it was last modified
ls -a                         # lists all files in 'long format', including hidden files (name beginning with '.')
ln -s <filename> <link>       # creates symbolic link to file
readlink <filename>           # shows where a symbolic links points to
tree                          # show directories and subdirectories in easilly readable file tree
mc                            # terminal file explorer (alternative to ncdu)
touch <filename>              # creates or updates (edit) your file
mktemp -t <filename>          # make a temp file in /tmp/ which is deleted at next boot (-d to make directory)
cat <filename>                # displays file raw content (will not be interpreted)
cat -n <filename>             # shows number of lines
nl <file.sh>                  # shows number of lines in file
cat filename1 > filename2     # Copy filename1 to filename2
cat filename1 >> filename2    # merge two files texts together
any_command > <filename>      # '>' is used to perform redirections, it will set any_command's stdout to file instead of "real stdout" (generally /dev/stdout)
more <filename>               # shows the first part of a file (move with space and type q to quit)
head <filename>               # outputs the first lines of file (default: 10 lines)
tail <filename>               # outputs the last lines of file (useful with -f option) (default: 10 lines)
vim <filename>                # opens a file in VIM (VI iMproved) text editor, will create it if it doesn't exist
mv <filename1> <dest>         # moves a file to destination, behavior will change based on 'dest' type (dir: file is placed into dir; file: file will replace dest (tip: useful for renaming))
cp <filename1> <dest>         # copies a file
rm <filename>                 # removes a file
find . -name <name> <type>    # searches for a file or a directory in the current directory and all its sub-directories by its name
diff <filename1> <filename2>  # compares files, and shows where they differ
wc <filename>                 # tells you how many lines, words and characters there are in a file. Use -lwc (lines, word, character) to ouput only 1 of those informations
sort <filename>               # sorts the contents of a text file line by line in alphabetical order, use -n for numeric sort and -r for reversing order.
sort -t -k <filename>         # sorts the contents on specific sort key field starting from 1, using the field separator t.
rev                           # reverse string characters (hello becomes olleh)
chmod -options <filename>     # lets you change the read, write, and execute permissions on your files (more infos: SUID, GUID)
gzip <filename>               # compresses files using gzip algorithm
gunzip <filename>             # uncompresses files compressed by gzip
gzcat <filename>              # lets you look at gzipped file without actually having to gunzip it
lpr <filename>                # prints the file
lpq                           # checks out the printer queue
lprm <jobnumber>              # removes something from the printer queue
genscript                     # converts plain text files into postscript for printing and gives you some options for formatting
dvips <filename>              # prints .dvi files (i.e. files produced by LaTeX)
grep <pattern> <filenames>    # looks for the string in the files
grep -r <pattern> <dir>       # search recursively for pattern in directory
head -n file_name | tail +n   # Print nth line from file.
head -y lines.txt | tail +x   # want to display all the lines from x to y. This includes the xth and yth lines.

sed 's/<pattern>/<replacement>/g' <filename> # replace pattern in file with replacement value to std output the character after s (/) is the delimeter 
sed -i 's/<pattern>/<replacement>/g' <filename> # replace pattern in file with replacement value in place
echo "this" | sed 's/is/at/g' # replace pattern from input stream with replacement value

### DIRECTORY COMMANDS


mkdir <dirname>               # makes a new directory
rmdir <dirname>               # remove an empty directory
rmdir -rf <dirname>           # remove a non-empty directory
mv <dir1> <dir2>              # rename a directory from <dir1> to <dir2>
cd                            # changes to home
cd ..                         # changes to the parent directory
cd <dirname>                  # changes directory
cp -r <dir1> <dir2>           # copy <dir1> into <dir2> including sub-directories
pwd                           # tells you where you currently are
cd ~                          # changes to home.
cd -                          # changes to previous working directory

### SSH, SYSTEM INFO & NETWORK COMMANDS


ssh user@host            # connects to host as user
ssh -p <port> user@host  # connects to host on specified port as user
ssh-copy-id user@host    # adds your ssh key to host for user to enable a keyed or passwordless login

whoami                   # returns your username
su <user>                # switch to a different user
su -                     # switch to root, likely needs to be sudo su -
sudo <command>           # execute command as the root user
passwd                   # lets you change your password
quota -v                 # shows what your disk quota is
date                     # shows the current date and time
cal                      # shows the month's calendar
uptime                   # shows current uptime
w                        # displays whois online
finger <user>            # displays information about user
uname -a                 # shows kernel information
man <command>            # shows the manual for specified command
info <command>           # shows another documentation system for the specific command
help                     # shows documentation about built-in commands and functions
df                       # shows disk usage
du <filename>            # shows the disk usage of the files and directories in filename (du -s give only a total)
resize2fs                # ext2/ext3/ext4 file system resizer
last <yourUsername>      # lists your last logins
ps -u yourusername       # lists your processes
kill <PID>               # kills the processes with the ID you gave
killall <processname>    # kill all processes with the name
top                      # displays your currently active processes
lsof                     # lists open files
bg                       # lists stopped or background jobs ; resume a stopped job in the background
fg                       # brings the most recent job in the foreground
fg <job>                 # brings job to the foreground

ping <host>              # pings host and outputs results
whois <domain>           # gets whois information for domain
dig <domain>             # gets DNS information for domain
dig -x <host>            # reverses lookup host
wget <file>              # downloads file
netstat                  # Print network connections, routing tables, interface statistics, masquerade connections, and multicast memberships

time <command>           # report time consumed by command execution


### VARIABLES


varname=value                # defines a variable
varname=value command        # defines a variable to be in the environment of a particular subprocess
echo $varname                # checks a variable's value
echo $$                      # prints process ID of the current shell
echo $!                      # prints process ID of the most recently invoked background job
echo $?                      # displays the exit status of the last command
read <varname>               # reads a string from the input and assigns it to a variable
read -p "prompt" <varname>   # same as above but outputs a prompt to ask user for value
column -t <filename>         # display info in pretty columns (often used with pipe)
let <varname> = <equation>   # performs mathematical calculation using operators like +, -, *, /, %
export VARNAME=value         # defines an environment variable (will be available in subprocesses)
export -f  <funcname>        # Exports function 'funcname'
export var1="var1 value"     # Export and assign in the same statement
export <varname>             # Copy Bash variable
declare -x <varname>         # Copy Bash variable

array[0]=valA                # how to define an array
array[1]=valB
array[2]=valC
array=([2]=valC [0]=valA [1]=valB)  # another way
array=(valA valB valC)              # and another

${array[i]}                  # displays array's value for this index. If no index is supplied, array element 0 is assumed
${#array[i]}                 # to find out the length of any element in the array
${#array[@]}                 # to find out how many values there are in the array

declare -a                   # the variables are treated as arrays
declare -f                   # uses function names only
declare -F                   # displays function names without definitions
declare -i                   # the variables are treated as integers
declare -r                   # makes the variables read-only
declare -x                   # marks the variables for export via the environment
declare -l                   # uppercase values in the variable are converted to lowercase
declare -A                   # makes it an associative array

${varname:-word}             # if varname exists and isn't null, return its value; otherwise return word
${varname:word}              # if varname exists and isn't null, return its value; otherwise return word
${varname:=word}             # if varname exists and isn't null, return its value; otherwise set it word and then return its value
${varname:?message}          # if varname exists and isn't null, return its value; otherwise print varname, followed by message and abort the current command or script
${varname:+word}             # if varname exists and isn't null, return word; otherwise return null
${varname:offset:length}     # performs substring expansion. It returns the substring of $varname starting at offset and up to length characters

${variable#pattern}          # if the pattern matches the beginning of the variable's value, delete the shortest part that matches and return the rest
${variable##pattern}         # if the pattern matches the beginning of the variable's value, delete the longest part that matches and return the rest
${variable%pattern}          # if the pattern matches the end of the variable's value, delete the shortest part that matches and return the rest
${variable%%pattern}         # if the pattern matches the end of the variable's value, delete the longest part that matches and return the rest
${variable/pattern/string}   # the longest match to pattern in variable is replaced by string. Only the first match is replaced
${variable//pattern/string}  # the longest match to pattern in variable is replaced by string. All matches are replaced

${#varname}                  # returns the length of the value of the variable as a character string

*(patternlist)               # matches zero or more occurrences of the given patterns
+(patternlist)               # matches one or more occurrences of the given patterns
?(patternlist)               # matches zero or one occurrence of the given patterns
@(patternlist)               # matches exactly one of the given patterns
!(patternlist)               # matches anything except one of the given patterns

$(UNIX command)              # command substitution: runs the command and returns standard output

typeset -l <x>                 # makes variable local - <x> must be an interger

### FUNCTIONS


# The function refers to passed arguments by position (as if they were positional parameters), that is, $1, $2, and so forth.
# $@ is equal to "$1" "$2"... "$N", where N is the number of positional parameters. $# holds the number of positional parameters.


function functname() {
  shell commands
}

unset -f functname  # deletes a function definition
declare -f          # displays all defined functions in your login session


### FLOW CONTROLS


statement1 && statement2  # and operator
statement1 || statement2  # or operator

-a                        # and operator inside a test conditional expression
-o                        # or operator inside a test conditional expression

### STRINGS

str1 == str2               # str1 matches str2
str1 != str2               # str1 does not match str2
str1 < str2                # str1 is less than str2 (alphabetically)
str1 > str2                # str1 is greater than str2 (alphabetically)
str1 \> str2               # str1 is sorted after str2
str1 \< str2               # str1 is sorted before str2
-n str1                    # str1 is not null (has length greater than 0)
-z str1                    # str1 is null (has length 0)

### FILES

-a file                   # file exists or its compilation is successful
-d file                   # file exists and is a directory
-e file                   # file exists; same -a
-f file                   # file exists and is a regular file (i.e., not a directory or other special type of file)
-r file                   # you have read permission
-s file                   # file exists and is not empty
-w file                   # your have write permission
-x file                   # you have execute permission on file, or directory search permission if it is a directory
-N file                   # file was modified since it was last read
-O file                   # you own file
-G file                   # file's group ID matches yours (or one of yours, if you are in multiple groups)
file1 -nt file2           # file1 is newer than file2
file1 -ot file2           # file1 is older than file2

### NUMBERS

-lt                       # less than
-le                       # less than or equal
-eq                       # equal
-ge                       # greater than or equal
-gt                       # greater than
-ne                       # not equal

if condition
then
  statements
[elif condition
  then statements...]
[else
  statements]
fi

for x in {1..10}
do
  statements
done

for name [in list]
do
  statements that can use $name
done

for (( initialisation ; ending condition ; update ))
do
  statements...
done

case expression in
  pattern1 )
    statements ;;
  pattern2 )
    statements ;;
esac

select name [in list]
do
  statements that can use $name
done

while condition; do
  statements
done

until condition; do
  statements
done

### COMMAND-LINE PROCESSING CYCLE


# The default order for command lookup is functions, followed by built-ins, with scripts and executables last.
# There are three built-ins that you can use to override this order: `command`, `builtin` and `enable`.

command  # removes alias and function lookup. Only built-ins and commands found in the search path are executed
builtin  # looks up only built-in commands, ignoring functions and commands found in PATH
enable   # enables and disables shell built-ins

eval     # takes arguments and run them through the command-line processing steps all over again


### INPUT/OUTPUT REDIRECTORS


cmd1|cmd2  # pipe; takes standard output of cmd1 as standard input to cmd2
< file     # takes standard input from file
> file     # directs standard output to file
>> file    # directs standard output to file; append to file if it already exists
>|file     # forces standard output to file even if noclobber is set
n>|file    # forces output to file from file descriptor n even if noclobber is set
<> file    # uses file as both standard input and standard output
n<>file    # uses file as both input and output for file descriptor n
n>file     # directs file descriptor n to file
n<file     # takes file descriptor n from file
n>>file    # directs file description n to file; append to file if it already exists
n>&        # duplicates standard output to file descriptor n
n<&        # duplicates standard input from file descriptor n
n>&m       # file descriptor n is made to be a copy of the output file descriptor
n<&m       # file descriptor n is made to be a copy of the input file descriptor
&>file     # directs standard output and standard error to file
<&-        # closes the standard input
>&-        # closes the standard output
n>&-       # closes the ouput from file descriptor n
n<&-       # closes the input from file descriptor n

|tee <file># output command to both terminal and a file (-a to append to file)


### PROCESS HANDLING


# To suspend a job, type CTRL+Z while it is running. You can also suspend a job with CTRL+Y.
# This is slightly different from CTRL+Z in that the process is only stopped when it attempts to read input from terminal.
# Of course, to interrupt a job, type CTRL+C.

myCommand &  # runs job in the background and prompts back the shell

jobs         # lists all jobs (use with -l to see associated PID)

fg           # brings a background job into the foreground
fg %+        # brings most recently invoked background job
fg %-        # brings second most recently invoked background job
fg %N        # brings job number N
fg %string   # brings job whose command begins with string
fg %?string  # brings job whose command contains string

kill -l               # returns a list of all signals on the system, by name and number
kill PID              # terminates process with specified PID
kill -s SIGKILL 4500  # sends a signal to force or terminate the process
kill -15 913          # Ending PID 913 process with signal 15 (TERM)
kill %1               # Where %1 is the number of job as read from 'jobs' command.

ps           # prints a line of information about the current running login shell and any processes running under it
ps -a        # selects all processes with a tty except session leaders

trap cmd sig1 sig2  # executes a command when a signal is received by the script
trap "" sig1 sig2   # ignores that signals
trap - sig1 sig2    # resets the action taken when the signal is received to the default

disown <PID|JID>    # removes the process from the list of jobs

wait                # waits until all background jobs have finished
sleep <number>      # wait # of seconds before continuing

pv                  # display progress bar for data handling commands. often used with pipe like |pv
yes                 # give yes response everytime an input is requested from script/process


### TIPS & TRICKS


# set an alias
cd; nano .bash_profile
> alias gentlenode='ssh admin@gentlenode.com -p 3404'  # add your alias in .bash_profile

# to quickly go to a specific directory
cd; nano .bashrc
> shopt -s cdable_vars
> export websites="/Users/mac/Documents/websites"

source .bashrc
cd $websites


### DEBUGGING SHELL PROGRAMS


bash -n scriptname  # don't run commands; check for syntax errors only
set -o noexec       # alternative (set option in script)

bash -v scriptname  # echo commands before running them
set -o verbose      # alternative (set option in script)

bash -x scriptname  # echo commands after command-line processing
set -o xtrace       # alternative (set option in script)

trap 'echo $varname' EXIT  # useful when you want to print out the values of variables at the point that your script exits

function errtrap {
  es=$?
  echo "ERROR line $1: Command exited with status $es."
}

trap 'errtrap $LINENO' ERR  # is run whenever a command in the surrounding script or function exits with non-zero status

function dbgtrap {
  echo "badvar is $badvar"
}

trap dbgtrap DEBUG  # causes the trap code to be executed before every statement in a function or script
 ...section of code in which the problem occurs...
trap - DEBUG  # turn off the DEBUG trap

function returntrap {
  echo "A return occurred"
}

trap returntrap RETURN  # is executed each time a shell function or a script executed with the . or source commands finishes executing

### COLORS AND BACKGROUNDS
note: \e or \x1B also work instead of \033
Reset
Color_Off='\033[0m' # Text Reset

### Regular Colors
Black='\033[0;30m'  # Black
Red='\033[0;31m'    # Red
Green='\033[0;32m'  # Green
Yellow='\033[0;33m' # Yellow
Blue='\033[0;34m'   # Blue
Purple='\033[0;35m' # Purple
Cyan='\033[0;36m'   # Cyan
White='\033[0;97m'  # White

### Additional colors
LGrey='\033[0;37m'  # Light Gray
DGrey='\033[0;90m'  # Dark Gray
LRed='\033[0;91m'   # Light Red
LGreen='\033[0;92m' # Light Green
LYellow='\033[0;93m'# Light Yellow
LBlue='\033[0;94m'  # Light Blue
LPurple='\033[0;95m'# Light Purple
LCyan='\033[0;96m'  # Light Cyan


### Bold
BBlack='\033[1;30m' # Black
BRed='\033[1;31m'   # Red
BGreen='\033[1;32m' # Green
BYellow='\033[1;33m'# Yellow
BBlue='\033[1;34m'  # Blue
BPurple='\033[1;35m'# Purple
BCyan='\033[1;36m'  # Cyan
BWhite='\033[1;37m' # White

### Underline
UBlack='\033[4;30m' # Black
URed='\033[4;31m'   # Red
UGreen='\033[4;32m' # Green
UYellow='\033[4;33m'# Yellow
UBlue='\033[4;34m'  # Blue
UPurple='\033[4;35m'# Purple
UCyan='\033[4;36m'  # Cyan
UWhite='\033[4;37m' # White

### Background
On_Black='\033[40m' # Black
On_Red='\033[41m'   # Red
On_Green='\033[42m' # Green
On_Yellow='\033[43m'# Yellow
On_Blue='\033[44m'  # Blue
On_Purple='\033[45m'# Purple
On_Cyan='\033[46m'  # Cyan
On_White='\033[47m' # White

### Example of usage
echo -e "${Green}This is GREEN text${Color_Off} and normal text"
echo -e "${Red}${On_White}This is Red test on White background${Color_Off}"
option -e is mandatory, it enable interpretation of backslash escapes
printf "${Red} This is red \n"