Shell

Terminal emulator

Terminal emulator is the prerequisite of shell. An open-source terminal emulator is terminator. Install it with

sudo apt install terminator

The terminal emulator provides color and font support for shell. For example, the agnoster theme of zsh requires the powerline font. To install it, run

sudo apt install fonts-powerline

and then set the fonts in the terminator.

Swap keys

Usually, if someone wants to swap Caplock and left Ctrl since the latter is more useful, some desktop provides an GUI tool such as gnome-tweak in GNOME or the setting in KDE that support this function. For the desktops that do not have an GUI tool, a shell method is to use xmodmap and the GUI helper xkeycaps. After change the keys in xkeycaps, write out the configuration file of xmodmap in the HOME diretory, and add the follows in the shell

xmodmap ~/.xmodmap-uname -n``

xmodmap will trigger the key swap when open a terminal.

Bash

Bash has two style interface: emacs style and vi style. Emacs style is the default style. Vi style can type to exit insert mode and use vim keybindings. The follows are the keymappings in for its emacs style.

CTRL+a/e    # Move to begin/End of the line
CTRL+b/f    # Backward/forward
CTRL+c      # Halt
CTRL+d      # Logout when no editing command
            # Display the completion for the commmand
CTRL+g      # Abort the editing command
CTRL+h      # Delete the character before cursor
CTRL+j/m    # <RETURN>
CTRL+k/w    # Delete to the end/begin of the line
CTRL+l      # Clears the terminal except the current line
CTRL+n/p    # <DOWN>/<UP>
CTRL+o      # <RETURN>, then display next line in the history

CTRL+s/q    # Suspend/Resume shell output
CTRL+r      # Search backward
CTRL+t      # Transpose two characters
CTRL+u      # Delete the line
CTRL+v      # Enable to insert control strings
CTRL+y      # Yank (for CTRL+k/u/w)
CTRL+z      # Stop the current command, resume with 'fg' in the foreground

CTRL+x (    # Recording a keyboard macro
CTRL+x )    # Stop recording
CTRL+x e    # Recall the record

CTRL+x CTRL+e   # Invoke text editor in $EDITOR 
                # Recommandation: export EDITOR=vim
                # In vim, you can type the command
                # Type 'ZZ' in normal mode to input multi-line commands for bash

CTRL+S      # Search forword

ALT+b/f     # Backward/Forward one word
ALT+c       # Upper character
ALT+d       # Delete word
ALT+h       # 'run-help' program 
ALT+l/u     # Upper, to the end/begin of the line
ALT+n/p     # Search history to match the edited command
ALT+r       # Revert changes of the command from the history
ALT+t       # Transpose two words
ALT+.       # Paste last word from the last command
ALT+?       # List completions

!!          # Repeat the last command
!<n>        # Refer to command line 'n'
!<string>   # Refer to command starting with 'string'

File

touch <file>        # Update timestamp
                    # Make a new file
mktemp -t <file>    # Make a temp file in /tmp/ 
cat <file>          # Display <file>
nl <file>           # Number of lines 
wc <file>           # Word count
more/less <file>    # Display a file and type q to quit
head/tail <file>    # Output the first/last lines of file 
diff                # Compare two files
sort <file>         # Sort the file line by line
rev <file>          # Reverse strings in file line by line
grep <pat> <file>   # Find for the string in the <file>
sed                 # Stream editor

ln -s <file> <link> # Symbolic link to <file>
readlink <link>     # Display the file name of the symbolic link 

Disk

df                  # Disk free
df -h               # Echo the result with human-readable output
du                  # Disk used
du -h -d 1 . 2&> /dev/null | grep "G\s"
                    # Echo the size of directories and files of the 
                    # current folder. Ignore errors. Grep a space 
                    # after 'G'. Useful command to show the large 
                    # directories and files

Process and Job

jobs                # Builtin: List jobs
ps                  # List processes in snapshot
top                 # Real-time list processes
kill/killall        # Builtin: Kill process with PID/name
bg                  # Builtin: List background jobs
fg                  # Builtin: Bring the background job to foreground
time <command>      # Builtin: Execution time
wait                # Builtin: Wait all background jobs to finish
sleep <num>         # wait <num> seconds before continuing
yes                 # Always yes

Variable

var=value           # Variables, NO <SPACE>
export VAR=value    # Environment variable to child, NO <SPACE>
$@{#var@}             # Length of var

list=(a b c)        # List
$@{list[@@]@}          # All elements in list

read <var>          # Read from input and assign it to <var>
let <var> = <expr>  # let a result of <expr> 
declare <var>       # Declare variable and its attribute

	Parameter Set and Not Null	Parameter Set but Null	Parameter Not Set
$@{parameter:-word@}	substitute parameter	substitute word	substitute word
$@{parameter:=word@}	substitute parameter	assign word	assign word
$@{parameter:?word@}	substitute parameter	error, exit	error, exit
$@{parameter:+word@}	substitute parameter	substitute null	substitute null
$@{parameter-word@}	substitute parameter	substitute null	substitute word
$@{parameter=word@}	substitute parameter	substitute null	assign word
$@{parameter?word@}	substitute word	substitute null	error, exit
$@{parameter+word@}	substitute word	substitute word	substitute null

$@{variable#pattern@}     # Remove Smallest Prefix Pattern
$@{variable##pattern@}    # Remove Largest Prefix Pattern
$@{variable%pattern@}     # Remove Smallest Suffix Pattern    
$@{variable%%pattern@}    # Remove Largest Suffix Pattern     

$0      # The program
$1      # The first parameter
$2      # The second parameter
$#      # The number of parameters
$@      # All parameters in a list

IF Condition

test expression
[ expression ]
[[ expression ]]

String

str1 == str2    # Match
str1 != str2    # Not match
str1 < str2     # Less
str1 > str2     # Greater
-z str1         # Null
-n str1         # Not null

File

-a file         # File
-b file         # Block file
-c file         # character file
-d file         # Directory
-e file         # File
-f file         # Regular file
-r file         # Read
-s file         # Not empty
-w file         # Write
-x file         # Execute
-N file         # File was modified
-O file         # File belongs to user
-G file         # File belongs to user group

Numbers

-lt         # Less than
-le         # Less than or equal
-eq         # Equal
-ge         # Greater than or equal
-gt         # Greater than
-ne         # Not equal

Input/Output Redirector

|           # Pipe
< file      # Redirect input
> file      # Redirect output
>> file     # Redirect output and append to a file
<> file     # uses file as both standard input and standard output
2>&1        # Redirect error output to standard output
n<&m        # Redirect input file n to m
n>&m        # Redirect output file n to m 

cat << EOF # Here document
    Text
EOF

tee <file>  # output input to both terminal and a file 

Debug

Run the script

bash -n scriptname  # Check errors without running
bash -v scriptname  # Print shell input lines as they are read.
bash -x scriptname  # Print commands and their arguments as they are executed.

Set options in script

set -o noexec       # Check errors without running
set -o verbose      # Print shell input lines as they are read.
set -o xtrace       # Print commands and their arguments as they are executed.

eval

With eval, bash can print the value of a variable as well as its name In this case, the value of BLACK is a string to control the string’s color The value of COLOR is the string “BLACK”

BLACK=’\e[30m’ COLOR=BLACK eval echo -e ‘$’@{$@{COLOR@}@}$@{COLOR@}

Color

BLACK='\e[30m'
RED='\e[31m'
GREEN='\e[32m' 
YELLOW='\e[33m'
BLUE='\e[34m'
PURPLE='\e[35m'
CYAN='\e[36m'
WHITE='\e[37m'
LBLACK='\e[90m'
LRED='\e[91m'
LGREEN='\e[92m'
LYELLOW='\e[93m'
LBLUE='\e[94m'
LPURPLE='\e[95m'
LCYAN='\e[96m'
LGREY='\e[97m'

echo -en "$@{RED@}RED"