DSI Session 2 - Unix lesson 3 notes

Lesson3notes.md

Notes from Shell Lesson 3

Doing things faster in bash

Keyboard shortcuts

• A good (albeit comprehensive) cheatsheet is found here: https://gist.github.com/tuxfight3r/60051ac67c5f0445efee
• You'll eventually find a subset of the shortcuts that you use most often.
• My personal list is:
  • Ctrl + A / Home key: move to the beginning of the line
  • Ctrl + E / End key : move to the end of the line
  • Ctrl + K : delete from cursor to end of the line
  • Ctrl + W : delete the current word
  • Ctrl + Left/Right arrow : move cursor faster (one word at a time)

Last command recall

• Use the Up arrow (or Ctrl + P) to pull up previous commands
• Type !! and press Enter to directly execute the last command
• Use Ctrl + R to search your previous commands

Autocomplete (tab)

• Typing a few characters then pressing your keyboard's TAB key autocompletes either:
  • the command (if you're typing a command)
  • files in the current directory (if you're providing a file argument to a command)
• If there are multiple files that match what you've typed so far, press tab several times to see a list of options

Shell scripts

• Several shell commands in a text file
• Typically named with .sh file extensions
• Run scripts by either:
  1. Setting them to executable with chmod +x myscript.sh, then executing them with ./myscript.sh
  2. Passing the script to bash as an argument (eg. bash myscript.sh)
• Write scripts in a text editor (eg. VSCode or nano)
• All bash scripts should start with #!/bin/bash on the first line to tell the computer what to use to run it

Adapting script behaviour based on context

Variables

• Define variables with this notation:
  • Note: no space between variable name, equal sign, and assigned value

varname="varvalue"

• Use variables with this notation:
  • Note: the $ is required when using bash variables

echo $varname

myfirstscript.sh

#!/bin/bash

# This is a comment
myvar="World"
echo "Hello $myvar!"

Parameters

• Special variables named $1, $2, etc...
• Set by bash to the arguments provided to the command
  • (not tested) Note on terminology:
    • A "parameter" is the name of the variable in the context of the function (eg. $1)
    • An "argument" is the value provided to the function / script that gets assigned to each parameter
    • Sometimes these two words are used interchangeably

Suppose we have a shell script named myparamscript.sh

#!/bin/bash
echo "Hello $1!"

Then we can change what it does when we run it in the shell

$ bash myparamscript.sh World
Hello World!

$ bash myparamscript.sh Bash
Hello Bash!

$ bash myparamscript.sh Everyone
Hello Everyone!

Parameter Expansion

Parameter names can be made unambiguous using curly braces. This notation is called Parameter Expansion.

#!/bin/bash

# This is a comment
myvar="World"
echo "Hello ${myvar}s!"

Parameter Expansion can also be used to substitute or set default values:

$ myvar=
$ echo ${foo:-"default value"}
default value

String operations

$ myvar="Toronto needs more trees" 
$ echo ${myvar:8:5}
needs

$ echo ${i//needs/must have}
Toronto must have more trees

Command substitution

• Store output of a command into a variable

myvar=$(ls -1)
echo $myvar

Flow Control

&& and ||

• command1 && command2 will run command1, then run command2 only if command1 was successful
  • eg. prepfiles && analyzefiles
• command1 || command2 will run command1, then run command2 if command1 failed
  • eg. downloadfile || printerror

if/else

• Perform different actions based on the value of a condition

while/until

• Repeat actions while a condition is true or until a condition is true

#!/bin/bash
count=1
while [ $count -le 5 ]
do 
    echo $count
    count=$((count +1)) 

    mkdir "mynewdir$count"
    
done
echo "Finished."

#!/bin/bash
count=1
until [ $count -gt 5 ]
do 
    echo $count
    count=$((count +1)) 

    mkdir "mynewdir$count"
    
done
echo "Finished."

for

• Run commands within the loop for each item in a variable / returned from a command

Putting it all together

• This script renames all .txt files in a folder to .TXT

for i in $(find *.txt)
do 
    echo $i
    mv $i ${i//.txt/.TXT}
done