Understanding Bash Basics: Shell State, Variables, and Scripting Essentials
Explore the fundamental concepts of Bash scripting, including the management of shell state, variables, and script essentials. Learn about maintaining shell state, using shell variables for string substitution, and utilizing default script variables for enhanced functionality.
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Lecture Participation Poll #3 Log onto pollev.com/cse374 Or Text CSE374 to 22333 CSE 374: Intermediate Programming Concepts and Tools Lecture 3: Bash Basics 1
Administrivia HW 1 will be posted later today will post instructions on setup and HW to course webpage - will include access to Klaatu Linux accounts will announce posting via Ed board (which sends emails) - - Sorry about Canvas weirdness Working with UW IT to fix panopto and zoom tabs - Lecture 2 video is now on Panopto Recordings tab of Canvas Going forward goal is to have lectures recorded and uploaded as well as a live sign in link, but figuring that out - - CSE 374 AU 21 - KASEY CHAMPION 2
Shell State Each new instance of the bash shell maintains a state -Current location in the file system PATH variable -Whenever a command is executed there is a list of pre-defined locations bash looks -PATH holds the list of pre-designed directories -echo $PATH Bash rc file -A shell script that is automatically run whenever Bash starts up -Used to initialize your shell state -You can find example files shard online -Exists locally on your machine, not on the remote linux server Bash history file -.bash_history is a hidden file that automatically logs your command history https://astrobiomike.github.io/unix/modifying_your_path https://www.gnu.org/software/bash/manual/html_node/Bash-Startup-Files.html CSE 374 AU 20 - KASEY CHAMPION 3
Shell Variables Shell variables = string substitution -Declare variables in the shell to easily refer to a given string -All variables are strings Declare variables in the terminal with a name and a string value -<var name>= <var string> -EX: myvar= myvalue - Note: no white space allowed on either side of the = Refer to your variable using the $ symbol before the var name -$<var name> -EX: echo $myvar - myvalue Alias -Rename a bash command, create your own shortcut -alias <string>= substitution string - EX: alias cheer= echo hip hip horray! -Only exists within the currents state of your shell -Can store alias in bashrc file to preserve alias across all shells CSE 374 AU 20 - KASEY CHAMPION 4
Bash Script Variables When writing scripts you can use the following default variables $# - stores number of parameters entered Ex: if [$# -lt 1] tests if script was passed less than 1 argument $N - returns Nth argument passed to script Ex: sort $1 passes first string passed into script into sort command $0 command name Ex: echo $0 needs 1 argument prints <name of script> needs 1 argument $* returns all arguments $@ returns a space separated string containing all arguments $@ prevents args originally quoted from being read as multiple args CSE 374 AU 20 - KASEY CHAMPION 5
Alias defines a shortcut or alias to a command -alias <string>= substitution string - EX: alias cheer= echo hip hip horray! -technically just string replacement -Only exists within the currents state of your shell -Can store alias in bashrc file to preserve alias across all shells 6
Special Variables $HOME - sets home directory EX: $HOME=~/CSE374 would reset your home directory to always be CSE374 $PS1 - sets prompt $PATH - tells shell where to look for things printenv shows current state 7
Math in Bash Everything is interpreted as a string -No concept of integer values To do math use double parentheses to interpret as arithmetic expression -RES=$((1+2)) -$RES will be set to 3 -MYVAR=$((RES+2)) -$MYVAR will be 5 -K=$i + $j does not add numbers -k=$(($i+$j)) will add numbers CSE 374 AU 20 - KASEY CHAMPION 8
Quotes in the Shell Double quotes can be used to wrap a string with white space into a single argument -EX: myvar= Some string Single quotes tell the shell to treat the string as a literal -No variable expansion or command substitution -EX: echo $myvar - $myvar - echo $myvar - Some string CSE 374 AU 20 - KASEY CHAMPION 9
Demo: Variables and Aliases CSE 374 AU 20 - KASEY CHAMPION 10
