Introduction to Bash Programming Ellen Zhang

Previous three classes What have we learnt so far ?

Outline Shell command line syntax Shell special characters, metacharacters – Write our own echo command – Use echo to experiment with metacharacters – Quotation to protect metacharacters Practice problems Our first shell script – Passing arguments to shell script – Generate arguments by running a command

Shell command line A command ends with a newline, or a semicolon (;), or an ampersand (&) – date; – date; who – sleep 20&who Sending two commands through a pipe: – date; who | wc – What happens ? – ls –l | grep ^d &

Shell command line Use parenthesis to group commands – (date;who) | wc sleep command: pause for given amount of time, and then return – How to show a message in the console to yourself in 30 minutes ? “get up and do some exercise !” – (sleep 1800; echo “get up and do some exercise!” )& – We will learn how to put this into a loop …

Useful commands tee: copies its input to a file and to its standard output – (date; who) | tee save | wc

Special SHELL characters Characters in the command line that shell interpret them and substitute them – >, >>, <, |, &, ;, ! – #: making the rest of the line comments (must be the first letter of a word) – $: $var means value of variable var – *: match any string of zero or more characters in file name (except those starting with.) – ?, [aeiou], [1-9], [a-z], [^0z],[^a-z]

Remember echo command echo: display a line of text – echo Good morning, everyone ! Writing you own echo program ? – Remember how are command line options passed to a program ? main(int argc, char * argv[]) argc: number of command line arguments, including command itself argv: the arguments

Our own echo program #include main(int argc, char ** argv) { //printf (“Number of arguments:%d\n”,argc); //display all command line arguments, except the // command name itself for (int i=1;i<argc;i++) printf (“%s ”,argv[i]); }

Write a small program A program called pick – Syntax: pick arguments… – Function: pick command presents the arguments one at a time and waits after each for a response. The output of pick is those argument selected by y response. Any other response causes the argument to be discarded.

Some Experiments with echo Let’s now use echo command to experiment with metacharacters – echo Good morning, #everyone! – echo Good morning > everyone – echo < message_file – echo $SHELL, echo $PS1 – echo *, echo *.o If no matching is found, original string is passed on – echo *.pdf

What if I want to display special characters ? What happens if we run echo Hello to the world >> ! Use quotation to tell the shell to leave the metacharacters alone… – echo Hello to the world “>>” ! – echo “Hello to the world >> !” – echo “*”

Different quotation Strongest one: single quote character – Echo ‘***’ – Echo ‘$PS1’ Less strong quote: double quote character – Shell peeks inside for $, `…` and \ – echo “Your PS1 is set to $PS1” To display “Your $PS1 is …” – Echo “Your \$PS1 is set to $PS1” – Backslash,\, protect a single character following it

Echo multiple lines message When using quotes, shell knows the command is not finished when a newline is entered Echo]$ echo "Hello, > world, > I can write my own echo." Hello, world, I can write my own echo. To enter a long command line Echo]$ echo Hello world,\ > I can write my own echo. Hello world,I can write my own echo. Echo]$

Practice Problems Create a file named a*b without using any editor, and then delete the file Start a program named long_job in the background, sleep for 20 seconds and then check if the long_job process is still running or not – Hint: ps command lists all current processes – grep file: search for the given string in the file or standard input

Our first shell script How to find the ten largest file under your directory ? – Commands to use ? Save the command sequence into a file – Run the file script using: bash 10largest bash <10largest

Script: 10largest ls -Rl * | grep ^- | sort -k 5 -nr | head -10

Run script like a command Would be nicer if the command is invoked same way as other commands Default permission for a new created file is uc]$ cat >dd echo Hello World ! uc]$ ls -l dd -rw-r--r-- 1 zhang staff :44 dd

Change file permission Make the file executable – chmod +x 10largest – #all users now have the right to execute the file – chmod u+x 10largest – # the owner can execute the file Remember “file” command ? – What will “file 10largest” return ? – One can add an initial comment line: – #!/bin/bash

Pass argument to script 10largest script does not take arguments – Would be nice if we can specify the directory to check, i.e., not always check current directory – i.e. 10largest path_name We need to access the argument, and call commands with according directory – ls -Rl * path_name| grep ^- | sort -k 5 -nr | head - 10

Shell parameter variables If your script is invoked with parameters, shell set the following variables – $#: the number of parameters – $0: the command/script name – $1: the first parameter given to the script – $2,… – the list of all parameters Example: 10largest2

Script: 10largest2 #echo $# #echo $1 #echo $0 ls -Rl * $1 | grep ^- | sort -k 5 -nr | head -10 What if we call 10largest2 without argument ?

Say we want to generate arguments We have used filename expansion (*,? etc) as a way to generate arguments One can use a command to generate the argument – echo Welcome, `who am i`. The time is `date`. Write to all students – mail `cat student_account.txt` < SampleScript What happens if we enter `date` on command line ?

Use pick to choose from the list Remember our pick program ? Say we want to remove all files in a directory, but want to make sure before we delete each file – rm `pick *` mail `pick \`cat mailinglist`` < sampleCode

Summary Shell command line syntax Shell special characters, metacharacters – Write our own echo command – Use echo to experiment with metacharacters – Quotation to protect metacharacters Our first shell script – Passing arguments to shell script – Generate arguments by running a command