Lecture 7

Shell Scripts

Basic Bash

You can use many different shells to "run" your default terminal. In your labs, you played around with C-shell (or tcsh). So you are somewhat familiar with the commands: each of them can be executed as a stand alone one. For example you can use echo command that you know already. e.g.,
```
echo $PATH
```
The last echo command above prints the value of the PATH variable.
Note that we say echo $PATH but not echo PATH. Just PATH refers to the variable. $PATH refers to the value of that variable. In the first case you want to echo the value of the variable.
Simple bash scripts
The Shell language can be used to write scripts that control and run various jobs and do other things. Very convenient! For reasons I don't have time to go into (but look up a discussion in your "Unix Power Tools"), the C-shell is not optimal for this, and so we will be using the "Bourne Shell" or simply bash . Make sure that you prefix every shell script with the line
```
#!/bin/bash
```
The "#!" tells Linux that whatever follows is to be interpeted/executed using the /bin/bash command (i.e., the bash shell). Put your shell script in a file, say myscript.sh, and make the script executable by doing chmod u+x myscript.sh. To execute the script, just type its name as a command. However, just typing myscript.sh at the command prompt might not work. You would need to do ./myscript.sh, to indicate the path (i.e., the current directory) location of the script. If you would like the shell to find the path automatically, you must suitably set the PATH environment variable. Add . to your path, see the example C-shell from your lab. From this point onwards, you can get away with just typing myscript.sh.
- Here's the mandatory first bash script:
```
#!/bin/bash
echo "Hello World"
```
- This one prints numbers from 1 to 4:
```
#!/bin/bash

for i in 1 2 3 4 5 6
do
        echo $i

# note that the value of the variable has $ infront, and that 
# the "#" sign denotes a comment EXCEPT if followed by a '!". 

done
```
- If we omit the stuff including and after the "in" in a for loop, bash takes it to mean that we want the loop index to range over the command line arguments (given when the command is invoked). For instance,
```
#!/bin/bash
for ut
do
   mv $ut `echo $ut | tr "[a-z]" "[A-Z]"`
done

# The tr command translates letters, in our example from upper to lower case.
# Note the single quotes around the shell-interpetble command:
# Bash will execute the command inside ` ` and return the results.
# In other words, bash will first executes a shell command in `  ` and then
# copy the vlue of $ut to  the reulting value (upper-case file name). 
```
  Here, ut is a variable that ranges over the arguments. Lets put this shell script in a file called toupper and invoke toupper as: toupper home.accounts office.accounts. Then, the loop variable ut essentially first takes the value home.accounts, then takes the value office.accounts. The above shell script renames each file given as argument by replacing every lowecase letter with an upper case letter.
  
  An important point to note in the above command is the backquote (`). This is a cue to bash to execute the command enclosed within the backquotes and return the result. Without the backquotes, the above command will not produce the desired effect.
  
  You can even run the shell script on itself!; like so:
```
toupper toupper
```
  From this point, you will have to refer to the script as TOUPPER.
- The read command reads input from the terminal. Try the following shell script
```
#!/bin/bash

echo "Type your name"
read name

# let's add some color to life! (next line sets the fonts)
echo -en "\033[1;36;44m"

# outputs the name:
echo "Hi, $name, let's be friends!"
```

Lecture 7

Shell Scripts

Basic Bash

Simple bash scripts