An Introduction to the Command-Line (on Unix-like systems)

by Oliver; 2014

19. What is Scripting?

By this point, you should be comfortable using basic utilities like echo, cat, mkdir, cd, and ls. Let's enter a series of commands, creating a directory with an empty file inside it, for no particular reason:

$ mkdir tmp
$ cd tmp
$ pwd
/Users/oliver/tmp
$ touch myfile.txt  # the command touch creates an empty file
$ ls
myfile.txt
$ ls myfile_2.txt  # purposely execute a command we know will fail
ls: cannot access myfile_2.txt: No such file or directory

What if we want to repeat the exact same sequence of commands 5 minutes later? Massive bombshell—we can save all of these commands in a file! And then run them whenever we like! Try this:

$ nano myscript.sh

and write the following:

# a first script
mkdir tmp
cd tmp
pwd
touch myfile.txt
ls
ls myfile_2.txt

Gratuitous screenshot:

This file is called a script (.sh is a typical suffix for a shell script), and writing it constitutes our first step into the land of bona fide computer programming. In general usage, a script refers to a small program used to perform a niche task. What we've written is a recipe that says:

create a directory called "tmp"

go into that directory

print our current path in the file system

make a new file called "myfile.txt"

list the contents of the directory we're in

specifically list the file "myfile_2.txt" (which doesn't exist)

This script, though silly and useless, teaches us the fundamental fact that all computer programs are ultimately just lists of commands.

Let's run our program! Try:

$ ./myscript.sh
-bash: ./myscript.sh: Permission denied

WTF! It's dysfunctional. What's going on here is that the file permissions are not set properly. In unix, when you create a file, the default permission is not executable. You can think of this as a brake that's been engaged and must be released before we can go (and do something potentially dangerous). First, let's look at the file permissions:

$ ls -hl myscript.sh 
-rw-r--r-- 1 oliver staff 75 Oct 12 11:43 myscript.sh

Let's change the permissions with the command chmod and execute the script:

$ chmod u+x myscript.sh  # add executable(x) permission for the user(u) only
$ ls -hl myscript.sh 
-rwxr--r-- 1 oliver staff 75 Oct 12 11:43 myscript.sh

$ ./myscript.sh 
/Users/oliver/tmp/tmp
myfile.txt
ls: cannot access myfile_2.txt: No such file or directory

Not bad. Did it work? Yes, it did because it's printed stuff out and we see it's created tmp/myfile.txt:

$ ls
myfile.txt  myscript.sh  tmp
$ ls tmp
myfile.txt

An important note is that even though there was a cd in our script, if we type:

$ pwd
/Users/oliver/tmp

we see that we're still in the same directory as we were in when we ran the script. Even though the script entered /Users/oliver/tmp/tmp, and did its bidding, we stay in /Users/oliver/tmp. Scripts always work this way—where they go is independent of where we go.

If you're wondering why anyone would write such a pointless script, you're right—it would be odd if we had occasion to repeat this combination of commands. There are some more realistic examples of scripting below.

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