Episode 10 Shell scripts

We create shell scripts using the hash-bang header, and also look at permissions, variables, and loops in the shell.

29 September 2015

[Rhythmic, dark electronic intro music]


Welcome back to Command Line TV, this is episode 10.

Today we’re going to talk about shell scripts and permissions.

But first do we have any follow-up from last time?


We have one follow-up question. We did redirection in the last episode,

and the one issue we had was that – when we did the single > it would always overwrite the files.

We had one workaround which we used, set -o noclobber.

The question we had was, can it be turned off or do we just have to create a new terminal.


Ah, right. So if you set -o noclobber already in your terminal but then you want to undo that.

Generally i think you would have to restart the terminal.

As long as you haven’t set it in your .bashrc where it would read it

again and apply that again then that would take us back to a default where it will clobber things.

But another useful trick is that you can temporarily –

like just for one command – say I really do mean to overwrite this file as I redirect.

And the way you would do that is – let’s work here on an output file.

So I’ll just cat into output.txt and I can say some message and then –

cat > output.txt

what was it to stop the input?




Control-D, yup. So now I’ve got that output file, output.txt and if I try

to do that again it can’t overwrite the existing file because I have

cat > output.txt

noclobber set already in this terminal. But what I can do to override

that temporarily is just use > followed immediately by the pipe character,

no space in there: >|. That is not a pipe actually –

cat >| output.txt

I mean the character is pipe, but it’s not the feature of the shell called pipe.

It’s redirection but the pipe says yes I really mean to overwrite this file.

So now it will let you do that. This is another line. Control-D.

And if I look at the content of that file it only has the second one.

The first content got overwritten.


Today we’ll be doing shell scripts. To start off, what exactly is a shell script? What does it do?


So a script is just a way of automating something. Whenever you have a

series of commands that you might want to execute, instead of remembering

to do each one independently and getting all the options right and syntax,

you can write down those commands into a file and then execute that file.

And that’s what we call a script.

So the basic way to do that is first we create a text file.

And we’ve learned a couple ways to do that with whatever text editor you want to use.

One of them that we did before was nano so I’m going to type nano and

let’s call this just hello – it does not necessarily need an extension

nano hello

so just because it’s a text file doesn’t mean you have to put .txt.

But when it’s a shell script sometimes people will put a .sh at the end,

to say that this is meant to be executed with the shell.

But it’s really up to you what the file name is, or what extension it has.

So I’ll do that. And now the first line of your script is very special.

It always starts with what we call hash-bang, or sometimes it’s called sh-bang for short, I guess.

But hash-bang is pound sign and the exclamation point.

And this is a special signal to the operating system that this text file

can be executed using some command interpreter. And then you put the

location of the command interpreter after the hash-bang.


So if it’s a shell script, that means that the command interpreter is your shell.

It’s usually /bin/sh or /bin/bash or you could even write scripts that

are particular about using some other shell, like zsh or whatever.

We still haven’t looked at other shells besides bash but we’ll eventually do that.

Let’s keep this as /bin/sh – a very generic shell.

And now any commands that I type after this will not execute now,

but they’ll execute when I run the entire script. So I can say something like –

echo is just a very simple command that allows you to put out a message on the screen –

echo "Hello, world!"

so I can say echo and put a message there. And then let’s just for

example say “Your files are” and then I’ll run a command like ls.

echo "Your files are:"

So it’s just sequencing together these two echo commands and then the ls.

So I’ll save that – control-O, filename to write – oh it should’ve –

I thought it would get that from the command line. So I’ll say hello there.

[laughs] Oh – that’s fine, overwrite I guess. Oh it’s a directory, interesting.

So that’s why that didn’t work. I already have a directory called hello,

so I can’t also have a file called hello. So we’ll go ahead and use the hello.sh.

And then we can exit.

So if I ls from here, there is my hello.sh file that got created.

I’m almost ready to execute that as a script, but we have to now look at these permissions.

The x permission means that you’re allowed to execute something.

Directories will already have executable permission because when you cd

into a directory that is thought to be like executing it.

So the executable permission is what allows you to cd into directories.

But these other files that are not directories do not have executable permission.

I need this to be executable in order to run the script.

So there’s a command we’re going to do called chmod.

And we’ll talk about all the options of this command in a moment.

But chmod just means to change the permissions which are also called ‘modes’.

So mod comes from modes. The +x means turn on execute permissions, and then the filename.

chmod +x hello.sh

So after doing that if I get my ls listing, you see that this hello.sh

first of all it turned green, which means it’s executable. And it has the x permission out there.

So now I can run that and to run it, because it’s in the current directory

I use . for the current directory – ./hello.sh.


And there it is, so it says “Hello world” and “Your files are” and then it

runs ls so it gives you a listing of the files.


So when we first made this file you did hash-bang and then you had to

specify which shell is going to run it. Can you specify multiple shells or

is there something particular about that?


There has to be just one shell that’s expected to execute this.

So I can’t – let’s bring up nano again – hello.sh

nano hello.sh

I can’t put multiple shells here, or have multiple hash-bang lines or whatever.

But what this means is that this – it’s telling the system that this is

the shell that should run the script. That doesn’t mean that you have to

run it from that shell to begin with. So I could be in any shell,

and if I specify ./hello.sh it will then invoke the correct shell to run it.

So in other words if I’m currently using bash it will invoke sh.

If I’m currently using zsh it will invoke sh – because that’s what the header said to do.

It’s equivalent to – like if I do head on that –

head hello.sh

it’s equivalent to taking this line without the hash-bang and then putting

the name of the file you specified after that. So this is just another way to run it –

/bin/sh ./hello.sh

it’s more verbose, but it takes that shell that we specified and then the

file containing the script and it does exactly the same thing.

So that’s what that hash-bang notation is doing for us.

Also we call it shell scripts if the command interpreter you specify after the hash-bang is a shell,

like sh or bash or zsh. But it actually could be any programming language interpreter.

So if you’re familiar with Python or Ruby or some other interpreted

programming language you can do exactly the same thing.

Let’s just do a quick one like – I’ll say hello.py as an example.

nano hello.py

And in this case I want to put the path to the Python interpreter So

instead of /bin/sh my command interpreter is /usr/bin/python.

And now instead of using echo I’m going to use commands that Python supports.

So I can say like print “Hello from Python” or whatever.

print("Hello from Python.")

So I can write that to hello.py, exit, and then same thing – chmod +x hello.py.

chmod +x hello.py

And then I can run ./hello.py and this time – it’s still a script,


I just wouldn’t call it a shell script. It’s a Python script,

but it’s the same hash-bang notation to do it.


So do the scripts we write automatically have error-checking built in to it,

or is that something we have to set as an option.


It’s not very – I mean there is error-checking but it doesn’t necessarily do what you want.

So for example if you make an error in a script, it might then continue

trying all the rest of the commands anyway. And maybe it would be safer to just stop.

So let’s try that out – if I open up this hello script in nano,

nano hello.sh

and let’s say I make a mistake on this first command. I mistyped echo or something.

eco "Hello, world!"

So I’m going to write and exit, and then try to run that script.


What we see happen is – it does detect the error right when it gets to line 3,

but then it goes on with the rest of the script. Maybe you would rather quit.

So let’s come in here – an option you can set in a shell script that says

basically “quit on error” is you just say set -e at the top of your script like that.

set -e

Save that and exit. And now when I run it, it encounters that error and tells me about it,


but it does not continue with the rest of the commands.

So that’s probably a little safer to do that kind of detection.

There are some fancier things that you can do once you learn a little more about the shell syntax.

For example, if a certain command fails you can then provide an alternative.

So if this fails then do this instead. And maybe the alternative command is

just to issue an error message and quit, but it allows your script to be a

little bit more adaptable and friendlier. But that takes a little bit of

knowledge of programming and of the syntax of the shell.

We’ll start to get into that, but I don’t want to get that complex just yet.


So we used chmod to change the permission on that .sh file that we had.

We always see all these different letters – rwxd, maybe some dashes –

what are they all representing and what else can we do with them?


Right so this permission string, I’ll call it, that comes in the ls -l output.

I’m going to leave out the d part, that’s just whether it’s a directory or not.

But the rest of it are these 9 characters, right. So to demonstrate this

I’m going to open up an editor so I can just do some typing for us.

So rwxrwxrwx – that appears 3 times – those are the 9 characters.

And of course r is read, w is write, and x is execute. But why do they appear 3 times?

Well these apply to different classes of people on the system.

The first set of permissions is about the user, so the owner of that file

or the current user or whatever. The second set is about the group that owns that file.

The third set is everyone else, which we call “others” – anyone else.

So anyone that’s on this system but is not the owner or the group that owns the file.

We saw here – I guess I’ll save this as permissions.txt or something so I can come back to it.

We saw here also in the ls -l output that this is the owner of the file

and the group that owns the file. You can set up groups and stuff –

that’s a little beyond the scope of what we’re going to do right now.

But a user on the system can be a member of multiple groups,

so you might have different work groups that you coordinate with or whatever.

If I open that up again – the way that we specify those –

so when I did chmod I just did chmod +x and that means turn on all of

the execute permissions for everybody – for user, group, and others.

But you can also specify it in different ways. So I could say –

let’s say for user and other, turn on read and execute. And then you give your filename out here.

chmod uo+rx FILENAME

You could say something like, for group turn off write and execute and then you put the filename.

chmod g-wx FILENAME

So it’s just this little syntax of specifying which permissions to turn on and off.

And the first half of it, which is optional, is you can put u, g,

or o for user, group, or others. And then you have + or - to turn it on or off.

And then you can have r, w, or x for read, write, execute.

But there’s yet another notation that can be used for these permission strings.

And it comes up sometimes – it’s actually a numeric notation using the base 8, or octal.

So this is a little weird but what you do is you treat each of these as a bit.

So the bit is on or off, right. If it’s on we put the w, if it’s off we put the dash.

So something like that. And then maybe these three are all off.

Then underneath that you mentally put a 4, 2, 1 which are the first three powers of two.

So in binary notation you would use the first three powers of two here and

then just repeat them over and over again. Then you end up with an octal

number by just taking the sum in each group. So I’m going to split up the

three groups here to make it a little clearer. For this permission string that’s rx,

rw, and then blank – I would then add the 4 and the 1 because those are both on, to make 5.

Then in the next group I would add the 4 and 2 because those are on, to make 6.

Then none of these are on so that would be 0.

So that’s a numeric representation of exactly that permission string.

Let’s say I did chmod 560 filename and – let’s try that, I’ll save this first.

chmod 560 hello.sh

And we’ll do chmod 560 hello.sh. And now when I do ls -l I see exactly that set of permissions,

right? r-x and then rw- and then nothing. So that’s 5 and then 6 and then 0.

So you eventually kind of learn this numeric notation.

The ones that are most common are – let’s say – 644 and 755. So why are they the most common?

Well 644 means you’re turning on read and write for user,

and then you’re turning on read-only for group and read-only for others.

So this is kind of a file that can be read by anybody but only the user can write to it.

This is similar except you’re just turning on the execute bit for each of these.

So the 7 gets you rwx, 5 is 4+1 so that’s r-x and then r-x. So 644 and 755 are very common.

For files where you want to be a little more private –

if you want to make sure that only I can read this file and nobody else –

then you use zeroes for the rest. So maybe 600 would be rw- for me and nothing for anybody else.

Or if it’s executable then 700 so that’s rwx, blank, and blank.

Those octal notations are there and a lot of Unix people know and understand them very well.

But they are optional. If you’re more comfortable with it,

you can always just use the + and - with this other notation and that works just fine.


So that shell script we wrote at the beginning of the episode seemed very

similar to writing a program in C++ or any other programming language.

Does it support features such as variables and loops, just like these programming languages do?


Yeah, the shell is a full-blown programming language, it’s just kind of a weird one.

But it’s useful the way it’s designed. So it does support variables and

loops and if-then-else and all those sorts of things.

To do variables is fairly simple and I can just demonstrate it here on the command line,

as well as in a script. Let’s say that I want a variable called x – and I’ll give it a value.

If you want to use text with spaces in it, then as usual the spaces should be quoted somehow.

x="hello world"

So I can go like this – x becomes that string.

That just takes whatever value was on the right and stores it into that

variable called x and then to refer to that later on, I have to use $x, okay.

So you can’t just say x like if I were to echo x, it says x.

echo x

But if I echo $x that means go look up the variable by that name and it

echo $x

will give me the message that I stored into x.

So that’s a very simple example of a shell variable.

There are other kinds of variables that are used in the shell pretty commonly.

They’re called environment variables. An environment variable –

the only difference is that the value of an environment variable is passed

in to programs that you execute. So it can be used for example to control

different programs in different ways. For example, if you want to see all

the environment variables your shell is currently holding,

there’s a builtin that does that called env. When I type this it’s going


to give me a list of all the environment variables along with their values.

You see it’s got things here like what terminal I’m using,

so TERM is the environment variable and on the right side of the = is its value.

Or what SHELL am I currently using. Who is the USER that’s logged in.

Where is that user’s HOME directory. And then the PATH is a set of

directories it uses to find programs. So all of these are variables that are set in the shell.

By convention they are upper-case names, but they don’t have to be upper-case names –

that’s just usually the way it’s done. If I want to set a new variable –

you notice that for example my $x which I set to “hello world” –

that doesn’t appear in this list because that’s just a shell variable and

not an environment variable.

So one way to set an environment variable is – go back to that way that I

set x to “hello world” and all I do is I type export before it.

export x="hello world"

So this means I’m setting a variable in my shell and I can still do echo $x like I did before.

echo $x

But now that variable will get passed into programs that I run subsequently

and they can access that. So sometimes you’ll use environment variables to configure those programs.

If I do env now, you see that x shows up here as one of the environment variables.


So that’s just like having two different classes of variables that are used

in slightly different ways.

As for loops, I sometimes use loops directly on the command line as well as in shell scripts.

The simplest type of loop you can do is a for loop where you are iterating over a set of names,

a set of things. They could be filenames or they could just be any type of text at all.

So let’s say I put for x in hello there world;

what’s going to happen is that any words that appear after in but before

the ; will then be used to construct a list of values for x.

So x will take on each of those values, one at a time.

If I just do that, it’s waiting for me to continue the rest of my for loop.

Oh, I actually have to type do – forgot about that.

That do could’ve gone on the previous line after the ;.

Now I can type certain commands in here like – "The word is $x" for example.

for x in hello there world;
  echo "The word is $x"
  echo "$x $x $x"

And then let’s just repeat that word a bunch of times. And then I type done.

The done matches with the do, that creates the body of my loop and it

will execute those two echo commands 3 times –

each time x will take on a different word from that list.

So the first time x is “hello” and it gives us those two lines. And then the next word and so on.

So that’s just a little for loop and you can type that in a shell script

or just on the command line like I did. A common way to use it is with files.

If I want to do like *; do – now x is going to take on the value of

every filename in the current directory, because that’s what the wildcard will do.

So I can do like echo "file is $x", then done. And it’s just showing me

for x in *; do
  echo "File is $x"

all of those files by executing that echo command one file at a time.


That only happens in your path, correct? Sorry not in your path – the current directory you’re in?


Just in the current directory. And some of these are directories themselves,

so django and hello are directories and I can’t really distinguish that here.

Some shells have a way to use a wildcard that only matches regular files

and not directories and that sort of thing. But that’s one way to use a for loop.


So now that we’ve gone over the basics of shell scripts,

what are some more advanced things we can do with it?

Something that could actually be put to use on a daily basis?


Let’s do a practical example like that. I keep going back to image processing –

we did ImageMagick as a theme of an episode – and it’s something I have to do pretty often.

So for example let’s say that you’re creating a little game application,

either a web app or on a mobile phone or something.

And you find or create a set of playing cards, like the faces of cards.

You need to use them at different sizes depending on whether this is on a

tablet or a phone device or something.

The first thing is I have this .zip file here and I’m going to unzip that.

So probably you have a command called unzip to deal with that.

if not, you know how to look for it in the package manager and install it.

unzip Playing\ Cards.zip

So I’m going to unzip and it’s asking me if I want to overwrite some

files that I forgot to delete from the last time I did this [laughs] so I’ll say yes,

it’s okay to overwrite all of those. What’s a little weird here is it

created a sub-folder called “Playing Cards” where it put a bunch of stuff,

but it also created this folder __MACOSX and we don’t need that,

it’s just when you zip something on a Mac you often get these extra files

which can hold some Mac-specific settings in them, but we don’t care about any of them right now.

So I’m going into my playing cards folder and in there I see two

cd Playing\ Cards

sub-folders for .png format or .svg. so we’ll go into PNG.

cd PNG-cards-1.3

And here’s just a big pile of .png files for the faces of all these different cards.

So if I go and look at those, using my eog viewer for example, that’s what they look like.

eog *.png

And these are 500 by 7-something pixels. So that’s the size.

Now if I want to use them at different sizes – let’s say I want to resize them at like 50% and 33%,

25%. And I want to just batch-create all of those assets of those different sizes.

Let’s create a script called resize.sh. And we’ll start with the

nano resize.sh

hash-bang and set -e so it stops on error. And then I’m going to create a

little loop for the sizes that I want – so 50, 33, and 25.

set -e
for z in 50 33 25; do
  echo "Creating $z..."
  mkdir -p cards$z
  cp *.png cards$z
  mogrify -geometry $z% cards$z/*.png

That size will get put into the z variable and then I can do stuff with that.

So I’ll say “creating $z” so I have a little status message,

because a lot of the other commands I’m going to do don’t really output

anything if they’re successful. So at least I’ll be able to see those messages.

Next let’s do a mkdir and I want to mkdir cards$z so it’ll say

cards50 or cards33 or whatever. And in case that directory already exists,

I’d like it to keep going, so the -p option – I think we learned this,

having to do with creating parent directories – that will also have the effect of,

if the directory already exists it just moves on.

Then I want to make a copy of all of the .png files from the current

directory and move them into cards$z. Finally I’m going to use mogrify

at geometry specified by $z%, so 50%, 33%, whatever –

and apply that to all of the .png files in the sub-directory.

So this way it’ll leave the .png files in the current directory –

they’ll stay at full size and I’m creating copies of them to shrink them.

One thing I forgot up here on my for loop is a do

and then down here I have the matching done. So I think that looks alright.

We’re going to try it out. So write to file and exit. Let’s try ./resize.sh.


Oh – forgot one thing!


The chmod.


That chmod +x yeah. So there we go. Now it should be able to execute that script.

chmod +x resize.sh

And it’ll go through the 50, the 33, and the 25. And as it did that,

it created these sub-folders here in blue. And we can browse those the same way.

eog cards25

So here are the small images – they ended up at 125x182. And let’s do 33 – they’re at 165x240.

eog cards33

So we have our assets now resized in a couple of different ways.

And that’s just a very practical example of using a script and a for loop

and ImageMagick and cp and a lot of the things we’ve learned so far.


So today we learned about shell scripts as well as permissions.

We also touched base on environment variables. We also learned about for

loops and how to incorporate them into our scripts. Thanks for joining us.


See you next time!

[Dark electronic beat]

[Captions by Christopher League]