First Shell Script — Creating and Running It

Creating a Script — vi and #!/bin/sh

A shell script bundles the commands you'd normally type into a terminal into a file so you can run them all at once.

Instead of typing a fixed set of steps by hand every time, you write them into a file, and you can repeat the same work accurately as many times as you want.

Here you'll learn the basics of creating and running a shell script.

Open the file with vi hello.sh, press i to enter insert mode and type the body, then press Esc and save and close with :wq (the basic vi operations are the same ones you learned last time).

Make the first line #!/bin/sh, a shebang, which specifies which shell runs the script.

Lines that start with # are ignored at runtime as comments.

The only exception is the first line: even though #!/... starts with #, it isn't a comment — it's specially interpreted as the directive for which shell to run.

vi note.sh        # open the script in the editor
# press i to enter insert mode and write:
#   #!/bin/sh
#   # show a short message
#   echo "build step 1"
#   echo "build step 2"
# press Esc and save and quit with :wq
chmod +x note.sh  # add executable permission
./note.sh         # prints build step 1 / build step 2

You'll build a small tool that records the date you ran it and the working directory.

① Open hello.sh with vi hello.sh and press i to enter insert mode.

② On the first line write #!/bin/sh, then add a comment line stating what the script is for and the commands that print the current date and working directory (you can copy and paste the contents from the answer panel).

③ Press Esc and save and quit with :wq.

④ Add executable permission to the hello.sh you created.

⑤ Run ls -l hello.sh and confirm that an x appears at the left edge, showing the executable permission changed.

⑥ Run the script and confirm that the date and working directory are printed. (If it runs correctly, an explanation will appear.)

Running It — chmod +x and ./

A file you've only just created can't be run as is.

Once you add executable permission with chmod +x file, you can run it in the form ./file.

./ is the way to refer to a file in the current directory; without it, the shell only searches for the command within PATH and won't find it.

vi greet.sh        # open in the editor, write #!/bin/sh and echo hi, save with :wq
ls -l greet.sh      # no executable permission yet
chmod +x greet.sh   # add executable permission
ls -l greet.sh      # an x appears at the left edge
./greet.sh          # prints hi

There's also a way to run it without executable permission

When you want to run it without adding executable permission, there's also a way to hand the file to a shell, like sh file.

In this course we use the basic form of adding executable permission and running with ./file.

You can check whether executable permission was added by whether an x appears at the left edge of ls -l.

You'll go through the flow of creating a script, adding executable permission, and running it.

① Open run.sh with vi run.sh, write #!/bin/sh on the first line and a command that prints a short message on the next line, then save with :wq.

② Check the current permissions with ls -l run.sh (there's no executable permission yet).

③ Add executable permission to the run.sh you created.

④ Run ls -l run.sh again and confirm that an x appeared at the left edge.

⑤ Run the script with ./ and confirm that the message is printed.

Building a Small Tool That Records Runs

The create, grant-permission, and run flow you learned can be used directly in small real-world tools.

Here you'll build a logging tool that appends "when the script was run" to a single log file.

When you append with >>, lines pile up every time you run it, so you can look back at the run history later.

You'll build a small tool that records the time you ran it to a log file.

① Open who.sh with vi who.sh, press i to enter insert mode, and write #!/bin/sh on the first line.

② Add a comment line stating what the script is for, append the current time to a log file (>>), and then write a command to show the whole log.

③ Press Esc, save with :wq, then add executable permission to the file.

④ Run the script and confirm that the recorded line appears in the log.