Mistake this is a duplicate [duplicate] - bash

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How to obtain the first letter in a Bash variable?
(7 answers)
Closed 3 years ago.
I am trying to my a custom terminal command. I just learned I am supposed to do it using the Unix script? I don't really know much of what that is and am still trying to figure it out. What I do know is that $1 is an arg is it possible to make it a variable and then get the first letter like you could in python?
EX:
str = 'happy'
str[0] = 'h'

Answer 1

You're asking a few different things here.
I am trying to my a custom terminal command.
That could mean a few different things, but the most obvious meaning is that you want to add an executable to your path so that when you type it at the terminal, it runs just like any other executable on your system. This requires just a few things:
the executable permission must be set.
the file must specify how it can be executed. For interpreted programs such as bash scripts or python scripts, you can do so by beginning the file with a "shebang line" that specifies the interpreter for the file.
the file must be in one of the locations specified by your $PATH.
I just learned I am supposed to do it using the Unix script?
there's no such thing as a "unix script", but what you seem to be referring to is a "shell script". Though these are commonly associated with unix, they're no more inherently a unix script than any other language. A shell, such as bash, sh, or any other, is just an interpreted language that is designed so that it is convenient to be used interactively by a human as well as being programmatically executed as part of a saved file.
I don't really know much of what that is and am still trying to figure it out.
Let's get into some specifics.
First I edit a file called 'hello-world' to contain:
#!/bin/bash
echo "Hello, world!"
Note that this filename has no "extension". Though heuristics based on file extension are sometimes used (espeically in windows) to determine a file type, unix typically sees a file "extension" as part of the arbitrary file name. The thing that makes this a potentially executable bash script is the specification of that interpreter on the shebang line.
We can run our script right now from bash, just as we could if we wrote a python script.
$ bash hello-world
hello, world!
To make the bash implicit, we mark the file as executable. This enables the linux operating system to consult the beginning "magic bytes" of the file to determine how to run it. Thes beginning bytes might signify an ELF file (a compiled executable, written in eg C, C++, or go). Or, it might be #! which just so happens means , "read the rest of this first line to determine the command to run, and pass the rest of this file into that command to be interpreted.
$ chmod +x hello-world
ls -l will show us the "permissions" on the file (more accurately called the "file mode", hence chmod rather than chperm) . The x stands for executable, so we have enabled the use of the leading bytes to determine method of execution. Remember, the first two bytes of this file, and the rest of that first line, then specify that this file should be "run through bash" so to speak.
$ ls -l hello-world
-rwxr-xr-x 1 danfarrell staff 33 Dec 27 20:02 hello-world
Now we can run the file from the current directory:
$ ./hello-world
hello, world!
At this point, the only difference between this command and any other on the system, is that you have to specify its location. That's because my current directory is not in the system path. In short, the path (accessible in a unix shell via the $PATH variable) specifies an ordered list of locations that should be searched for a specified command whose location is not otherwise specified.
For example, there's a very common program called whoami. I can run it directly from my terminal without specifying a location of the executable:
$ whoami
danfarrell
This is because there's a location in my $PATH in which the shell was able to find that command. Let's take a closer look. First, here's my path:
$ echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/go/bin
And there's also a convenient program called whereis which can help show which path elements supply a named executable:
$ whereis whoami
/usr/bin/whoami
Sure enough, whoami is in one of the elements of the $PATH. (Actually I shared a simplified $PATH. Yours might be somewhat longer).
Finally, then, we can get to the last thing. If I put hello-world in one of the $PATH elements, I will be able to invoke it without a path. There are two ways to do this: we can move the executable to a location specified in the path, or we can add a new location to the path. For simplicity's sake I'll choose the first of these.
$ sudo cp hello-world /usr/local/bin/
Password:
I needed to use sudo to write to /usr/local/bin because it's not accessible as my user directly - that's quite standard.
Finally, I've achieved the goal of being able to run my very important program from any location, without specifying the executable's location.
$ hello-world
hello, world!
$ which hello-world
/usr/local/bin/hello-world
It works! I've created what might be described as a "custom terminal command".
What I do know is that $1 is an arg is it possible to make it a variable and then get the first letter like you could in python?
Well, one option would be to simply write the custom terminal command in python. If python is available,
$ which python
/usr/bin/python
You can specify it in a shebang just like a shell can be:
#!/usr/bin/env python
print("hello, world!"[0])
$ hello-world
h
it works!
Okay, confession time. I actually used #!/usr/bin/env python, not /usr/bin/python. env helps find the correct python to use in the user's environment, rather than hard coding one particular python. If you've been using python during the very long running python 2 to python 3 migration, you can no doubt understand why I"m reticent to hard code a python executable in my program.
It's certainly possible to get the first letter of a string in a bash script. But it's also very possible to write a custom command in a program other than shell. Python is an excellent choice for string manipulation, if you know it. I often use python for shell one-liners that need to interact with json, a format that doesn't lend itself well to standard unix tool stream editing.
Anyway, at the expense of incurring SO community's ire by reanswering an "already answered" question, I'll include a version in shell (Credit goes to David C Rankin)
#!/bin/bash
echo "${1:0:1}"
$ hello-world hiworld
h