Sådan arbejder du med variabler i Bash

Variabler er vigtige, hvis du vil skrive scripts og forstå, hvad den kode, du er ved at klippe og indsætte fra internettet, vil gøre for din Linux-computer. Vi sætter dig i gang!

Variabler 101

Variabler er navngivne symboler, der repræsenterer enten en streng eller en numerisk værdi. Når du bruger dem i kommandoer og udtryk, behandles de som om du havde skrevet den værdi, de har, i stedet for navnet på variablen.

For at oprette en variabel skal du blot angive et navn og en værdi for den. Dine variabelnavne skal være beskrivende og minde dig om den værdi, de har. Et variabelnavn kan ikke starte med et tal og kan heller ikke indeholde mellemrum. Det kan dog starte med en understregning. Bortset fra det kan du bruge enhver blanding af alfanumeriske tegn med store og små bogstaver.

Eksempler

Her opretter vi fem variabler. Formatet er at skrive navnet, ligetegnet =og værdien. Bemærk, at der ikke er et mellemrum før eller efter ligetegnet. At give en variabel en værdi omtales ofte som at tildele en værdi til variablen.

Vi opretter fire strengvariabler og en numerisk variabel, this_year:

mig = Dave
my_boost = Linux
ham = Popeye
his_boost = Spinat
dette_år = 2019

Brug echokommandoen for at se værdien i en variabel . Du skal gå foran variabelnavnet med et dollartegn, $når du refererer til den værdi, den indeholder, som vist nedenfor:

ekko $ my_name
ekko $ my_boost
ekko $ dette_år

Lad os bruge alle vores variabler på én gang:

ekko "$ my_boost er til $ mig som $ his_boost er til $ ham (c) $ dette_år"

Værdierne for variablerne erstatter deres navne. Du kan også ændre værdierne for variabler. For at tildele en ny værdi til variablen  my_boostgentager du bare, hvad du gjorde, da du tildelte den første værdi, sådan:

my_boost = Tequila

Hvis du kører den forrige kommando igen, får du nu et andet resultat:

ekko "$ my_boost er til $ mig som $ his_boost er til $ ham (c) $ dette_år"

Så du kan bruge den samme kommando, der refererer til de samme variabler og få forskellige resultater, hvis du ændrer værdierne i variablerne.

Vi taler om at citere variabler senere. For nu er her nogle ting at huske:

  • En variabel i enkelte citater ' behandles som en bogstavstreng og ikke som en variabel.
  • Variabler i anførselstegn "  behandles som variabler.
  • For at få værdien tilbage i en variabel skal du angive dollartegnet $.
  • En variabel uden dollartegn $ indeholder kun navnet på variablen.

Du kan også oprette en variabel, der tager dens værdi fra en eksisterende variabel eller et antal variabler. Følgende kommando definerer en ny variabel kaldet drink_of_the_Year,og tildeler den de kombinerede værdier af my_boostog this_yearvariablerne:

drink_of-the_Year = "$ my_boost $ this_year"
ekko drikke_af_året

Sådan bruges variabler i scripts

Scripts ville være helt lukket uden variabler. Variabler giver den fleksibilitet, der gør et script til en generel snarere end en specifik løsning. For at illustrere forskellen er her et script, der tæller filerne i /devbiblioteket.

Skriv dette i en tekstfil, og gem den derefter som fcnt.sh(for "filtælling"):

#! / bin / bash folder_to_count = / dev file_count = $ (ls $ folder_to_count | wc -l) echo $ file_count files in $ folder_to_count

Før du kan køre scriptet, skal du gøre det eksekverbart, som vist nedenfor:

chmod + x fcnt.sh

Skriv følgende for at køre scriptet:

./fcnt.sh

Dette udskriver antallet af filer i /devbiblioteket. Sådan fungerer det:

  • En kaldet variabel folder_to_counter defineret, og den er indstillet til at holde strengen “/ dev.”
  • Another variable, called file_count, is defined. This variable takes its value from a command substitution. This is the command phrase between the parentheses $( ). Note there’s a dollar sign $ before the first parenthesis. This construct $( ) evaluates the commands within the parentheses, and then returns their final value. In this example, that value is assigned to the file_count variable. As far as the file_count variable is concerned, it’s passed a value to hold; it isn’t concerned with how the value was obtained.
  • The command evaluated in the command substitution performs an ls file listing on the directory in the folder_to_count variable, which has been set to “/dev.” So, the script executes the command “ls /dev.”
  • The output from this command is piped into the wc command. The -l (line count) option causes wc to count the number of lines in the output from the ls command. As each file is listed on a separate line, this is the count of files and subdirectories in the “/dev” directory. This value is assigned to the file_count variable.
  • The final line uses echo to output the result.

But this only works for the “/dev” directory. How can we make the script work with any directory? All it takes is one small change.

How to Use Command Line Parameters in Scripts

Many commands, such as ls and wc, take command line parameters. These provide information to the command, so it knows what you want it to do. If you want ls to work on your home directory and also to show hidden files, you can use the following command, where the tilde ~ and the -a (all) option are command line parameters:

ls ~ -a

Our scripts can accept command line parameters. They’re referenced as $1 for the first parameter, $2 as the second, and so on, up to $9 for the ninth parameter. (Actually, there’s a $0, as well, but that’s reserved to always hold the script.)

You can reference command line parameters in a script just as you would regular variables. Let’s modify our script, as shown below, and save it with the new name fcnt2.sh:

#!/bin/bash  folder_to_count=$1  file_count=$(ls $folder_to_count | wc -l)  echo $file_count files in $folder_to_count

This time, the folder_to_count variable is assigned the value of the first command line parameter, $1.

The rest of the script works exactly as it did before. Rather than a specific solution, your script is now a general one. You can use it on any directory because it’s not hardcoded to work only with “/dev.”

Here’s how you make the script executable:

chmod +x fcnt2.sh

Now, try it with a few directories. You can do “/dev” first to make sure you get the same result as before. Type the following:

./fnct2.sh /dev
./fnct2.sh /etc
./fnct2.sh /bin

You get the same result (207 files) as before for the “/dev” directory. This is encouraging, and you get directory-specific results for each of the other command line parameters.

To shorten the script, you could dispense with the variable, folder_to_count, altogether, and just reference $1 throughout, as follows:

#!/bin/bash   file_count=$(ls $1  wc -l)   echo $file_count files in $1

Working with Special Variables

We mentioned $0, which is always set to the filename of the script. This allows you to use the script to do things like print its name out correctly, even if it’s renamed. This is useful in logging situations, in which you want to know the name of the process that added an entry.

The following are the other special preset variables:

  • $#: How many command line parameters were passed to the script.
  • [email protected]: All the command line parameters passed to the script.
  • $?: The exit status of the last process to run.
  • $$: The Process ID (PID) of the current script.
  • $USER: The username of the user executing the script.
  • $HOSTNAME: The hostname of the computer running the script.
  • $SECONDS: The number of seconds the script has been running for.
  • $RANDOM: Returns a random number.
  • $LINENO: Returns the current line number of the script.

You want to see all of them in one script, don’t you? You can! Save the following as a text file called, special.sh:

#!/bin/bash  echo "There were $# command line parameters" echo "They are: [email protected]" echo "Parameter 1 is: $1" echo "The script is called: $0" # any old process so that we can report on the exit status pwd echo "pwd returned $?" echo "This script has Process ID $$" echo "The script was started by $USER" echo "It is running on $HOSTNAME" sleep 3 echo "It has been running for $SECONDS seconds" echo "Random number: $RANDOM" echo "This is line number $LINENO of the script"

Type the following to make it executable:

chmod +x special.sh

Now, you can run it with a bunch of different command line parameters, as shown below.

Environment Variables

Bash uses environment variables to define and record the properties of the environment it creates when it launches. These hold information Bash can readily access, such as your username, locale, the number of commands your history file can hold, your default editor, and lots more.

To see the active environment variables in your Bash session, use this command:

env | less

If you scroll through the list, you might find some that would be useful to reference in your scripts.

How to Export Variables

When a script runs, it’s in its own process, and the variables it uses cannot be seen outside of that process. If you want to share a variable with another script that your script launches, you have to export that variable. We’ll show you how to this with two scripts.

First, save the following with the filename script_one.sh:

#!/bin/bash  first_var=alpha second_var=bravo  # check their values echo "$0: first_var=$first_var, second_var=$second_var"  export first_var export second_var  ./script_two.sh  # check their values again echo "$0: first_var=$first_var, second_var=$second_var"

This creates two variables, first_var and second_var, and it assigns some values. It prints these to the terminal window, exports the variables, and calls script_two.sh. When script_two.sh terminates, and process flow returns to this script, it again prints the variables to the terminal window. Then, you can see if they changed.

The second script we’ll use is script_two.sh. This is the script that script_one.shcalls. Type the following:

#!/bin/bash  # check their values echo "$0: first_var=$first_var, second_var=$second_var"  # set new values first_var=charlie second_var=delta  # check their values again echo "$0: first_var=$first_var, second_var=$second_var"

This second script prints the values of the two variables, assigns new values to them, and then prints them again.

To run these scripts, you have to type the following to make them executable:

chmod +x script_one.sh chmod +x script_two.sh

And now, type the following to launch script_one.sh:

./script_one.sh

This is what the output tells us:

  • script_one.sh prints the values of the variables, which are alpha and bravo.
  • script_two.sh prints the values of the variables (alpha and bravo) as it received them.
  • script_two.sh changes them to charlie and delta.
  • script_one.sh prints the values of the variables, which are still alpha and bravo.

What happens in the second script, stays in the second script. It’s like copies of the variables are sent to the second script, but they’re discarded when that script exits. The original variables in the first script aren’t altered by anything that happens to the copies of them in the second.

How to Quote Variables

You might have noticed that when scripts reference variables, they’re in quotation marks ". This allows variables to be referenced correctly, so their values are used when the line is executed in the script.

If the value you assign to a variable includes spaces, they must be in quotation marks when you assign them to the variable. This is because, by default, Bash uses a space as a delimiter.

Here’s an example:

site_name=How-To Geek

Bash sees the space before “Geek” as an indication that a new command is starting. It reports that there is no such command, and abandons the line. echo shows us that the site_name variable holds nothing—not even the “How-To” text.

Try that again with quotation marks around the value, as shown below:

site_name="How-To Geek"

This time, it’s recognized as a single value and assigned correctly to the site_name variable.

echo Is Your Friend

It can take some time to get used to command substitution, quoting variables, and remembering when to include the dollar sign.

Inden du trykker på Enter og udfører en linje med Bash-kommandoer, skal du prøve den med echoforan den. På denne måde kan du sørge for, hvad der vil ske, er, hvad du vil have. Du kan også fange eventuelle fejl, du måtte have lavet i syntaksen.