In our first tutorial on command line wizardry, we coated easy redirection and the fundamentals of sed, awk, and grep. In the present day, we will introduce the ideas of easy variable substitution and loops—once more, with a particular concentrate on the Bash command line itself, moderately than Bash scripting.
If you must write a script for repeated use—notably one with vital logical branching and analysis—I strongly suggest a “actual language” as an alternative of Bash. Fortunately, there are many choices. I am personally an enormous fan of Perl, partially as a result of it is obtainable on just about any *nix system you may ever encounter. Others may fairly select, say, Python or Go as an alternative, and I would not decide.
The actual level is that we’re specializing in the command line itself. Every part under is one thing you may simply be taught to assume in and use in actual time with just a little observe.
Setting and getting values in Bash
Bash handles variables a bit oddly in comparison with any of the “actual” languages I’ve used. Specifically, you have to check with the identical variable with completely different syntax when setting its worth versus when retrieving it.
Let’s check out a quite simple instance:
[email protected]:~$ hey="Hiya, world!"
[email protected]:~$ echo $hey
Hiya, world!
In the event you’ve by no means labored with variables in Bash earlier than, you may assume there is a typo within the above instance. We set the worth of hey however learn again its worth as $hey. That is not a mistake—you do not use the main $ when defining/setting a variable, however you have to use it when studying from the identical variable later.
Destroying variables
If we have to clear that variable, we are able to use the unset command—however, once more, we should check with hey as an alternative of $hey:
[email protected]:~$ echo $hey
Hiya, world!
[email protected]:~$ unset $hey
bash: unset: `Hiya,': not a sound identifier
bash: unset: `world!': not a sound identifier
[email protected]:~$ unset hey
[email protected]:~$ echo $hey
Serious about the error messages we get when making an attempt incorrectly to unset $hey ought to make the issue clear. After we unset $hey, we’re passing the worth of hey to unset moderately than passing the variable identify itself. Meaning precisely what you assume it does:
[email protected]:~$ hey="Hiya, world!"
[email protected]:~$ myhello="hey"
[email protected]:~$ unset $myhello
[email protected]:~$ echo $hey
Though the swap backwards and forwards between referencing hey and $hey is deeply unsettling for those who’re solely conversant in “actual” languages, Bash is at the least comparatively constant about when it makes use of every type. With just a little observe, you may get used to this—even for those who by no means really feel really snug about it.
Variable scope
Earlier than we transfer on from the fundamentals of variables, we have to discuss variable scope. Mainly, a Bash variable is restricted in scope to solely the present course of. It will not move your variables onto any youngster processes you provoke. We are able to see this in motion most simply by calling bash from bash:
[email protected]:~$ hey="Hiya, world!"
[email protected]:~$ echo $hey
Hiya, world!
[email protected]:~$ bash
[email protected]:~$ echo $hey
[email protected]:~$ exit
exit
[email protected]:~$ echo $hey
Hiya, world!
In our unique Bash session, we set the variable hey to equal “Hiya, world!” and entry it efficiently with the echo command. However after we begin a brand new bash session, hey is just not handed onto that youngster session. So after we once more attempt to echo $hey, we get nothing. However after exiting the kid shell, we are able to as soon as once more efficiently echo $hey and obtain the worth we initially set.
If you must carry variables set in a single session into a toddler course of, you must use the export command:
[email protected]:~$ unset hey
[email protected]:~$ hey="Hiya, future kids!"
[email protected]:~$ export hey
[email protected]:~$ bash
[email protected]:~$ echo $hey
Hiya, future kids!
As you may see, export efficiently flagged our variable hey for passing all the way down to youngster processes—on this case, one other occasion of bash. However the identical approach works, in the identical approach, for calling any youngster course of that references setting variables. We are able to see this in motion by writing a really small Perl script:
[email protected]:~$ cat perlexample
#!/usr/bin/perl
print "$ENV{hey}n";
exit 0;
[email protected]:~$ hey="Hiya from Perl!"
[email protected]:~$ ./perlexample
[email protected]:~$ export hey
[email protected]:~$ ./perlexample
Hiya from Perl!
Similar to our earlier instance of a kid session of bash, our Perl script cannot learn our hey setting variable until we first export it.
The very last thing we’ll say about export is that it may be used to set the worth of the variable it is exporting, in a single step:
[email protected]:~$ export hey="Hello once more!"
[email protected]:~$ ./perlexample
Hello once more!
Now that we perceive methods to set and skim values of variables on the command line, let’s transfer on to a really helpful operator: backticks.
Capturing command output with backticks
[email protected]:~$ echo check
check
[email protected]:~$ echo `echo check`
check
The backticks operator captures the output of a command. Sounds easy sufficient, proper? Within the above instance, we exhibit that the output of the command echo check is, after all, “check”—so after we echo `echo check` we get the identical outcome.
We are able to take this a step additional and as an alternative assign the output of a command right into a variable:
[email protected]:~$ du -hs ~
13G /dwelling/me
[email protected]:~$ myhomedirsize=`du -hs ~`
[email protected]:~$ echo $myhomedirsize
13G /dwelling/me
Lastly, we are able to mix this with the teachings from our first tutorial and strip the primary column from du‘s output to assign to our variable:
[email protected]:~$ myhomedirsize=`du -hs ~ | awk '{print $1}'`
[email protected]:~$ echo $myhomedirsize
13G
At this level, we perceive how variables and the backticks operator work. So let’s discuss easy looping constructions.
For loops in Bash
[email protected]:~$ y="1 2 3"
[email protected]:~$ for x in $y ; do echo "x equals $x" ; achieved
x equals 1
x equals 2
x equals 3
The above instance concisely demonstrates the best way a easy for loop works in Bash. It iterates via a provided sequence of things, assigning the present merchandise from the listing to a loop variable. We used semicolons to separate the for loop itself, the do command that marks the inside of the loop, and the achieved which lets us know the loop is over. Alternatively, we might additionally enter them as technically separate traces.
[email protected]:~$ y="1 2 3"
[email protected]:~$ for x in $y
> do
> echo "x=$x"
> achieved
x=1
x=2
x=3On this instance, the main angle brackets aren’t one thing you kind—they seem to be a immediate provided by Bash itself. This allows you to know you are still inside a loop regardless of having hit Enter. And that is essential, as a result of you may have as many instructions as you want contained in the loop itself!
[email protected]:~$ y="1 2 3"
[email protected]:~$ for x in $y ; do echo "x equals $x" ; echo "x=$x" ; echo "subsequent!" ; achieved
x equals 1
x=1
subsequent!
x equals 2
x=2
subsequent!
x equals 3
x=3
subsequent!
As you may see, we are able to string as many instructions as we would like collectively inside our for loop. Throughout the loop, every command could check with the loop variable if it likes—or it could actually ignore the loop variable totally.
Up to now, we have solely checked out numbers in sequences, however the for loop does not care about {that a} bit. Any sequence of things, in any order, will work:
[email protected]:~$ y="cats canine bears"
[email protected]:~$ for x in $y ; do echo "I like $x" ; achieved
I like cats
I like canine
I like bears
Lastly, you do not have to provide the listing with a variable; you too can embed it straight within the loop:
[email protected]:~$ for x in "cats canine bears" ; do echo "I like $x" ; achieved
I like cats
I like canine
I like bears
