More Ways to Use Angle Brackets in Bash

Preface

In this article, we will continue to explore more other uses of angle brackets.

In the previous article, we introduced angle brackets (<>) and some of their uses. In this article, we will continue to explore more other uses of angle brackets.

By using <, you can achieve a "cheating" effect and make other commands think that the output of a command is a file.

For example, when backing up files, if you are not sure whether the backup is complete, you need to confirm whether a directory already contains all the files copied from the original directory. You can try this:

diff <(ls /original/dir/) <(ls /backup/dir/)

The diff command is a tool for comparing the differences between two files line by line. In the above example, < is used to make diff think that the output results of the two ls commands are files, so that the differences between them can be compared.

Note that there is no space between < and (...).

I tried to execute the above command in my image directory and its backup directory, and the output was the following:

diff <(ls /My/Pictures/) <(ls /My/backup/Pictures/)
5d4 < Dv7bIIeUUAAD1Fc.jpg:large.jpg

The < in the output indicates that the file Dv7bIIeUUAAD1Fc.jpg:large.jpg exists in the directory on the left (/My/Pictures) but not in the directory on the right (/My/backup/Pictures). In other words, a problem may have occurred during the backup process, resulting in the file not being successfully backed up. If diff displays no output, the files in the two directories are identical.

Seeing this, you may think that since you can use < to provide the output content of some command lines as a file to a command that needs to accept a file format, then in the "sorting favorite actors" example in the previous article, you can skip some intermediate steps and directly perform the sort operation on the output content.

Indeed, this example can be simplified to this:

sort -r <(while read -r name surname films;do echo $films $name $surname ; done < CBactors)

Here String

In addition, there is another way to use the redirection function of angle brackets.

I believe everyone is familiar with the usage of using echo and pipe (|) to pass variables. If you want to convert a string variable to all uppercase, you can do this:

myvar="Hello World" echo $myvar | tr '[:lower:]' '[:upper:]' HELLO WORLD

The tr command can convert a string to a certain format. In the above example, tr is used to convert all lowercase letters in a string to uppercase letters.

It is important to understand that the focus of this transfer process is not the variable, but the value of the variable, which is the string Hello World. Such a string is called a HERE string, which means "this is the string we are going to process." But for the above example, you can also use a more intuitive way to handle it, as follows:

tr '[:lower:]' '[:upper:]' <<< $myvar

This shortcut does not require the use of echo or pipes, but instead uses the angle brackets we have been talking about.

Summarize

Using these two simple symbols, < and >, it turns out that so much can be accomplished. Bash once again provides many options for flexibility in work.

Of course, our introduction is far from over, because there are many other symbols that can bring more convenience to Bash commands. But if you don't fully understand them, Bash commands full of symbols will just look like a bunch of gibberish. I'll be explaining more Bash symbols like this in the future, so see you next time!

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