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A collection of Unix/Linux ‘grep’ command examples

Linux grep commands FAQ: Can you share some Linux/Unix grep command examples?

Sure. The name grep means "general regular expression parser", but you can think of the grep command as a "search" command for Unix and Linux systems: it's used to search for text strings and more-complicated "regular expressions" within one or more files.

I think it's easiest to learn how to use the grep command by showing examples, so let's dive right in.

Sorting ‘ls’ command output by filesize alvin July 14, 2017 - 11:15am

I just noticed that some of the MySQL files on this website had grown very large, so I wanted to be able to list all of the files in the MySQL data directory and sort them by filesize, with the largest files shown at the end of the listing. This ls command did the trick, resulting in the output shown in the image:

ls -Slhr

The -S option is the key, telling the ls command to sort the file listing by size. The -h option tells ls to make the output human readable, and -r tells it to reverse the output, so in this case the largest files are shown at the end of the output.

A Linux shell script to find large files alvin July 14, 2017 - 11:14am

I made a mistake in configuring logrotate on a new Linux system, and almost ran into a problem because of that. Fortunately I saw the problem before it became a BIG problem, but as a result, I decided to add a script to my Linux system to check for large files, typically log files that have grown out of control for one reason or another.

Here then is a simple Linux shell script I named LargeFileCheck.sh, which searches the filesystem for files that are larger than 1GB in size:

A Linux shell script to rename files with a counter and copy them

As a quick note, I used this shell script to copy many files with the same name into a directory named tmpdir, giving them all new names during the copy process:

count=1
for i in `cat myfiles`
do
    fname=`basename $i`
    cp $i tmpdir/${count}-${fname}
    count=`expr $count + 1`
done

How to copy text from the MacOS Terminal to the clipboard

If you ever need to copy text (or a text file) from the MacOS Terminal to the Mac clipboard, I can confirm that the macOS pbcopy command works. It reads from STDIN and copies the text to the clipboard, so commands like these work:

$ echo "foo bar baz" | pbcopy

$ cat /etc/passwd | pbcopy

Scala code to read a text file to an Array (or Seq)

As a quick note, I use code like this read a text file into an Array, List, or Seq using Scala:

def readFile(filename: String): Seq[String] = {
    val bufferedSource = io.Source.fromFile(filename)
    val lines = (for (line <- bufferedSource.getLines()) yield line).toList
    bufferedSource.close
    lines
}

How to use ‘awk’ to print columns from a text file (in any order)

Printing columns of information from Unix text files is easy, especially using commands like awk, perl, and more recently, ruby. This short tutorial shows my old-school awk way of doing this.

awk column printing examples

Suppose you have a file named foo with contents like this:

1 2 3
a b c

You can easily use awk to print columns of information from this file. Here are a few examples that show how to print the data columns from the file:

What is the Unix/Linux “bit bucket”?

Unix/Linux FAQ: What is the “bit bucket”?

The bit bucket is a way of referring /dev/null. Sending output to the /dev/null device file is like sending output directly to the trash. That’s why you see code like this a lot of times:

aCommand 2> /dev/null

That’s a way of saying, “Run the command aCommand and send it’s error output to the bit bucket.” In use like this, “error output” refers to STDERR, and redirecting STDERR to the bit bucket is the same as throwing it into the trash (or throwing it into a black hole, if you prefer).

Another way you can demonstrate this is by sending STDOUT to the bit bucket. In this next example, I send the output from the ls command to the bit bucket:

ls -l > /dev/null

If you run that command, you won’t see any output because the standard output — STDOUT — is redirected to /dev/null. There’s no practical reason for doing this in the real world; I just wanted to demonstrate that you can redirect both STDOUT and STDERR to the bit bucket, if you ever need to.

For more information, here’s a short tutorial on How to redirect Unix STDOUT and STDERR to the same location.