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blog - fun with the Linux command line.md

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+# Linux tips, one-liners and magic-tricks
+
+## Truncate a text file
+
+This is good for emptying huge log files without actually deleting and re-creating the file
+
+```bash
+echo -n '' > {target file}
+```
+
+For example
+
+```bash
+echo -n '' > /var/www/vhosts/horriblewebsite.com/statistics/logs/error_log
+```
+
+---
+
+## Grep One-Liners
+
+How to find specific things with grep
+
+### find what looks like an IP Address
+
+```bash
+egrep -o "[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}"
+```
+
+### find what looks like an email address
+
+```bash
+egrep "\w+([._-]\w)*@\w+([._-]\w)*\.\w{2,4}"
+```
+
+### find what looks like a URL
+
+```bash
+egrep -o "(http|https)://[a-zA-Z0-9./?=_-]*"
+```
+
+---
+
+## mempigs
+
+A one-liner that will return the top 10 RAM consuming processes on a Linux machine
+
+```bash
+ps aux  | awk '{print $6/1024 " MB\t\t" $11 " " $12}'  | sort -n | tail -10
+```
+
+Alias it with something like this:
+
+```bash
+alias mempigs="ps aux | awk '{print \$6/1024 \" MB\t\t\" \$11 \" \" \$12}'  | sort -n | tail -10"
+```
+
+Then you only have to type **mempigs** to get a quick overview of what's eating all the memory.
+
+---
+
+## Filter comments and blank lines with grep
+
+There are times when you may want to check the contents of a configuration file, but it's littered with explanatory comments. Normally, documentation in a file is a good thing, but sometimes you just want to see the relevant settings.
+
+Filter out comments with something like this:
+
+```bash
+grep -v \# /etc/some-config-file.conf
+```
+
+The grep flag -v excludes the next argument from the returned results. Because the pound sign is a common bash idiom that denotes a comment, you need to escape it in the above example, or everything in the command after the pound sign would be considered a comment!
+
+So, you've filtered out all the comments. But now you're left with lines separated by blank lines where the comments used to be.
+
+How to remove those blank lines? Add another grep to the command:
+
+```bash
+grep -v \# /etc/some-config-file.conf | grep -v -e '^$'
+```
+
+The last grep uses the -v again to remove the next argument, but in this case, we are removing the regex (denoted by -e) `'^$'` which looks for the beginning of a line followed by the end of the line (in other words a line with no content, a blank line)
+
+---
+
+## Shrink and convert PDFs with ghostscript  
+
+REQ: ghostscript
+
+```bash
+gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf
+```
+
+If the `-dPDFSETTINGS=/screen` setting is too low quality to suit your needs, replace it with `-dPDFSETTINGS=/ebook` for better quality, but slightly larger PDFs. Delete the setting altogether for the high quality default.