Overview

Let’s say you’re at the command line, and you’re running a long process like a kernel recompile. On some machines, that can take up to an hour. An average user might sit and wait. A clever user might background the job, or use the function keys to open a new TTY login. A screen user, however, has already thought ahead. Had screen been launched first (or automatically – more on that later) that user would just hit the proper key combo to create a new shell. And another, and another, if desired. Each one operates independently of the others and all output will remain on the appropriate screen session.

Some of you who like to use the ALT-CTRL-(F1 through F4) method may wonder how screen is any better. Several reasons actually, some of which would be:

1. Not limited to number of function keys
2. Ability to name each screen
3. Ability to detach/reattach screen sessions
4. “Status Bar” support
5. Multi-user shared screens
6. Split screens

A LOTS more.

Getting Started

As screen is included in nearly every major Linux distro, this guide will not cover installation. The simplest way to launch screen is to just open a terminal and type

screen

You might get a welcome screen, but otherwise you may not notice anything different. It just looks like a normal terminal right?

What you’re actually seeing is the default screen session, and you can think of it as a layer over your shell. Should you exit or detach the session, you’d be returned to your normal screen-less shell.

Adding and Navigating Screens

For starters, you’ll need to know one major key combo. This is the “magic” key combo that lets you access all the individual commands. What is this magical combination? Ctrl-A. For example, Ctrl-A followed by c will create a new screen. Ctrl-A and n will move to the next screen in line (if one exists). The following includes many of the most common action keys (all keys are case-sensitive):

• c – Create a new screen
• n – Move to next screen
• p – Move to previous screen
• S – Split screen into stacked regions
• | – Split to side-by-side regions (might not work on all systems)
• d – Detach screen
• A – Set screen title

Attaching/Detaching Screens

While there are many great uses for screen, this function could be called screen’s bread and butter. The ability to detach a screen, leaving it running, and pick it back up later is invaluable.

Let’s say you’re running the Minecraft server from last week, and it’s running on an old PC under your bed. Normally if you wanted to, say, enter commands at the server console, you’d have to pull out the machine, dig out a keyboard and monitor, run your commands, and push it all back. Had the server software been started in a screen session, you can just SSH into the server, re-attach the screen, and do what you need. When done, detach the screen, and the server will keep running even after you close the SSH session. Days/weeks/months later, you can log back into the server and reattach that screen to enter new commands.

.screenrc

Finally, we could not cover screen without at least a brief mention of the ~/.screenrc file. There are many many uses for this file, but as this is an introductory guide, we will not cover all possible options. Instead, we’ll just point out the well-loved hardstatus setting.

This option gives you a fantastically useful info bar that will persist across all screen sessions. Typically, it’s placed at the bottom and holds info such as time, hostname, and title/number of the current screen.

You can find example hardstatus codes all over the net with your preferred search engine, but here are a few to get you started.

#1

hardstatus alwayslastline
hardstatus string '%{= kG}[ %{G}%H %{g}][%= %{= kw}%?%-Lw%?%{r}(%{W}%n*%f%t%?(%u)%?%{r})%{w}%?%+Lw%?%?%= %{g}][%{B} %d/%m %{W}%c %{g}]'

#2

hardstatus alwayslastline
hardstatus string "%{+b Rk}(%{-b g}$LOGNAME@%H%{+b R}) (%{-b g}%C %a%{+b R}) %{-b g} %n %t %h"

#3

hardstatus alwayslastline
hardstatus string '%{= M} %H%{= G} %l %= %{= w}%-w%{+b r}%n*%t%{-b r}%{w}%+w %= %{c}%d %D %{B}%c '

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The Basics

As noted above, wget is most commonly used to quickly grab a file from somewhere on the web.

wget http://mydomain.com/file.zip

Which would simply grab the file and save it to the current directory.

If you want to save to a different filename or different location, you use the -O flag.

#Remember it’s a capital "O" not zero or small "o"

wget http://download.maketecheasier.com/Firefox_shortcut_keys.pdf -O Documents/ffkeys.pdf

You may be downloading multiple files, in which case you may want to specify a location for all downloads. Just use the -P flag (or –directory-prefix=LOCATION) to specify where they go.

As you can see, that’s a lot of output. Let’s try the -q option to clean it up, making it more suitable for scripts.

wget -q http://mydomain.com/file.zip

Similarly, you can use the -nv option for just a little output, but not as much as the default.

Fancy stuff

What if your download failed, and you want to resume? What if the file already exists, and you don’t want to overwrite it? There are options to handle those as well as several other situations.

To resume a broken download, you’d use the -c flag (or –continue)

If you need to make sure your command (or script) doesn’t overwrite any existing files, use the -nc option (for no-clobber)

There are times when you can’t be certain if filenames will be case-sensitive on both ends, but the –ignore-case flag will negate that problem.

To limit the download rate, use the –limit-rate=RATE option, as demonstrated below.

wget –limit-rate=20000 http://ftp.us.debian.org/debian-cd/5.0.7/amd64/iso-cd/debian-507-amd64-netinst.iso

FTP Options

As noted earlier, wget supports FTP as well. If you just specify a FTP site, like

wget ftp://ftp.us.debian.org/debian-cd/5.0.7/amd64/iso-cd/debian-507-amd64-netinst.iso

wget will assume you want an anonymous login. If that’s not the case, you can manually specify things like username and password with the following flags:

• –ftp-user=USER Specifies the username for login
• –ftp-password=PASS Specifies password
• –no-passive-ftp Disables passive transfer mode.

As well as a few others for advanced use cases.

Timeouts, Retries, and Failed Downloads

Finally, wget comes with several options relating to server connection problems and timeouts. Not all failures can be dealt with of course, but the following flags are all intended to help deal with server issues:

• –tries=NUMBER Specify number of times to retry download
• –retry-connrefused Retry download even if connection is refused by server.
• –continue Resumes an incomplete download, used as -c above
• –timeout=SECONDS Global setting – how long to wait before timeouts
• –wait=SECONDS How long to wait between successful downloads (if repeating)

Conclusion

For such a simple, basic, built-in utility, wget has a surprising amount to offer. Next time you find yourself writing an internet-aware shell script, or needing to get that missing driver file on your broken computer, give wget a shot. If you’ve got any interesting stories about how wget has got you out of a jam, let us know in the comments below.

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There are many options to use with the bash history, some of them I use nearly every time I open a command shell. Today I’ll be covering some of the most useful aspects of the history, but I certainly won’t be able to cover them all.

Up/Down Arrows

Many of you probably know this one, but for anyone who doesn’t, it’ll make your life a lot easier. Any time you’re in the command shell, simply use the up and down arrows on your keyboard to move through the list of previously entered commands. Next time you enter a long command that turns out to have a typo, you don’t have to retype the whole thing just hit the up arrow and make whatever edits are needed.

!!

A similar feature to the one listed above is the double bang (!!). Those characters entered in the shell represent the last typed command. Let’s say you’re trying to run a command only to find out that you need root privileges to do it. Instead of retyping the whole command, you can simply enter

sudo !!

Bash will substitute the !! for the previous command, as in the example below.

This also works further backward, you can do something like

!-5

to recall the command 5 entries back in the history.

!(word)

Perhaps a few days ago you typed a long, complex command into your shell, for example a series of options for “wterm”. You can find and re-enter than command by using the history’s built in search. You simply add the first few letters of the command after the ! and bash will find it.

Be careful with this one. You may end up running something other than what you expected. Don’t use this for potentially dangerous commands like “rm” or “fdisk”.

!?(word)

This is similar to the last feature in that it searches the history for the word entered, but unlike !(word) this will find the word anywhere in the command, not just the beginning.

Be careful with this one. You may end up running something other than what you expected. Don’t use this for potentially dangerous commands like “rm” or “fdisk”.

Ctrl-R

This one may be my personal favorite history tool. It’s a bit like the !? item above, but interactive. In your command shell, hit Ctrl-R, and it will begin a search. As you type, bash will search the history and show you the results as you type. When it shows the command you want, simply hit enter and it will run that command. This can be safer than things like !? because you can see what the command will be before you run it, so you don’t have to guess or rely on memory.

Manual Search

Perhaps you don’t like the way Ctrl-R works, or you want to see ALL the commands you typed, or just the ones that contain a particular word. The history command will show you your full command history, along with the history number of each command (hold that thought, we’ll cover those numbers in a moment).

You can also specify the number of items the history command will show. To see the last 10 entries in the history, you could type

history 10

To see all history entries that contain a particular word, you can use grep to filter the results, like below.

!(number)

When you use the history command to view your command history list, you’ll notice each item in the list has a number associated with it. You can use that to recall that particular command. For example, item 87 could be run again by entering

!87

into your shell.

Word replacement

This one’s pretty great. How many times have you written out some big long command only to realize you put, say, hdd when you wanted hdc? Well bash has you covered. You can replace a word in the previous command with another using “^”, as in the example below

Managing Your History

By default, the history is saved in ~/.bash_history. You can deal with this file however you see fit, but there are some useful things you could try to manage it automatically. Namely, setting the environment variables HISTSIZE and HISTFILESIZE. To set the number of entries that can be saved in the history file to 1000, enter

export HISTFILESIZE=1000

into your shell. This will limit the file to 1000 entries. If you want to make this change permanent, put that export command into your .bashrc or .bash_profile file.

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