This was originally published on my blog back in 2007, I am glad to see that this has aged well. I still find personally using many of the tricks mentioned here even today.
Quick Command editing
-
^orig^repl^
replace 'orig' in previous command with 'repl' and execute the new command.
File Name Generation
-
cp filename{,.bak}
Copy 'filename' as 'filename.bak. -
mkdir -p dir{1,2,3}/subdir{1,2}
Createdir1/subdir1
,dir1/subdir2
,dir2/subdir1
,dir2/subdir2
,dir3/subdir1
, anddir3/subdir2
directories
Completion
No I am not talking about completing command/file names. I am talking about completing arguments to various commands, completing filenames based on application. This is one of the touted features of
'zsh', but unknown to many is the fact that it is also available in bash.
All you need to do is install bash-completion.
With bash completion installed you can use the TAB
key to complete arguments to common commands like rpm, apt, find, grep
etc. Also bash-completion will complete host-names
for ssh, scp
, by looking up hosts inside your $HOME/.ssh/authorized_keys
file. For rpm
based distros, bash-completion will even lookup package names already installed.
The feature I find most handy is file-name completion based on the context of the command. e.g. if you type tar -zxvf
and then press the TAB key twice, you will get a list of only files ending in .tar.gz or .tgz
rather than a list of all files in the directory.
Socket Communication
I bet a lot of you haven't heard of this, but bash can indeed perform basic socket communication via /dev/tcp, /dev/udp
. These are pseudo devices that bash uses to communicate with network sockets. In fact if you did ls -l /dev/tcp /dev/udp
you will get a 'No such file or directory' error message.
So how to use them, we below I present 2 examples.
One to quickly check headers from a HTTP Server. Here is a simple function you can put in your $HOME/.bashrc
that will check for headers of HTTP server.
headers() {
server=$1; port=${2:-80}
exec 5<> /dev/tcp/$server/$port
echo -e "HEAD / HTTP/1.0\nHost: ${server}\n\n" >&5;
cat 0<&5;
exec 5>&-
}
Simply invoke it by
headers <servername or ip> <port>
The port number defaults to 80 if not provided.
Second example is a quick way to check if a port is open or not. You can always use netcat or telnet, but I find this easier.
testPort() {
server=$1; port=$2; proto=${3:-tcp}
exec 5<>/dev/$proto/$server/$port
(( $? == 0 )) && exec 5<&-
}
Again invoke it by
testPort <servername or ip> <port> <protocol>
The protocol can be either tcp or udp and defaults to tcp.
Arithmetic Evaluations
Bash can perform arithmetic evaluations. They are much easier to using expr
tool. Below are some examples.Note the absense of $ prefix for variable names.
((count++)) #increment value of variable 'count' by one.
((total+=current)) # set total = total+current.
((current>max?max=current:max=max)) # ternary expression.
Top comments (14)
Neat tips! thanks
Here's mine:
We all know
!!
but it's mostly just used tosudo !!
I find
!$
(which is last argument of the last command) incredibly useful:There's a whole slew of
!
commands but!$
is one I use all the time.You can also use
esc
+.
instead of!$
. I find more natural and useful if you need to fix a typo.Even better! Thanks
P.S. For posterity, all of these work just as well in zsh.
Funny
$_
is the exact same as!$
. There's always more to learn 😀.Great write-up!
Quick note, if you are using the command edit/replace feature and are using a conditional command construct (
echo "first command" && echo "second command!"
) the^foo^bar
syntax only replaces the first instance of the searched text.To replace every instance of that searched text, you have to use
!!:gs/foo/bar
; going back to my first example:To replace all the instances of
command
in...echo "first command" && echo "second command!"
...you have to execute...
!!:gs/command/echo
...to output:
Further reading: Stack exchange
Yup I found out about it sometime after I wrote the original post almost 10 years ago. Thanks for the reminder!
Small correction: bash completion for SSH looks for
$HOME/.ssh/config
, notauthorized_keys
Yikes my bad. Actually it should be
$HOME/.ssh/known_hosts
file for auto completing host names. You won't find host names in$HOME/.ssh/config
unless you've explicitly put them there, but every host you connect to gets saved inknown_hosts
by default.Yes, it does, but bash-completion does not read hosts from
known_hosts
- at least on my Ubuntu 16.04 machine.It will, however, read both hostnames and host aliases from
config
.Nice writeup! I definitely have look more into
/dev/*
!I learn something new that unix tools can do basically every day 😄 In case someone is interested, I try to share those findings on a separate Twitter account: twitter.com/qvlio
So cool! I also use
$_
for the arguments of the last command,echo "something"
$_
will be "something"This is a great article!
If you are a fan of opensource, feel free to contribute to the Introduction to Bash Scripting open-source eBook on GitHub!
+1 for the first one. 👌👌
I love shells. They are so powerful. If I can't use one on any of my devices I feel naked