Bhaskar Karambelkar's Blog

Bash Shell Tricks

 

Tags: bash unix


Are you sure you are squeezing out all, that the bash shell can provide? Presented below are a few obscure things that bash can do, but chances are you may not have heard of them.

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.

Other neat bash tricks

  • ^orig^repl^ replace ‘orig’ in previous command with ‘repl’ and execute the new command.
  • cp filename{,.bak} Copy ‘filename’ as ‘filename.bak.
  • mkdir -p dir{1,2,3}/subdir{1,2} Create dir1/subdir1, dir1/subdir2, dir2/subdir1, dir2/subdir2, dir3/subdir1, and dir3/subdir2 directories