Shell Enhancements

Some tips and tricks for the Linux shell, mainly for the bash shell.

history

The bash history is a powerful feature for the work on the console of a Linux system.

Removing and avoiding Duplicates in the History

Manual Deduplication

export HISTCONTROL=ignoreboth:erasedups
history -a
history -w
awk '!a[$0]++' $HOME/.bash_history > $HOME/.bash_history.tmp && mv $HOME/.bash_history.tmp $HOME/.bash_history

This of course does not remove the duplicates that are already/still hold in memory. Here a reboot or at least a re-login would help.

Make it permanent

Add

export HISTCONTROL=ignoreboth:erasedups

to /etc/rc.local or to $HOME/.bashrc and reboot.

Some distributions may already set $HISTCONTROL in $HOME/.bashrc.

An enhancement is described here. This leads to a $HOME/.bashrc like this:

# don't put duplicate lines or lines starting with space in the history.
# See bash(1) for more options
HISTCONTROL=ignoreboth:erasedups

# don't put simple commands to the history
HISTIGNORE=cd:ls:"ls -la":exit

# append to the history file, don't overwrite it
shopt -s histappend

# merge the history for all sessions on close
function historymerge {
    history -n; history -w; history -c; history -r;
}
trap historymerge EXIT

# for setting history length see HISTSIZE and HISTFILESIZE in bash(1)
HISTSIZE=5000
HISTFILESIZE=15000

Command Aliases

The bash can be enhanced through aliases that will be defined in .bash_aliases like this:

alias <name>="<command sequence>"

Here are the aliases that I use regularly:

alias histfind="history | grep $*"
alias md="mkdir -p"
alias mdcd="source mdcd.sh"

• histfind searches a recently used command in the command history.

• md creates a new directory and all missing parent directories in one go.

• mdcd creates a new directory and changes to it afterwards. This alias requires a shell script named mdcd.sh somwhere on the path.

  The script looks like this:

   #!/bin/bash
  
   mkdir -p "$1"
   eval cd "$1"