Category Archives: GNU/Linux

GNU/Linux, the free/open-source operation system.

Speed Test: Check the Existence of a Command in Bash and Zsh

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Last updated on December 10, 2016

In both bash and zsh, there are multiple methods to check whether a command exists. In this post, a set of speed tests will be performed on them to find the fastest way in each of the two shells (NOT to compare the two shells). We will test 5 different methods (foobar is the command to test for existence in the list):

  • type foobar &> /dev/null
  • hash foobar &> /dev/null
  • command -v foobar &> /dev/null
  • which foobar &> /dev/null
  • (( $+commands[foobar] )) (zsh only)

All the methods listed above will have a return status of zero if the command foobar exists, otherwise non-zero. That is, after replacing testing-command by any of the commands listed above, you can test the existence of the command foobar by executing testing-command && echo exist || echo non-exist.

Throughout this post, ls will be the command that is used for testing existence, which does exist on the system which runs the tests. The test environment is Debian Jessie with bash 4.3.30 and zsh 5.0.7 on Intel Xeon processor E3-1240 v3 (8 MB Cache, 3.4 GHz). The test scripts are also available at the end of the post.

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Restore the Previously Canceled Command in Zsh

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Last updated on December 10, 2016

In zsh, it is often annoying that we can't easily restore the command we just canceled by Ctrl-C: canceled commands are not recorded into the history file and thus cannot be restored by searching previous commands. To make the restoration possible, zsh provides a variable ZLE_LINE_ABORTED which keeps a record of the last command that was canceled—everything looks so simple. However, for some reasons, such as canceling stuck tab completion, we often push Ctrl-C for more than once—but ZLE_LINE_ABORTED would become empty if Ctrl-C is used on an empty line! Thanks to the great extensibility of zsh, we can solve this issue by tweaking zle-line-init (add the following to your ~/.zshrc):

function zle-line-init {
  # Your other zle-line-init configuration ...

  # Store the last non-empty aborted line in MY_LINE_ABORTED
  if [[ -n $ZLE_LINE_ABORTED ]]; then
    MY_LINE_ABORTED="$ZLE_LINE_ABORTED"
  fi

  # Restore aborted line on the first undo.
  if [[ -n $MY_LINE_ABORTED ]]; then
    local savebuf="$BUFFER" savecur="$CURSOR"
    BUFFER="$MY_LINE_ABORTED"
    CURSOR="$#BUFFER"
    zle split-undo
    BUFFER="$savebuf" CURSOR="$savecur"
  fi
}
zle -N zle-line-init
  • Line 1: Define the zle-line-init widget which will be executed every time when the a new command line is ready to take input.
  • Line 5-7: If ZLE_LINE_ABORTED is non-empty, save it to MY_LINE_ABORTED.
  • Line 10-16: If MY_LINE_ABORTED is non-empty, the initial undo will restore the contents in MY_LINE_ABORTED. Also see man zshzle for an explanation of split-undo.
  • Line 18: Install the widget zle-line-init.

Now type any command and push Ctrl-C twice and undo (bound to Ctrl-/ by default): your canceled command is back!

Note that if you use zsh-autosuggestions this code snippet somehow breaks it. Adding _zsh_autosuggest_widget_clear before the end of zle-line-init would fix it.

References

Make the less Command More Powerful

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Last updated on December 25, 2024

Due to its speed and simplicity, GNU less is probably the most common default terminal pager on various GNU/Linux distributions—you may have probably used it explicitly via the less command, or implicitly when you execute the man command or git diff. Although the default configuration of less does not really offer much except for a basic text viewer, it is actually much more powerful than most people think. Here a few improvements over the default configuration are offered.

For macOS/OS X users: consider installing a newer version of less and other GNU command line utilities. To do so, you can follow the instructions here.

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Control the LED on a USB WiFi Adapter on Linux

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Last updated on October 11, 2018

First, find the corresponding directory of this LED device in /sys/class/leds, such as /sys/class/leds/device_name, and switch your working directory to this directory. The device_name may be similar to the device driver of your wifi adapter. You should have a file named trigger and a file named brightness in this directory. cat trigger shows you available triggers to trigger the LED light, with the current trigger surrounded by brackets. Different triggers will turn on and off the LED in different cases. Use echo new-trigger > trigger to change trigger. cat brightness shows the current brightness of the LED. Use echo N > brightness to change brightness, where N is an integer.

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