Random useful Linux stuff

Exiting VIM

:wq or :x

The only commands you need to know - then switch to Emacs.

Random Linux stuff

sha256sum file | diff - shafile             # generate sha-256
export PATH=$PATH:~/opt/bin:~/opt/node/bin  # export PATH
history -c                                  # reset history
reset

time

time a.out
# real 0m0.003s  # user + system: often 'stopwatch'
                 # not 'accurate' - fork(), program might
                 # not be running at all
# user 0m0.000s  # in userland
# sys  0m0.001s  # kernel time e.g. syscalls, interrupts

Adding user to another group

usermod -aG additional_groups username

• Be sure you don't omit the -a flag, otherwise it will also remove the user from groups not in additional_groups

• List of groups in /etc/groups

Adding your scripts to PATH

• Create your ~/bin directory

• In your .bashrc, add it to your PATH=$PATH:~/bin (this is appending it to PATH)

• Add a shebang #!/bin/bash to your script

  • Better to use #!/usr/bin/env bash

• Give it exec permissions: chmod +x script

Redirection

./somescript >/dev/null 2>&1

To break it up:

• 2 is the handle for standard error or STDERR

• 1 is the handle for standard output or STDOUT

• 2>&1 is asking to direct all the STDERR as STDOUT, (i.e. to treat all the error messages generated from the script as its standard output).

  • Now we already have >/dev/null at the end of the script which means all STDOUT will be written to /dev/null.

  • Since STDERR is now going to STDOUT (because of 2>&1) both STDERR and STDOUT go to /dev/null.

File handlers

Integer value	Name	<unistd.h> symbolic constant	<stdio.h> file stream
0	Standard input	STDIN_FILENO	stdin
1	Standard output	STDOUT_FILENO	stdout
2	Standard error	STDERR_FILENO	stderr