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Setup | Shells | Markdown and IDEs | Git |Virtual Environments | Task Management | Advanced Shells

Shells

A shell is a computing environment where commands can be interpreted, evaluated, and its output displayed (i.e., an instance of a read–eval–print loop (REPL)). A good shell provides access to a rich set of commands and allows simple programming of commands, which can be used to create powerful scripts and tools.

ls -R | grep ":$" | sed -e "s/:$//" -e "s/[^-][^\/]*\//--/g" -e "s/^/ /" -e "s/-/|/"

But with great power comes great bullshittery. Commands and their options can be terse, inconsistent, and difficult to learn. A steep learning curve often prevents novices from enjoying the eventual payoff. If you've hardly used a command line environment before, you might want to go review this more thorough tutorial: software carpentry: shell-novice---this page is more of a discussion of common tasks and mistakes, advanced topics, and resources.

You may also want to reference the online book, the Unix Workbench.

Shell Basics

Commands

99% of the reason you use shells is to run useful commands.

Essential commands.
  • ls: list content of a directory.
  • cd: change directories to a new path.
  • mkdir: make a new directory.
  • pwd: output current directory
  • cp: copy files
  • rm: rm files
  • touch: make a new file/update status**
  • cat: output the contents of a file.
  • head: output the first lines of a file.
  • tail: output the last lines of a file.
  • grep: search files for a key phrase.
  • wget: retrieve file from the web.
  • cut: extract output of a file (columns)
  • awk and sed: Magic commands for extracting, searching, and transforming content.
Combining commands

Command can run sequentially or conditionally:

command1 ; command2
(command1 ; command2) # in a sub-shell
command1 || command2  # do command2 only if command1 fails
command1 && command2  # do command2 only if command1 succeeds

Note: In Windows, ; does not work in Cmd, but does in Powershell. Use && for the most portable operation.

Try running this command that combines these shell commands.

echo "Hello World" > shells-test.txt && cat shells-test.txt

Now, try using the || operator.

cat shells-test.txt || echo "backup plan"

See what happens in this case.

cat filedoesnotexist.txt || echo "backup plan"
Command I/O

The UNIX shell commands push data from sources through filters along pipes. Normally, each command runs as a process and reads and writes data the following way:

  • Standard input (stdin): get information from keyboard.
  • Standard output (stdout): write information as output to console.
  • Standard error (stderr): write error information as output to console.

Pipes and redirects change stdin and stdout from default sources. For example, we can change the stdin of a process to be piped from the output of another process. Or rather than printing to the console, we can get a process to write to a file.

command              # default standard in and standard out
command < inputFile  # redirect of inputFile contents to command as standard in
command > outputFile # redirect command output to outputFile as standard out
command1 | command2  # pipes output of command1 as standard in to command2

Neat trick: Copy the value of a file into your clipboard!

Windows: clip < file.txt Mac: pbcopy < file.txt

pbcopy < ~/.ssh/id_rsa

clip < %HOME%/.ssh/id_rsa

📒 Online Exercise

Click the following to start the exercise.

Resources

Bite Size Command Line Zine

You might find Julia Evan's zine useful: Bite Size Command Line!.

Here is an example for the lsof command. More examples can be found here.

lsof

Command Line Fu

A list of command line examples for interesting tasks:
http://www.commandlinefu.com/commands/browse

Create a graphical directory tree from your current directory.

ls -R | grep ":$" | sed -e 's/:$//' -e 's/[^-][^\/]*\//--/g' -e 's/^/ /' -e 's/-/|/'

Explain shell

What does tar -zxvf ph.tar.gz do?

http://explainshell.com/explain?cmd=tar+-zxvf

image