shell.md

layout	title	presenter	video
lecture	Shell and Scripting	Jon	aspect id 56.25 dbDRfmH5uSI

The shell is an efficient, textual interface to your computer.

The shell prompt: what greets you when you open a terminal. Lets you run programs and commands; common ones are:

• cd to change directory
• ls to list files and directories
• mv and cp to move and copy files

But the shell lets you do so much more; you can invoke any program on your computer, and command-line tools exist for doing pretty much anything you may want to do. And they're often more efficient than their graphical counterparts. We'll go through a bunch of those in this class.

The shell provides an interactive programming language ("scripting"). There are many shells:

• You've probably used sh or bash.
• Also shells that match languages: csh.
• Or "better" shells: fish, zsh, ksh.

In this class we'll focus on the ubiquitous sh and bash, but feel free to play around with others. I like fish.

Shell programming is a very useful tool in your toolbox. Can either write programs directly at the prompt, or into a file. #!/bin/sh + chmod +x to make shell executable.

Working with the shell

Run a command a bunch of times:

for i in $(seq 1 5); do echo hello; done

There's a lot to unpack:

• for x in list; do BODY; done
  • ; terminates a command -- equivalent to newline
  • split list, assign each to x, and run body
  • splitting is "whitespace splitting", which we'll get back to
  • no curly braces in shell, so do + done
• $(seq 1 5)
  • run the program seq with arguments 1 and 5
  • substitute entire $() with the output of that program
  • equivalent to

    for i in 1 2 3 4 5

• echo hello
  • everything in a shell script is a command
  • in this case, run the echo command, which prints its arguments with the argument hello.
  • all commands are searched for in $PATH (colon-separated)

We have variables:

for f in $(ls); do echo $f; done

Will print each file name in the current directory. Can also set variables using = (no space!):

foo=bar
echo $foo

There are a bunch of "special" variables too:

• $1 to $9: arguments to the script
• $0 name of the script itself
• $# number of arguments
• $$ process ID of current shell

To only print directories

for f in $(ls); do if test -d $f; then echo dir $f; fi; done

More to unpack here:

• if CONDITION; then BODY; fi
  • CONDITION is a command; if it returns with exit status 0 (success), then BODY is run.
  • can also hook in an else or elif
  • again, no curly braces, so then + fi
• test is another program that provides various checks and comparisons, and exits with 0 if they're true ($?)
  • man COMMAND is your friend: man test
  • can also be invoked with [ + ]: [ -d $f ]
    • take a look at man test and which "["

But wait! This is wrong! What if a file is called "My Documents"?

• for f in $(ls) expands to for f in My Documents
• first do the test on My, then on Documents
• not what we wanted!
• biggest source of bugs in shell scripts

Argument splitting

Bash splits arguments by whitespace; not always what you want!

• need to use quoting to handle spaces in arguments for f in "My Documents" would work correctly
• same problem somewhere else -- do you see where? test -d $f: if $f contains whitespace, test will error!
• echo happens to be okay, because split + join by space but what if a filename contains a newline?! turns into space!
• quote all use of variables that you don't want split
• but how do we fix our script above? what does for f in "$(ls)" do do you think?

Globbing is the answer!

• bash knows how to look for files using patterns:
  • * any string of characters
  • ? any single character
  • {a,b,c} any of these characters
• for f in *: all files in this directory
• when globbing, each matching file becomes its own argument
  • still need to make sure to quote when using: test -d "$f"
• can make advanced patterns:
  • for f in a*: all files starting with a in the current directory
  • for f in foo/*.txt: all .txt files in foo
  • for f in foo/*/p??.txt all three-letter text files starting with p in subdirs of foo

Whitespace issues don't stop there:

• if [ $foo = "bar" ]; then -- see the issue?
• what if $foo is empty? arguments to [ are = and bar...
• can work around this with [ x$foo = "xbar" ], but bleh
• instead, use [[: bash built-in comparator that has special parsing
  • also allows && instead of -a, || over -o, etc.

Composability

Shell is powerful in part because of composability. Can chain multiple programs together rather than have one program that does everything.

The key character is | (pipe).

• a | b means run both a and b send all output of a as input to b print the output of b

All programs you launch ("processes") have three "streams":

• STDIN: when the program reads input, it comes from here
• STDOUT: when the program prints something, it goes here
• STDERR: a 2nd output the program can choose to use
• by default, STDIN is your keyboard, STDOUT and STDERR are both your terminal. but you can change that!
  • a | b makes STDOUT of a STDIN of b.
  • also have:
    • a > foo (STDOUT of a goes to the file foo)
    • a 2> foo (STDERR of a goes to the file foo)
    • a < foo (STDIN of a is read from the file foo)
    • hint: tail -f will print a file as it's being written
• why is this useful? lets you manipulate output of a program!
  • ls | grep foo: all files that contain the word foo
  • ps | grep foo: all processes that contain the word foo
  • journalctl | grep -i intel | tail -n5: last 5 system log messages with the word intel (case insensitive)
  • who | sendmail -t me@example.com send the list of logged-in users to me@example.com
  • forms the basis for much data-wrangling, as we'll cover later

Bash also provides a number of other ways to compose programs.

You can group commands with (a; b) | tac: run a, then b, and send all their output to tac, which prints its input in reverse order.

A lesser-known, but super useful one is process substitution. b <(a) will run a, generate a temporary file-name for its output stream, and pass that file-name to b. For example:

diff <(journalctl -b -1 | head -n20) <(journalctl -b -2 | head -n20)

will show you the difference between the first 20 lines of the last boot log and the one before that.

Job and process control

What if you want to run longer-term things in the background?

• the & suffix runs a program "in the background"
  • it will give you back your prompt immediately
  • handy if you want to run two programs at the same time like a server and client: server & client
  • note that the running program still has your terminal as STDOUT! try: server > server.log & client
• see all such processes with jobs
  • notice that it shows "Running"
• bring it to the foreground with fg %JOB (no argument is latest)
• if you want to background the current program: ^Z + bg (Here ^Z means pressing Ctrl+Z)
  • ^Z stops the current process and makes it a "job"
  • bg runs the last job in the background (as if you did &)
• background jobs are still tied to your current session, and exit if you log out. disown lets you sever that connection. or use nohup.
• $! is pid of last background process

What about other stuff running on your computer?

• ps is your friend: lists running processes
  • ps -A: print processes from all users (also ps ax)
  • ps has many arguments: see man ps
• pgrep: find processes by searching (like ps -A | grep)
  • pgrep -af: search and display with arguments
• kill: send a signal to a process by ID (pkill by search + -f)
  • signals tell a process to "do something"
  • most common: SIGKILL (-9 or -KILL): tell it to exit now equivalent to ^\
  • also SIGTERM (-15 or -TERM): tell it to exit gracefully equivalent to ^C

Flags

Most command line utilities take parameters using flags. Flags usually come in short form (-h) and long form (--help). Usually running CMD -h or man CMD will give you a list of the flags the program takes. Short flags can usually be combined, running rm -r -f is equivalent to running rm -rf or rm -fr. Some common flags are a de facto standard and you will seem them in many applications:

• -a commonly refers to all files (i.e. also including those that start with a period)
• -f usually refers to forcing something, like rm -f
• -h displays the help for most commands
• -v usually enables a verbose output
• -V usually prints the version of the command

Also, a double dash -- is used in built-in commands and many other commands to signify the end of command options, after which only positional parameters are accepted. So if you have a file called -v (which you can) and want to grep it grep pattern -- -v will work whereas grep pattern -v won't. In fact, one way to create such file is to do touch -- -v.

Exercises

1. If you are completely new to the shell you may want to read a more comprehensive guide about it such as BashGuide. If you want a more in-depth introduction The Linux Command Line is a good resource.

2. PATH, which, type

   We briefly discussed that the PATH environment variable is used to locate the programs that you run through the command line. Let's explore that a little further

   • Run echo $PATH (or echo $PATH | tr -s ':' '\n' for pretty printing) and examine its contents, what locations are listed?
   • The command which locates a program in the user PATH. Try running which for common commands like echo, ls or mv. Note that which is a bit limited since it does not understand shell aliases. Try running type and command -v for those same commands. How is the output different?
   • Run PATH= and try running the previous commands again, some work and some don't, can you figure out why?

3. Special Variables

   • What does the variable ~ expands as? What about .? And ..?
   • What does the variable $? do?
   • What does the variable $_ do?
   • What does the variable !! expand to? What about !!*? And !l?
   • Look for documentation for these options and familiarize yourself with them

4. xargs

   Sometimes piping doesn't quite work because the command being piped into does not expect the newline separated format. For example file command tells you properties of the file.

   Try running ls | file and ls | xargs file. What is xargs doing?

5. Shebang

   When you write a script you can specify to your shell what interpreter should be used to interpret the script by using a shebang line. Write a script called hello with the following contentsmake it executable with chmod +x hello. Then execute it with ./hello. Then remove the first line and execute it again? How is the shell using that first line?

     #! /usr/bin/python
   
     print("Hello World!")

   You will often see programs that have a shebang that looks like #! usr/bin/env bash. This is a more portable solution with it own set of advantages and disadvantages. How is env different from which? What environment variable does env use to decide what program to run?

6. Pipes, process substitution, subshell

   Create a script called slow_seq.sh with the following contents and do chmod +x slow_seq.sh to make it executable.

     #! /usr/bin/env bash
   
     for i in $(seq 1 10); do
             echo $i;
             sleep 1;
     done

   There is a way in which pipes (and process substitution) differ from using subshell execution, i.e. $(). Run the following commands and observe the differences:

   • ./slow_seq.sh | grep -P "[3-6]"
   • grep -P "[3-6]" <(./slow_seq.sh)
   • echo $(./slow_seq.sh) | grep -P "[3-6]"

7. Misc

   • Try running touch {a,b}{a,b} then ls what did appear?
   • Sometimes you want to keep STDIN and still pipe it to a file. Try running echo HELLO | tee hello.txt
   • Try running cat hello.txt > hello.txt what do you expect to happen? What does happen?
   • Run echo HELLO > hello.txt and then run echo WORLD >> hello.txt. What are the contents of hello.txt? How is > different from >>?
   • Run printf "\e[38;5;81mfoo\e[0m\n". How was the output different? If you want to know more, search for ANSI color escape sequences.
   • Run touch a.txt then run ^txt^log what did bash do for you? In the same vein, run fc. What does it do?

{% comment %}

TODO

1. parallel

• set -e, set -x
• traps

{% endcomment %}

1. Keyboard shortcuts

   As with any application you use frequently is worth familiarising yourself with its keyboard shortcuts. Type the following ones and try figuring out what they do and in what scenarios it might be convenient knowing about them. For some of them it might be easier searching online about what they do. (remember that ^X means pressing Ctrl+X)

   • ^A, ^E
   • ^R
   • ^L
   • ^C, ^\ and ^D
   • ^U and ^Y