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Presentation notes from JMU Unix Users Group meetings

Bash scripting

A little bit of background

A lot of the background for bash in general is given in the bash talk, so we won’t retread all of that ground, but forgive me if I repeat some things.

Source-ing vs Executing

There are two ways to run a shell script. The easy way through source and the slightly-more-work way of chmod and ./. There is a difference though!

source simply takes every line of the input file and runs it as a command on your shell. This means that all variables and side effects from the code remain on your shell when the script finishes executing. This can be useful but for most situations is not what you really want.

Making a file executable and using ./ executes the commands in a subshell, meaning it’s basically a different shell who’s side effects do not carry back to your main shell. This is most exemplified in the cd command. It is perfectly reasonable to cd inside a script. If you call source on a script that uses cd you will end up wherever the script cd‘d to. Using ./ however will not change your current working directory after returning.

Anyways it’s generally good practice to use ./. Here’s how you do it if you don’t know:

chmod +x yourfile.sh
./yourfile.sh

Syntax

Variables

Setting a variable is as easy as

VARIABLE="foo"
echo $VARIABLE

Variable replacements

Replacing values in strings in bash is very simple, just takes putting the variable name in the string as long as you are surrounding your string with ". ' describes a literal string however, and will not replace variables or expressions contained within.

echo "$VARIABLE bar"
# foo bar
echo '$VARIABLE bar'
# $VARIABLE bar

Expressions in strings

Putting an expression inside a string is similarly easy: simply surround the expression with $(expression), or `expression`

echo "I'm in $(pwd)"
echo "I'm still in `pwd`"

Brace expansion

Bash allows you to expand things inside braces pretty reasonably:

echo {A,B}     # A B
echo {A,B}.js  # A.js B.js
echo {1..5}     # 1 2 3 4 5

The last type allows step size with another .. set:

echo {0..50..5}

#### eval

Brace expansions do not work directly in strings however, but we can use the eval command to fix that:

echo "{1..5}.js"        # {1..5}.js
eval echo "{1..5}.js"   # 1.js 2.js 3.js 4.js 5.js

eval basically runs the command twice, first through the bash processor, and second as the actual command. Here, bash expands the brace expansion syntax before sending to echo.

Conditionals

The [ ... ] command is an alias for test (/bin/test) and returns either 0 (true) or 1 (false), much of the description is listed in man 1 test. [[ ... ]] is a superset of the test functionality, with a few added functions, though this functionality is not POSIX standard and therefore varies based on the system. I will be working based on bash’s implementation of [[ (found here). But your results may vary if using something else.

Some useful operators are:

There are a ton of file-specific options, but I wont go through them all here.

Note we do not need to include the $ when using a variable in a conditional.

There are also algebraic conditionals, defined with (( ... )). An easy example would be (( NUM == 5 )), if NUM does equal 5, this “function” returns 0.

Writing an actual conditional statement is simple, but slightly different from other languages you’ve likely used.

NUM=5
if (( NUM == 0 )); then
  echo "We have 0 problems! Good job!"
elif (( NUM == 1 )); then
  echo "We have 1 problem!"
else
  echo "We have $NUM problems!"
fi

Note the fi syntax, this will be a recurring theme in bash for other control structures.

Conditional execution

Remember how programming languages like java will lazily execute if statement conditions, let’s use the java example:

if(foo() || bar())
  //code
if(func() && meth())
  //code

statements using || will stop executing as soon as a true value is found, and conversely, && stops as soon as a false is found. Bash has a similar feature that is used to control execution since every bash function returns a ‘boolean’ based on whether the exectuion succeedes.

git commit && git push
# if commit returns 0, push after. Don't push otherwise
git push || echo "Push failed!"
# if push returns 0, continue, otherwise print failure

Loops

Basic loops can be C style or more python-esque, with no change in functionality:

# C like
for ((i = 0 ; i < 10 ; i++)); do
  echo $i
done

# Python like
for i in {1..10}; do
  echo $i
done

These two loops do not do the same thing however. The C-style iterator goes 0-9, and the other goes 1-10 inclusive. The second is also compatible with step size in the same way as the first. This also iterates in a similar way to python would, as the things in the bracket do not need to be numbers. It will simply set i equal to each index in the array and iterate for that index. So for i in {a, b, c, d, e}; do would work as expected.

While loops also exist and look like you’d expect:

while true; do
  ...
done

There is also an easy way to do text input from a file in a loop:

< file.txt | while read line; do
  echo $line
done

Functions

Functions are pretty simple, defined with either just declaring myfunc() { ... or with the function keyword.

func() {
  echo "bar"
}

Bash functions don’t have returns in the normal sense. A function that prints to stdout without being captured will simply print directly to stdout, however calling the function as if it is a executable like we did above will print it in the same way. The return keyword exists in bash, but since every function or script in bash must give a return code, it is used to define the return code of the function instead of any value. 0 is success and returned by default, and any other number from 1-255 is valid but considered failure. This return value can be accessed after the function finishes with the $? variable.

Arguments in bash functions also don’t work like other languages. You never define parameters in the () part of a method declaration, this means you need to check the number of paramaters passed to the function. This is done with some more variables:

This is the same syntax for command line arguments inside the script as a whole. You need to pass any command line args into the function, but other global variables without naming conflicts will stay in scope.

Data structures

These are fairly simple, albeit with strange definitions, so I’m just going to write out how to use them.

Arrays
Arr=('one' 'two' 'three')

echo ${Arr[0]}      # "one"
echo ${Arr[1]}      # "two"
echo ${Arr[2]}      # "three"
echo ${Arr[@]}      # all elements, space-separated
echo ${#Arr[@]}     # length of array
echo ${#Arr}        # String length of 1st element
echo ${#Arr[3]}     # String length of Nth element
echo ${Arr[@]:1:2}  # range from position 1, length 2

for i in "${arrayName[@]}"; do
  echo $i
done
Associative Arrays

AKA dictionaries

declare -A dict

dict[foo]="this"
dict[bar]="is"
dict[baz]="ez"

echo ${dict[foo]}   # this
echo ${dict[@]}     # all values
echo ${!dict[@]}    # all keys
echo ${#dict[@]}    # number of elements

unset dict[baz]     # delete baz

for val in "${dict[@]}"; do
  echo $val
done

for key in "${!dict[@]}"; do
  echo "${dict[$key]}"
done

Examples

Lets do some quick examples to show some simple things you can do with bash.

Dotfiles Installer

This is the first thing I wrote a bash script for and makes installing scripts on a new install of linux super easy. This one specifically also keeps all your dotfiles in your github repo folder which is nice.

#!/bin/bash

dir=$(pwd)
files="bashrc zshrc tmux.conf vimrc"
echo "current directory is ${dir}"
echo "moving current dotfiles to backup directory and installing new ones"

mkdir -p $dir/old

for file in $files; do
  echo "moving $file"
  mv ~/.$file $dir/old
  echo "creating symlink to ${file} in home directory"
  ln -s $dir/$file ~/.$file
done