Some things I've found interesting and helpful as I've slowly learned how to make practical use of bash scripts one silly project at a time. More geared towards making clunky scripts for automating file system stuff than advanced use of the shell for workflow optimization etc.
⚠️ Warning: This is a work in progress and very much a reflection of a self-taught perspective!
- what is bash
- check your shell
- comments printing & variables
- if statements
- greater than & less than
- declaring & invoking functions
- other notes
Bash is referred to as a Unix or command line language and is an abbreviation of Bourne Again Shell. This is a reference to an earlier similar Unix shell command language (Bourne Shell) which was named after Stephen Bourne who developed it in the 1970's at Bell Labs. Bash was specifically developed as part of the Free Software Movement / GNU as a free alternative to other similar languages that were changing to for-profit models.
Bash is a syntax for communicating directly with Unix systems. Unix systems differentiate themselves from prior systems because they are built for running multiple processes concurrently. Historically on prior systems like DOS multiple processes could not be run on a single machine at the same time, and multiple users could not work on / share the same system concurrently.
We don't have that much need for multiple users to share the same machine concurrently anymore (this was from back when computers were larger, more expensive and were often shared by multiple workers at different terminals) but we do take for granted that our computers can do more than one task at a time, and that we can open multiple shells in terminal windows, VS Code workspaces etc. We didn't always have these luxuries!
These are some very basic commands that can be run directly in your shell application (terminal etc). They can also be run as part of scripts if you save them in a file ending in sh (ie test-script.sh) and make use of them as individual lines or in more complex scenarios.
# confirm that you're running bash and not zsh...
echo $0
echo $SHELL
# switch into bash if you're in zsh (use control+d to exit)...
bash
# switch into zsh if you're in bash (just for fun?)...
zsh📌 Fun Fact: You can open independent nested instances of bash inside of bash inside of zsh inside of bash etc etc.
# make comments with single hash-tags like this!
# print text...
echo hello world!
# declare variables (cant use spaces)...
a=42
b=hello
c='hello world!'
# check variables using echo and the special $ charactor...
echo $a
echo $b
echo $c
# re-assign and exising variable...
a=2501
# echo environment variables (case-sensitive)...
echo $USER
echo $SHELL
# check all environment variables...
export
# using variables as commands...
e=echo
$e hello!
f='echo goodbye!'
$f
# using list elements as commands...
l=('hello there!' echo)
${l[1]} ${l[0]}
These can be written directly in the shell but are probably best written in a script file and then run. Make a file ending in sh (my-script.sh), write any valid bash syntax, save it, exit your editor and then run the file by typing $ bash my-script.sh which will run all the files commands.
# some basic conditionals...
if [ 'something' ]
then
echo 'a string is truthy!'
fi
if []
then
echo 'this wont print!'
fi
if [ 1 = 1 ]
then
echo 'one equals one!'
fi
if [ 1 = 42 ]
then
echo 'this wont print!'
fi
if [ 'cat' = 'cat' ]
then
echo 'cat equals cat!'
fi
if [ 1 = 1 ]
then echo 'one line then command!'
fi
if [ 1 = 1 ]; then echo 'one line conditional!'; fi
# quotes are not needed if spaces aren't in variables...
a="cat"
b="cat"
if [ $b = $a ]
then
echo 'true!'
fi
# quotes are needed if spaces aren't in variables...
a="my cat"
b="my cat"
if [ "$b" = "$a" ]
then
echo 'true!'
fi
# double equals is just alternative syntax...
if [ 1 == 1 ]
then
echo 'double equals works!'
fi
# all equals comparisons are loose...
if [ "1" == 1 ]; then echo 'true!'; fi
if [ "1" = 1 ]; then echo 'also true!'; fi
# some shorter conditional syntax examples...
# if this file doesn't exist then create it...
[ ! -d tom.txt ] && touch tom.txt
# if this file does exist then delete it...
[ -d tom.txt ] && rm tom.txt if [ 1 -gt 0 ]
then
echo '1 is greater than 0!'
fi
if [ 0 -gt 1 ]
then
echo 'this wont print!'
fi
if [ 0 -lt 1 ]
then
echo '0 is less than 1!'
fi
if [ 1 -eq 1 ]
then
echo 'we can use -eq instead of "="'
fi
# note that quotes don't matter...
if [ "1" -gt 0 ]; then echo 'true!'; fi
if [ "1" -eq 1 ]; then echo 'also true!'; fi
if [ "1" -lt 0 ]; then echo 'not true!'; fi
# print all commands as they are run...
set -x
# pause to capture user input as a variable...
read input
echo $input
# define a function foo...
foo () {
echo "foo!"
}
# invoke the new function...
foo
# make and call a function that takes arguments...
bar () {
for arg in "#@"
do
echo $arg
done
}
foo hello there world
To be expanded on later with examples etc maybe.
\ #breaks lines visually
; # command separator
[...] # test brackets
test # utility interchangeable with test brackets
[[...]] # korn shell brackets for boolean logic / regex
foo[1-5].txt # wild card range
foo & # run (foo) as background process
sort < foo.txt # input redirection
ls > foo.txt # output redirection
cat foo.txt | sort # pipes output as input
!! # reruns previous command
${foo} # parameter expansion
${foo:2:6} # slice string from index 2 to 6
echo "\$100" # print $ as literal / non-special character
touch hi-{tom,erin}.txt # brace expansion
$ # introduces parameter expansion,
# command substitution or arithmetic expansion
${parameter} # optional protective curly brackets
${foo:-bar} # if parameter foo is unset use string
# ...