Bash (scripting, not shell)

Created: 2016-03-25 15:31:50 -0700 Modified: 2022-07-04 15:09:22 -0700

Basics

Good Gist here with some general guidelines
ShellCheck is a script analysis tool for shell scripts
Safe scripting (e.g. by exiting on any error): reference
- For any of the following, just put a line at the top of the script, e.g. “set -Eeuo pipefail”.
  - ↑ In fact, that command is a good start for most Bash scripts to make it work like how you’re used to with most programming languages.
- set -e: exit immediately when a command fails
- set -o pipefail: sets exit code of a pipeline (piped commands) to the rightmost failing command
- set -u: treat unset variables as an error and exit immediately
- set -x: print every command before executing it. It’s like Batch’s “@echo on”
.bash_profile vs. .bashrc (reference, reference for zsh): in short: bash_profile is for commands that should only run once (like setting your $PATH), bashrc is good for commands that should run every time you start a new shell.
Interactive vs. login shell (reference): in short: a login shell is when you logged into the system through a shell, e.g. SSHing into a machine. An interactive shell is like when you start a new terminal on macOS.
Check if a command was successful (reference)

some_command
if [ $? -eq 0 ]; then
    echo OK
else
    echo FAIL
fi

You can also do this

some_command
retval=$?
do_something $retval
if [ $retval -ne 0 ]; then
    echo "Return code was not zero but $retval"
fi

Return
- “return” isn’t required from all functions
Arguments
- Reading script-level arguments is the same as reading function-level arguments: $1,$ 2, etc.
- I find it nice to name arguments in functions anyway:

add_two_numbers() {
    OPERAND_ONE=$1
    OPERAND_TWO=$2

    (( result = OPERAND_ONE + OPERAND_TWO ))
}

add_two_numbers 5 6
echo Result is: $result

Comparing arguments is easy

if [[ "$1" != "deploy" && "$1" != "delete" ]]; then
    echo "You need to specify either 'deploy' or 'delete'"
    exit 1
fi

Checking if an argument is empty (reference)

if [ -z "$1" ] # Note: to check if it's not empty, just do if [ ! -z "$1" ]
  then
    echo "No argument supplied"
fi

To pass in an empty argument, specify open and closing quotation marks like this:
- ./foo.sh "" second_arg "" fourth_arg
Using temporary buffers

Suppose you have sounds.json like this:

{
        "frog": "REPLACE_ME",
        "dog": "woof"
}

You can modify REPLACE_ME without having to make a temporary file by using a temporary buffer:

cat <(cat sounds.json | sed s/REPLACE_ME/ribbit/g)

↑ [09:24] Pneumatus: that sed command in the temporary buffers section, you know there’s an ‘-i’ arg to sed that lets you do in-place replacement? :)

[09:25] Pneumatus: sed -i ‘s/REPLACE_ME/ribbit/g’ sounds.json

[09:25] Pneumatus: or maybe the other way round

If/elif/end

# This is just a simple example that does nothing helpful:
if [[ "$arg" == "..." ]]; then
    # Go up 2 directories
    cd ../..
elif [[ "$arg" == "...." ]]; then
    # Go up 3 directories
    cd ../../..
else
    # Don't do anything
fi

Helpful snippets

Set error handling: set -Eeuo pipefail
- This was mentioned earlier, but it makes your environment more like a programming environment.
Write to stderr and then exit (reference)

die () {
    echo >&2 "$@"
    exit 1
}

This is useful for doing something like:

echo $1 | grep -E -q '^[0-9]+$ ’ || die “Numeric argument required, $1 provided”

Evaluating a scripting expression

You’re supposed to use $():

cur_date = $(date)

It used to be that you’d use backticks, e.g.

cur_date=date

(do not use this since it’s deprecated)

Evaluating an arithmetic expression (reference)

Simple example:

foo=5
bar=6
(( baz = foo + bar ))
echo $baz # This shows 11

For floating-point calculations, use bc.

$() vs.$ {}

[14:22] woodworker_: so $(something) will run the command and return whatever the command printed out$ {varname} is for using variable names when you do not have spaces so you can do ${VAR1}$ {VAR2}

So: ${VAR} =$ (echo $VAR)

“test”

”[ … ]” is syntactic sugar for the “test” command, meaning if you’re having problems with it, you can look through “man test”. Example usage:

if [ ${var}=YES ]

Accessing your bash_profile functions from a script

The best way I found was just to add “source ~/.bash_profile” to the top of whatever script you’re writing, e.g.:

~/.bash_profile:

#!/bin/bash

function foo() {

echo hi

}

test.sh:

#!/bin/bash

source ~/.bash_profile

foo

Using ssh-agent from within bash (notes reference, GitHub reference)

(just look at the references)

Using “sudo” inside of a script

Don’t do something like this:

sudo -iu bldeploy

Instead, do this:

run_as_bldeploy.sh

sudo -u bldeploy impl.sh

impl.sh

If you really wanted, you could follow the advice from here and do something like “sudo bash -c ‘cmd1; cmd2; cmd3’“. That link also has advice for using ‘here’ documents.

Regex

I had a string like this: <a bunch of garbage><a href=“http://127.0.0.1:8000/show/df8cd949661a”\>

I got the hash at the end by doing this:

if [[$curlOutput =~ show([a-zA-Z0-9]+)]]; then

bpasteHash=${BASH_REMATCH[1]}

else

echo “No match found”;

The key was not using “w” like you have in other languages. I don’t think that works in Bash to represent a letter character.