Bash/shell scripting

http://tldp.org/LDP/abs/html/index.html

Variables

`$?`

Return value of last run process

`$1`

The first argument passed to the script

`$0`

The name of the script

Declaring variables

To declare a variable within a script:

myvariable="value"

Note: do not put spaces before or after equals sign!

To declare a variable outside a script that is to be used within a script:

export myvariable="value"

Or pass it at the same time you call the script (which only declares it for the context of the script):

myvariable="value" ./myscript

Using variables

echo "This is my variable: ${myvariable}"

Check to make sure a variable is set and not null

if [ -n "$1" ]

Check the value of a variable

if [ "$LOCATION" == "home" ]; then

Spaces padding brackets and equals signs are required!
Use [] instead of [[]]
Always quote variable names, otherwise the script will crash if the variable isn’t set
Use == for equality and != for inequality for strings, -eq and -ne for integers. Other comparison operators

Output redirection

`>`

Redirect stdout

`>/dev/null`

Don’t show stdout

`2>`

Redirect stderr

`2>&1`

Direct stderr to stdout

`&>`

Redirect all output

Conditionals

If statements

if [ condition ]; then
elif...
fi

And/or

if [ condition ] && [ condition ] || [ condition ]; then
...

Case statements

Example from an init script:

case "$1" in
    start)
    start
    RETVAL=$?
    ;;
    stop)
    stop
    RETVAL=$?
    ;;
    restart)
    stop
    start
    ;;
esac

Check if a directory exists

if [ -d /path/to/folder ]; then
fi

Check if a file exists

if [ -f /path/to/file ]; then
fi

Check if a file exists and is readable

if [ -r /path/to/file ]; then
fi

Check if a file exists and is executable

if [ -x /path/to/file ]; then
fi

Command-line arguments

First argument

$1

Second argument

$2

Number of arguments

$#

Scripting tips

Environment variables

Make sure to export environment variables if they’re needed by a command in your script (and not just used internally by your script):

export http_proxy=http://proxy.example.org:3128/
export https_proxy=http://proxy.example.org:3128/

`cp`

Don’t ask before overwriting files:

/bin/cp -f

Because often vendors will alias cp so that it will always ask before overwriting files:

$ alias cp
alias cp='cp -i'

`curl`

Use -s to hide the progress bar:

curl -s ...

`ln`

Overwrite existing links and don’t fail if the link already exists:

ln -fs

`mkdir`

Create child directories and don’t fail if the directory already exists:

mkdir -p

`rm`

Don’t fail if the directory/file has already been deleted:

rm -f

`systemctl`

Use --no-pager to not page results, e.g.

systemctl status docker --no-pager || sudo systemctl start docker

`telnet`

sleep 0 | telnet server.example.org 22 || true

`wget`

Prefer curl to wget since curl will often be present when wget is not

Misc

For loops

list="item1 item2"
for item in $list; do
    do something with $item
done

`|| :`

Placed at the end of a command to force the exit status to 0 (success)

`$(dirname $0)`

Get name of directory script resides in

My wiki