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Posix Shell Scripting

Some neat tricks and methods for scripting.

Directory of the Script

At the start of my scripts is always the same line:

scriptdir=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)

Looks odd? Well, let's explain it and then you'll be enlightened. This is very portable

CDPATH= Unset the CDPATH environment variable to make sure we don't search outside of where we tell cd to go. If you man cd you'll possibly find that CDPATH is an environment variable that can be used like as a search list, much like the PATH environment variable. We don't want surprises.

cd -- Change directory. The double heiphen marks the end of options input to the cd command so that directories with odd names that start with a heiphen and will be mistaken for options instead of the directory name don't cause problems.

$() Use a subshell - substitutes the output of the command run in the subshell

dirname -- "$0" Output the directory name of the command line used to execute the shell script (which is in "${0}").

&& pwd Once the current working directory has been changed to the script's directory we run the pwd tool to get the absolute path of the program's working directory.

Looping

Various ways of looping depending on the data set.

Multiple Line Variables

Sometimes we end up with a multiline list in a variable and we want to do something for each item in the list. Looping over this list portably is straight forward using a HEREDOC directed into a while loop:

files=$(ls -1 /etc)

while read -r file; do
    echo "Do something with ${file}"
done << EOF
${files}
EOF

Output:

Do something with abrt
Do something with adjtime
Do something with aliases
Do something with alsa
Do something with alternatives
Do something with anaconda
...

Space Separated Variables

Looping over a space-separated list is a bit easier and can be done in a for loop. Can't have any spaces in the list items though.

packages="gcc gcc-g++ gdb autotools automake"
for package in ${packages}; do
    echo "Do something with ${package}"
done

Output:

Do something with gcc
Do something with gcc-g++
Do something with gdb
Do something with autotools
Do something with automake

Heredocs

These extremely useful things can be used in many places. Some examples of how they can be used below. I end up using them for writing configuration files often.

Write File from Heredoc

Variables are substituted normally when using a heredoc with variables inside it:

language=en-GB

cat << EOF > config.yaml
language: ${language}
EOF

Output

[brian@brian-2920x ~]$ cat config.yaml
language: en-GB

If you single quote the heredoc variables are not substituted:

```bash
language=en-GB

cat << 'EOF' > config.yaml
language: ${language}
EOF

Output

[brian@brian-2920x ~]$ cat config.yaml
language: ${language}