Variables and quoting

A variable is a name that holds a piece of text. In the shell everything is text: numbers, paths, lists of words. You assign with name=value and read with $name. Two rules trip everyone at first: there are no spaces around =, and you use the $ only when reading, never when assigning.

~% name=TempMV
~% echo "Welcome to $name"
Welcome to TempMV
~% name = TempMV
sh: name: not found
~% count=3
~% echo "$count files, next is $((count + 1))"
3 files, next is 4
~% echo "${name}Lab"
TempMVLab

name = TempMV fails because the shell reads name as a command and = and TempMV as its arguments. When a variable is glued to other letters, wrap the name in braces: ${name}Lab; otherwise the shell looks for a variable called nameLab. Variable names are case-sensitive, and by convention your own variables are lowercase while environment variables like HOME and PATH are uppercase.

The three kinds of quoting

The dangerous one is no quotes. After expanding $var, the shell splits the result on spaces and expands any * or ? it finds. If file="my notes.txt", then cat $file runs cat my notes.txt and complains about two missing files, while cat "$file" opens the right one. Get into the habit of writing "$var" every time; it is never wrong, and omitting it is the number one source of script bugs.

~% file="my notes.txt"
~% touch "$file"
~% cat $file
cat: can't open 'my': No such file or directory
cat: can't open 'notes.txt': No such file or directory
~% cat "$file" && echo "(empty, but it opened)"
(empty, but it opened)
~% echo 'Cost: $5 per *'
Cost: $5 per *
~% echo "Cost: \$5"
Cost: $5

Inside double quotes a backslash still escapes $, ", ` ` and \, so "\$5" prints a dollar sign. Inside single quotes nothing is special, not even a backslash, which is why you cannot put a single quote inside single quotes; close them, add \', and reopen: 'it'\''s'`.

Variables in files, and exporting them

A file that only contains assignments is a configuration file. Load it into a script with the dot command: . /root/lab/l82/config.sh. The lines run in the current shell, so the variables are now yours. A plain variable is private to the shell that set it; to let child processes (commands you run) see it, export it, as lesson 69 showed. Scripts receive the exported environment of the shell that started them, which is how $HOME and $PATH are already there when a script begins.