Practice Shell Scripting for Bash

Note

• Bash is the GNU Project's shell—the Bourne Again SHell.

Bash features

Variables

To create a variable: assign a value. Don't use whitespace around =

directory=reports

Values containing spaces: use quotes. Use double quotes - keep meaning of dollar sign intact.

filenames="notes.txt picture.jpg movie.mov"

To retrieve the value, prefix with $

echo $directory

Use braces to tell where your variable ends.

${variable}_report

Note Shell variables have no data type. They simply store strings. Naming: use letters, numbers, underscore. First letter: a letter or underscore. Case-sensitive. Predefined variables are uppercase. PATH, HOME, SECONDS, IFS, etc.

Taking arguments

Script can accept input from arguments

create_report.sh A5 myfolder

Use -- (end of options) - If your input is unpredictable you can run into problems when your variable contains a string which is starting with a -

grep -- "user_input"

Note Some commands don't support this - use printf instead of echo

printf "I will delete this file: %s\n" "$file_to_delete"

Return Values

Returned by program upon exit

Note 0-255, 0 means success Other values are error codes

Returned value from script - exit 0

Good habit Exit scripts with a correct value Always call exit with a value

Conditional Expressions

Test for return value of command

if touch /tmp/x; then ...

Perform test on strings, files, numbers

if [[ $str ]]; then
    ...
fi

Examples

Argument empty - user didn't pass first argument to the script

[[ ! $1 ]]

String not empty

[[ $str ]]

String equals "something"

[[ $str = "something" ]]

File exists

[[ -e $filename ]] 

File does NOT exist

[[ ! -e $file ]]

Directory exists

[[ -d $dirname ]]

AND operator

[[ -d $dir && $1 = "foo" ]]

OR operator

[[ $a || $b ]]

More info about possible comparisons

man test

Debugging

Use -x to show each line with variable values as the script runs

#!/bin/bash -x

Calling your script

If the script is not on your PATH: include the directory

./myscript.sh

If it is on your PATH: call it like a regular command

myscript.sh

Useful Commands

Making your scripts executable

Executable for owner only

chmod u+x filename

Executable for everyone

chmod a+x filename

Remove executable permission

chmod a-x filename