1.3. Bash

1.3.1. General

  • Shell scripts should be kept at a minimum and only for internal scripts. For every other script we should use Python.

  • Public shell scripts should use /bin/sh. Bash is not available on all Unix systems and ksh or other shells are not compatible with Bash syntax.

  • Use printf instead of echo when using /bin/sh as a shell, because echo behaviour is incompatible in Bash and Korn shells. Eg. use printf instead of echo -n and printf "blabla\n" instead of echo "blabla".

  • For internal scripts we should use /bin/bash, as the default /bin/sh sometimes points to a lesser shell such as Dash.

  • A function called main is required for scripts long enough to contain at least one other function.

  • Put all functions together in the file just below constants. Don’t hide executable code between functions.

  • Use uppercase for all GLOBAL variables.

  • In tests, enclose all uncontrolled string variables in double quotes to handle empty strings or spaces in variables.

  • Always double quote the $@ variable to keep all arguments with spaces.

  • Pipelines should be split one per line if they don’t all fit in one line.

  • Use $(command) instead of backticks. Nested backticks require escaping the inner ones with . The $(command) format doesn’t change when nested and is easier to read. See the example below

# This is preferred:
var="$(command "$(command1)")"

# This is not:
var="`command \`command1\``"

  • Always guard your Bash scripts from unexpected errors by using

set -o nounset

1.3.2. Path constants

When defining a directory path as a constant, you should include the trailing slash to make sure it is a directory and not a file. Two consecutive /’s are harmless in POSIX shells as long as they are not at the beginning of the path in some exotic environment such as Cygwin that actually uses this notation for SMB shares.

Always double quote path constants to handle files with spaces:

SOME_PATH=/some/path/
# and use it like this:
command "${SOME_PATH}/some.file"

# Another example
REMOTE_URI=chevah@chevah.com:/home/chevah/geck.chevah.com/vm/
scp "$LOCAL_FILE" "${REMOTE_URL}/file"

Instead of:

SOME_PATH=/some/path/
# and then:
command "${SOME_PATH}some.file"

1.3.3. Function Definition

Leave 2 blank lines between function definitions and always use local for local variable declaration.

Define local variables at the start of the function, in a distinct block. It is recommended to define a local variable named result to hold the value produced by calling the function.

Put the final returned result in a separate block.

Procedures are functions which have no result.

Since Bash only support returning numeric values, which are interpreted as exit codes, we will pass values between functions by using echo.

#
# Description of function 1.
#
# * $1 - description of first argument
# * $2 - description of second argument
# * return - description of return value.

function1() {
    local variable_which_is_local
    local result

    do_some_action_here
    do_more_action

    echo $result
}


#
# Description of procedure 1.
#
# It does this and this.
#
procedure1() {
    local something=$(function1 ARG1)

    do_something_else something
}

1.3.4. Case Syntax

case "$VARIABLE_NAME" in
    "option1")
        do specific
        ;;
    option2*)
        do generic
        ;;
    *)
        do default
        ;;
esac

1.3.5. IF/THEN/ELSE

if TEST; then
    call something
elif [ "$string" = OTHER_TEST ]; then
    call something_else
else
    call something_else_completely
fi

1.3.6. FOR

for CONDITION; do
    call something
done

1.3.7. WHILE/UNTIL

while TEST; do
    call something
done

1.3.8. References

Here are the pages I used to create this page.