Extended Arguments (xargs) Unix command

xargs builds and executes command lines from standard input.

While pipe command (|) takes the stdout of the previous command and forwards it to stdin of the next command, xargs takes space-separated strings from that stdin and convert them into arguments of xargs command.

Example:

$ echo "/dirA /dirB" | xargs ls

will be converted to:

ls /dirA /dirB

-n1 causes the command specified by xargs to be executed, taking one argument at a time from its input (arguments can be each in one line or simply separated by spaces or tabs) and running the command separately for each individual item.​ The -n1 option means "use just one argument per command invocation".​ Each item from standard input (for example, a filename from a list) will be passed separately to the command. As a result, the command will be run as many times as there are items in the input, once for each.

Example:

echo -e "a\nb\nc" | xargs -n1 echo

...will produce:

echo a

echo b

echo c

So each invocation receives only one argument from the input.​

This is useful when a command should process only one item at a time, such as deleting files one by one, or when handling commands that cannot accept multiple arguments simultaneously.

-I{} replaces {} in the command with the input item (in the following example that's lambda function name).

In the following example we use -I to replace {} with incoming argument for xarg and then we use $ positional parameters to interpolate inputs for sh:

Let's assume we have previously defined variables like...

AWS_PROFILE=...

AWS_REGION=...

SG_ID=...

aws lambda list-functions \

    --profile "$AWS_PROFILE" \

    --region "$AWS_REGION" \

    --query "Functions[].FunctionName" \

    --output text | \

xargs \

    -n1 \

    -I{} \

    sh -c \

        "aws lambda get-function-configuration \

            --profile \"\$1\" \

            --region \"\$2\" \

            --function-name \"\$3\" \

            --query \"VpcConfig.SecurityGroupIds\" \

            --output text 2>/dev/null | \

        grep \

            -w \"\$4\" && \

        echo \

            \"Found in Lambda function: \$3\"" \

    _ "$AWS_PROFILE" "$AWS_REGION" {} "$SG_ID"

The sh -c command allows passing multiple arguments, which are referenced as $1, $2, $3, and $4 inside the shell script.

The underscore (_) is used as a placeholder for the $0 positional parameter inside the sh -c subshell.

When you use sh -c 'script' arg0 arg1 arg2 ..., the first argument after the script (arg0) is assigned to $0 inside the script, and the rest (arg1, arg2, etc.) are assigned to $1, $2, etc.

In this context, _ is a common convention to indicate that the $0 parameter is not used or is irrelevant. It simply fills the required position so that $1, $2, $3, and $4 map correctly to "$AWS_PROFILE", "$AWS_REGION", {} (the function name), and "$SG_ID".

References:

xargs manual

bash - When to use xargs when piping? - Stack Overflow

xargs