Shell

Arguments, tests, and redirection

These are the pieces a short RHEL script actually uses. Quoting, a test, and where the output goes.

printf 'first %s\n' "$1"
printf 'count %s\n' "$#"
printf 'arg %s\n' "$@"

"$1" is the first argument. "$#" is how many arguments there were. "$@" is every argument, each as its own word. The quotes matter when an argument contains a space.

[[ -d /path ]]
[[ -f /path ]]
[[ -n "${1:-}" ]]
name=$(whoami)

[[ -d ]] is true when the path is a directory. [[ -f ]] is true when it is a regular file. $(whoami) runs the command and keeps what it printed. ${1:-} is an empty string when $1 was not set.

cmd > out.txt
cmd >> out.txt
cmd 2> err.txt
cmd > all.txt 2>&1
printf '%s\n' "$?"

> writes standard output to a file and replaces that file. >> appends. 2> is standard error. 2>&1 sends standard error to the same place as standard output, and it has to come after the > that sets that place. $? is the exit status of the last command. 0 means it worked.

set -euo pipefail

At the top of a script, -e exits when a command fails, -u errors when a variable is unset, and pipefail makes a pipeline fail if any stage fails. -e does not fire inside an if test.

On RHEL 7, 8, 9, and 10, sed is GNU sed. sed -i 's/old/new/' file edits the file in place. sed -i.bak keeps a copy named file.bak.