Skip to content

Scripting Essentials

A shell script is a file of commands with a shebang line, run as a program. RHCSA and daily operations need the same core: arguments, tests, conditionals, loops, functions and exit statuses; larger scripting topics belong in a dedicated Bash folder.

Track: RHCSA · Interview weight: Med

Must-Know Facts

Fact Value Verify with
Shebang First line, #!/bin/bash; chooses the interpreter head -1 script.sh
Run a script chmod +x script.sh; ./script.sh, or bash script.sh ls -l script.sh
Arguments $0 name, $1..$9, ${10}, $# count, "$@" all, kept separate ./args.sh a "b c"
Test commands [ ] (POSIX), [[ ]] (bash: patterns, no word splitting) help [[
Numeric comparison -eq -ne -lt -le -gt -ge, or (( a < b )) [ 5 -gt 3 ]
String comparison =, !=, -z (empty), -n (not empty) [ -z "$x" ]
File tests -e exists, -f file, -d directory, -r -w -x, -s not empty [ -d /etc ]
Read a file line by line while IFS= read -r line; do ...; done < file help read
Function variables local keeps them inside the function help local
Syntax check and lint bash -n script.sh; shellcheck script.sh (EPEL, Ubuntu universe) echo $?

Arguments

#!/bin/bash
echo "script: $0"
echo "count:  $#"
echo "first:  $1"
echo "all:    $*"
for arg in "$@"; do
    echo "arg:    [$arg]"
done
shift
echo "after shift, first: $1"

./args.sh web "db server" cache

Output:

script: ./args.sh
count:  3
first:  web
all:    web db server cache
arg:    [web]
arg:    [db server]
arg:    [cache]
after shift, first: db server

"$@" keeps db server as one argument; shift drops $1 and renumbers the rest.

Tests and Conditionals

[ is a command, so its arguments need spaces and quotes. Two common mistakes, with x=5:

[ $x > 3 ] && echo "test passed"
ls -l 3
unset y
[ $y = "a" ]; echo "rc=$?"
[ "$y" = "a" ]; echo "rc=$?"
[[ $y == a ]]; echo "rc=$?"

Output:

test passed
-rw-r--r-- 1 laborant laborant 0 Sep 16 13:48 3
bash: line 4: [: =: unary operator expected
rc=2
rc=1
rc=1

Inside single brackets, > is a redirection

Inside [ ], > is a redirection: the test ran as [ 5 ] (true) and created a file named 3. An unquoted empty variable disappears and leaves [ = a ]. Use -gt or (( x > 3 )) for numbers, and quote variables or use [[ ]].

A case statement matches one value against patterns:

#!/bin/bash
case "${1:-}" in
    start|stop|restart)
        echo "running: systemctl $1 app" ;;
    status)
        echo "app is active" ;;
    *)
        echo "usage: $0 {start|stop|restart|status}" >&2
        exit 2 ;;
esac

./svc.sh restart
./svc.sh reload; echo "rc=$?"

Output:

running: systemctl restart app
usage: ./svc.sh {start|stop|restart|status}
rc=2

Loops

for i in 1 2 3; do echo -n "$i "; done; echo
for ((i=0; i<3; i++)); do echo -n "$i "; done; echo
for f in /etc/host*; do echo "$f"; done
n=0; while [ $n -lt 3 ]; do n=$((n+1)); done; echo "n=$n"
until [ $n -eq 0 ]; do n=$((n-1)); done; echo "n=$n"

Output:

1 2 3 
0 1 2 
/etc/host.conf
/etc/hostname
/etc/hosts
n=3
n=0

A Complete Script

The script reads user names from a file, skips blanks and comments, reports each user, and exits non-zero if any is missing.

#!/bin/bash
set -euo pipefail

usage() {
    echo "usage: $0 <file-with-usernames>" >&2
    exit 2
}

user_status() {
    local user=$1
    if id "$user" &>/dev/null; then
        echo "$user: exists (uid $(id -u "$user"))"
    else
        echo "$user: missing"
        return 1
    fi
}

[[ $# -eq 1 ]] || usage
[[ -r $1 ]] || { echo "cannot read $1" >&2; exit 1; }

missing=0
while IFS= read -r name; do
    [[ -z $name || $name == \#* ]] && continue
    user_status "$name" || missing=$((missing + 1))
done < "$1"

echo "missing users: $missing"
(( missing == 0 ))

printf 'root\n# comment\n\nlaborant\nghost\n' > users.txt
./check-users.sh users.txt; echo "rc=$?"
./check-users.sh; echo "rc=$?"

Output:

root: exists (uid 0)
laborant: exists (uid 1001)
ghost: missing
missing users: 1
rc=1
usage: ./check-users.sh <file-with-usernames>
rc=2

Checking and Debugging

bash -n check-users.sh && echo "syntax ok"
printf 'if true\necho x\n' > bad.sh
bash -n bad.sh; echo "rc=$?"
bash -x ./svc.sh status

Output:

syntax ok
bad.sh: line 3: syntax error: unexpected end of file
rc=2
+ case "${1:-}" in
+ echo 'app is active'
app is active

bash -n checks syntax, not logic

bash -n parses without running; it catches a missing fi, esac or done (reported as unexpected end of file), not logic errors. bash -x traces each command. shellcheck finds unquoted variables and most of the mistakes above.

Common Errors

[: =: unary operator expected

Cause: an unquoted variable was empty, so [ received too few arguments.

Fix: quote it ([ "$y" = a ]) or use [[ $y == a ]].

Interview Checkpoints

L1: What is the difference between $@ and $*?

Say first: quoted, "$@" expands to each argument as a separate word, while "$*" joins them into one string.

Proof: ./args.sh web "db server" prints [db server] as one argument with "$@".

Follow-up: What happens to "db server" if $@ is not quoted?

L1: What is the difference between [ ] and [[ ]]?

Say first: [ is a command that follows normal word splitting; [[ is bash syntax that does not split variables and supports pattern and regex matching.

Proof: with y unset, [ $y = a ] errors while [[ $y == a ]] returns 1.

Follow-up: Which one works in /bin/sh on Ubuntu?

L2: Write a loop that prints each line of a file, keeping spaces.

Say first: while read with IFS= and -r.

Proof:

while IFS= read -r line; do printf '%s\n' "$line"; done < file.txt

Follow-up: Why is for line in $(cat file.txt) wrong?

L2: Write a script that creates users from a list, skipping those that exist.

Say first: loop over the file, test with id, create with useradd.

Proof:

while IFS= read -r u; do
    id "$u" &>/dev/null || sudo useradd -m "$u"
done < users.txt

Follow-up: How would you set an initial password non-interactively? (chpasswd.)

L3: A script works for most inputs but fails with unary operator expected for some.

Say first: an unquoted variable is empty for those inputs.

Proof: bash -x script.sh shows [ = value ]; quoting the variable fixes it.

Follow-up: Which tool reports this before the script runs? (shellcheck, SC2086.)

L3: A counter loop in a set -e script stops after the first iteration.

Say first: an arithmetic command returned status 1.

Proof: ((count++)) with count=0 evaluates to 0, which is false, so set -e exits.

Follow-up: Name two safe ways to increment. (count=$((count+1)), ((count+=1)) || true.)

Captured on Rocky Linux 10.2 and Ubuntu 24.04.4 LTS (iximiuz Labs microVMs, kernel 6.1.167), 2026-09.