Bash Scripting For Automation Practice Exam

Question 1 Bash Scripting For Automation

How do you read a file line by line?

while IFS= read -r line; do echo "$line"; done < file.txt. Setting IFS= preserves leading/trailing whitespace. Using the output of a command as a value: $(command) (preferred) or the legacy backticks `command`. $(...) nests cleanly and is recommended. $(( expression )) evaluates an integer expression, e.g. $(( a + b * 2 )). Supports +, -, *, /, %, ** (power), bitwise, and ternary. read -rp "Prompt: " answer. -r disables backslash escaping, -p provides a prompt.

Question 2 Bash Scripting For Automation

What is a Bash subshell trap?

(cd /tmp && cmd) changes directory only in the subshell; the parent's working directory is unchanged once the subshell exits. They are identical. source is a Bash alias for the POSIX dot operator .. ${var#pattern} removes shortest prefix, ${var##pattern} removes longest prefix, ${var%suffix} removes shortest suffix, ${var%%suffix} removes longest suffix. Enables alias expansion in non-interactive scripts. By default aliases are not expanded in scripts, so the line alias ll='ls -la' defined in ~/.bashrc won't apply.

Question 3 Bash Scripting For Automation

What does the `local` builtin do?

A shebang line, e.g. #!/usr/bin/env bash or #!/bin/bash, which tells the kernel which interpreter to invoke. A child shell spawned with ( commands ) or by backgrounding. Changes to variables, cd, and shell options do not affect the parent. Useful for isolating cd and temporary variable changes. Declares a variable with function-local scope, hiding any global of the same name. Always use local for variables inside functions to avoid leaking values. A named storage location for a value, assigned with NAME=value (no spaces around =) and referenced as $NAME or ${NAME}.

Question 4 Bash Scripting For Automation

How do you make a script only run on Linux, not macOS?

case "$(uname -s)" in Linux) ;; *) echo "Linux only"; exit 1 ;; esac. Or use [[ "$(uname)" == "Linux" ]] with a guard. joined=$(IFS=,; echo "${arr[*]}") or in Bash 4.4+ joined="${arr[*]}" with IFS=, set locally. For arbitrary joining use printf '%s\\n' "${arr[@]}" | paste -sd ,. -e exists, -f is regular file, -d is directory, -L is symlink, -r readable, -w writable, -x executable, -s size > 0, ! negates. Example: [[ -f "$file" ]]. Treats unset variables as an error when expanded, causing immediate exit. Helps catch typos and missing configuration.

Question 5 Bash Scripting For Automation

What is the `read` builtin with `-d`?

Toggles optional shell behaviour. Examples: shopt -s nocaseglob makes globs case-insensitive, shopt -s dotglob includes hidden files in globs, shopt -s globstar enables ** for recursive matching. -d DELIM sets the line delimiter instead of newline, useful for reading records terminated by NUL (read -d '') in find -print0 loops. A shebang line, e.g. #!/usr/bin/env bash or #!/bin/bash, which tells the kernel which interpreter to invoke. name() { commands; } or function name { commands; }. The body shares the caller's shell environment (no local scope by default).

Question 6 Bash Scripting For Automation

How do you iterate over command-line arguments?

for arg in "$@"; do echo "$arg"; done. Quoting "$@" preserves arguments containing spaces. Both expand the value, but ${VAR} disambiguates boundaries, e.g. ${VAR}suffix vs the ambiguous $VARsuffix. It also enables parameter expansion features like default values and substring operations. (cd /tmp && cmd) changes directory only in the subshell; the parent's working directory is unchanged once the subshell exits. sh explicitly invokes the sh interpreter and ignores the shebang, so Bash-specific features may break. ./script.sh honors the shebang and uses the listed interpreter.

Question 7 Bash Scripting For Automation

What is `declare -r`?

(cd /tmp && cmd) changes directory only in the subshell; the parent's working directory is unchanged once the subshell exits. Makes a variable read-only: any later assignment is an error. Equivalent to the readonly builtin, e.g. declare -r VERSION="1.0". Using the output of a command as a value: $(command) (preferred) or the legacy backticks `command`. $(...) nests cleanly and is recommended. case "$(uname -s)" in Linux) ;; *) echo "Linux only"; exit 1 ;; esac. Or use [[ "$(uname)" == "Linux" ]] with a guard.

Question 8 Bash Scripting For Automation

What is the `find` command's `-print0` for?

Internal Field Separator: the characters Bash uses for word splitting after unquoted expansions and for splitting records in read. Default is space, tab, newline. Parsing short command-line options like -a, -b value. Example: while getopts "ab:c" opt; do case $opt in ... esac; done. OPTARG holds an option's argument; $OPTIND is the next index to process. basename /a/b/c.sh prints c.sh, dirname /a/b/c.sh prints /a/b. Useful for working with file paths inside scripts. Prints filenames separated by a NUL byte so filenames with spaces or newlines are unambiguous. Pair with xargs -0 or while IFS= read -r -d '' f; do ...; done.

Question 9 Bash Scripting For Automation

How do you get the length of a string?

Both perform command substitution. $(...) is the modern form: easier to read, supports nesting ($(echo $(date))), and behaves more predictably inside double quotes. out=$(cmd 2>&1) or out=$(cmd 2>&1; echo "rc=$?"). For preserving them separately, use temp files: cmd >out 2>err. [[ -z "$s" ]] true if empty, -n true if non-empty, == or = equal (with glob on right), != not equal, < and > lexicographic compare. ${#var} returns the number of characters. For an array, use ${#arr[@]} for the number of elements.

Question 10 Bash Scripting For Automation

How do you ensure a script is run with Bash and not sh?

Makes a pipeline's return status the value of the rightmost command to exit non-zero (or zero if all succeed), instead of the last command only. Combined with set -e it surfaces failures hidden mid-pipeline. Start with #!/usr/bin/env bash and check at runtime: [[ -n "${BASH_VERSION:-}" ]] || { echo "Need bash"; exit 1; }. Both expand the value, but ${VAR} disambiguates boundaries, e.g. ${VAR}suffix vs the ambiguous $VARsuffix. It also enables parameter expansion features like default values and substring operations. Use arithmetic: n=$((10#$s)) forces base-10 interpretation and errors on non-digits. declare -i n=$s also forces integer semantics but silently ignores bad suffixes.

Question 11 Bash Scripting For Automation

What is the difference between `$(cmd)` and `cmd`?

Pattern-matching branching over a single value, e.g. case "$x" in\n start) ... ;;\n stop|halt) ... ;;\n *) default ;;\nesac. The right-hand side supports globs and | alternations. Both perform command substitution. $(...) is the modern form: easier to read, supports nesting ($(echo $(date))), and behaves more predictably inside double quotes. They are identical. source is a Bash alias for the POSIX dot operator .. Use the local builtin: local foo=bar. Without it, assignments are global.

Question 12 Bash Scripting For Automation

What is the `xargs` command for?

A named storage location for a value, assigned with NAME=value (no spaces around =) and referenced as $NAME or ${NAME}. set -euo pipefail — exit on error, undefined variables, and pipeline failures. Often paired with IFS=$'\n\t' to avoid word-splitting surprises. for f in a b c; do echo "$f"; done. The list can come from a glob, command substitution, or array. Builds and executes command lines from standard input. Safer: xargs -0 -n 1 -I {} cmd {} for NUL-delimited input and one argument per invocation.

Question 13 Bash Scripting For Automation

What is the difference between `$0` and `${BASH_SOURCE[0]}`?

Using the output of a command as a value: $(command) (preferred) or the legacy backticks `command`. $(...) nests cleanly and is recommended. $0 is the name used to invoke the script (may be bash, ./script.sh, or just script.sh). ${BASH_SOURCE[0]} is always the path of the script file currently executing, useful for resolving the script's own directory: dir=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd). Bash 4+: ${var^^} to upper, ${var,,} to lower. ${var^} and ${var,} capitalize/decapitalize the first character only. Start with #!/usr/bin/env bash and check at runtime: [[ -n "${BASH_VERSION:-}" ]] || { echo "Need bash"; exit 1; }.

Question 14 Bash Scripting For Automation

What is Bash?

Bash (Bourne Again SHell) is a Unix shell and command language written as a free software replacement for the Bourne shell (sh). It is the default login shell on most Linux distributions and macOS (until Catalina). -d DELIM sets the line delimiter instead of newline, useful for reading records terminated by NUL (read -d '') in find -print0 loops. Makes a pipeline's return status the value of the rightmost command to exit non-zero (or zero if all succeed), instead of the last command only. Combined with set -e it surfaces failures hidden mid-pipeline. Pattern-matching branching over a single value, e.g. case "$x" in\n start) ... ;;\n stop|halt) ... ;;\n *) default ;;\nesac. The right-hand side supports globs and | alternations.

Question 15 Bash Scripting For Automation

How do you make a variable local to a function?

VAR = value is a syntax error or runs the command VAR with args = and value. Whitespace around = in assignments is not allowed. Use the local builtin: local foo=bar. Without it, assignments are global. wc -l < file.txt to avoid the filename in output, or awk 'END{print NR}' file for portability across line endings. grep -c '' file also counts newlines. The special variable $? holds it. Zero means success; non-zero indicates failure.

Question 16 Bash Scripting For Automation

What is the `basename` and `dirname` pair?

./script.sh. The dot-slash is required because the current directory is not normally in $PATH. basename /a/b/c.sh prints c.sh, dirname /a/b/c.sh prints /a/b. Useful for working with file paths inside scripts. The null byte (\\0) cannot appear in Bash strings, since C-style null terminators end strings. find -print0 works around this for filenames; never printf a NUL into a Bash variable. for item in "${arr[@]}"; do echo "$item"; done. Quoting and [@] preserve elements that contain spaces.

Question 17 Bash Scripting For Automation

How do you read fields separated by a delimiter into variables?

read -rp "Prompt: " answer. -r disables backslash escaping, -p provides a prompt. Treats unset variables as an error when expanded, causing immediate exit. Helps catch typos and missing configuration. IFS=: read -r user _ uid _ _ home _ < /etc/passwd. The second positional is a placeholder variable name to skip fields. Filenames can contain spaces, newlines, or quotes that break naive parsing. Use globs (for f in *.txt) or find -print0 with xargs -0 or read -d ''.

Question 18 Bash Scripting For Automation

How do you log script output with timestamps?

exec > >(while IFS= read -r line; do printf '%s %s\\n' "$(date '+%F %T')" "$line"; done) 2>&1. Or pipe everything through ts '[%F %T]' from moreutils. declare -A m=([key1]=val1 [key2]=val2). Requires Bash 4+. Access with ${m[key1]}, all keys with ${!m[@]}, length with ${#m[@]}. out=$(cmd 2>&1) or out=$(cmd 2>&1; echo "rc=$?"). For preserving them separately, use temp files: cmd >out 2>err. [[ "$s" =~ ^[0-9]+$ ]] matches a digits-only string. Capture groups: if [[ "$s" =~ ^([A-Z]+)-([0-9]+)$ ]]; then echo "${BASH_REMATCH[1]}" "${BASH_REMATCH[2]}"; fi.

Question 19 Bash Scripting For Automation

What does `exec` do in a script?

After unquoted parameter, command, and arithmetic expansions, Bash splits the result into words on each character in IFS. Always quote expansions ("$var") to prevent it. sh explicitly invokes the sh interpreter and ignores the shebang, so Bash-specific features may break. ./script.sh honors the shebang and uses the listed interpreter. [[ -z "$s" ]] true if empty, -n true if non-empty, == or = equal (with glob on right), != not equal, < and > lexicographic compare. Without redirection it replaces the current shell process with a new command. With redirection (exec > file) it modifies the current shell's file descriptors, e.g. to send all subsequent output to a log.

Question 20 Bash Scripting For Automation

What is the `IFS` variable?

break exits the innermost loop; continue skips to the next iteration. Both accept an optional integer level, e.g. break 2 exits two levels. The special variable $? holds it. Zero means success; non-zero indicates failure. Internal Field Separator: the characters Bash uses for word splitting after unquoted expansions and for splitting records in read. Default is space, tab, newline. command -v cmd >/dev/null 2>&1 (preferred) or which cmd. Use it in an if to gate behaviour: if command -v docker >/dev/null; then ....

Question 21 Bash Scripting For Automation

How do you check if a string matches a regex in Bash?

[[ "$s" =~ ^[0-9]+$ ]] matches a digits-only string. Capture groups: if [[ "$s" =~ ^([A-Z]+)-([0-9]+)$ ]]; then echo "${BASH_REMATCH[1]}" "${BASH_REMATCH[2]}"; fi. set -e doesn't help if cd succeeds in the parent but a later command fails for another reason. Use cd /path || exit 1 or pushd /path || exit to make failure visible. 0 success, 1 general error, 2 misuse of shell builtin, 126 command found but not executable, 127 command not found, 128+N terminated by signal N (e.g. 130 = SIGINT/Ctrl-C, 137 = SIGKILL, 143 = SIGTERM). Export it: export VAR=value or VAR=value; export VAR. Without export, the variable is local to the current shell.

Question 22 Bash Scripting For Automation

How do you print to stderr from a script?

exec > >(while IFS= read -r line; do printf '%s %s\\n' "$(date '+%F %T')" "$line"; done) 2>&1. Or pipe everything through ts '[%F %T]' from moreutils. source (alias: .) executes the file in the current shell, so variable assignments and cd persist. ./script.sh runs in a new subshell, so its changes do not affect the parent. printf '%s\\n' "error" >&2 or echo "error" >&2. Send errors to stderr so they can be redirected separately from normal output. echo adds a trailing newline and may interpret backslash escapes depending on the shell. printf is portable, format-driven, and gives precise control. Prefer printf '%s\\n' "$var" for arbitrary data.

Question 23 Bash Scripting For Automation

What is the `shopt` builtin for?

for i in "${!arr[@]}"; do echo "$i=${arr[i]}"; done. ${!arr[@]} expands to the indices of arr. Both perform command substitution. $(...) is the modern form: easier to read, supports nesting ($(echo $(date))), and behaves more predictably inside double quotes. Bash 4+: ${var^^} to upper, ${var,,} to lower. ${var^} and ${var,} capitalize/decapitalize the first character only. Toggles optional shell behaviour. Examples: shopt -s nocaseglob makes globs case-insensitive, shopt -s dotglob includes hidden files in globs, shopt -s globstar enables ** for recursive matching.

Question 24 Bash Scripting For Automation

What is `PS4` in Bash?

Append &: long_job &. The shell prints the job ID and PID. Use wait %1 or wait $pid to synchronize. set -u does not affect this, but shopt -s nullglob makes empty globs expand to nothing, and shopt -s failglob causes the command to fail when no match is found. The prompt prefix used by set -x for trace output. Setting PS4='+ ${BASH_SOURCE}:${LINENO}: ' gives line-numbered traces that pinpoint where in a script each command runs. ${var/old/new} replaces first match, ${var//old/new} replaces all, ${var/#old/new} replaces at the start, ${var/%old/new} at the end.

Question 25 Bash Scripting For Automation

How do you redirect stdout and stderr separately?

Positional parameters $1, $2, etc. inside the function body. Quoted arrays can be passed by reference via declare -n refname=array (Bash 4.3+). exec > >(while IFS= read -r line; do printf '%s %s\\n' "$(date '+%F %T')" "$line"; done) 2>&1. Or pipe everything through ts '[%F %T]' from moreutils. for i in "${!arr[@]}"; do echo "$i=${arr[i]}"; done. ${!arr[@]} expands to the indices of arr. cmd > out.log 2> err.log. 2>&1 sends stderr to wherever stdout points (order matters: do redirections left to right).

Question 26 Bash Scripting For Automation

How do you compare two floating-point numbers in Bash?

Toggles optional shell behaviour. Examples: shopt -s nocaseglob makes globs case-insensitive, shopt -s dotglob includes hidden files in globs, shopt -s globstar enables ** for recursive matching. Filename pattern expansion: unquoted *, ?, [...] match filenames in the current directory. To disable, set -f; to force, set -f; set +f in a subshell. Use time: time cmd, or TIMEFORMAT='%R'; time { cmd1; cmd2; } to get the elapsed seconds in ${TIMEFORMAT}-aware contexts. Inside a script, SECONDS=0; cmd; echo "$SECONDS" gives elapsed seconds. Bash arithmetic only handles integers. Use awk or bc: awk -v a=1.5 -v b=1.4 'BEGIN{exit !(a>b)}', or (( $(echo "$a > $b" | bc -l) )).

Question 27 Bash Scripting For Automation

What is the difference between `echo` and `printf`?

After unquoted parameter, command, and arithmetic expansions, Bash splits the result into words on each character in IFS. Always quote expansions ("$var") to prevent it. break exits the innermost loop; continue skips to the next iteration. Both accept an optional integer level, e.g. break 2 exits two levels. echo adds a trailing newline and may interpret backslash escapes depending on the shell. printf is portable, format-driven, and gives precise control. Prefer printf '%s\\n' "$var" for arbitrary data. ${var#pattern} removes shortest prefix, ${var##pattern} removes longest prefix, ${var%suffix} removes shortest suffix, ${var%%suffix} removes longest suffix.

Question 28 Bash Scripting For Automation

What does `${VAR:?message}` do?

Prints message to stderr and exits the script with status 1 if VAR is unset or empty. Useful for required-input guards. sh explicitly invokes the sh interpreter and ignores the shebang, so Bash-specific features may break. ./script.sh honors the shebang and uses the listed interpreter. Filenames can contain spaces, newlines, or quotes that break naive parsing. Use globs (for f in *.txt) or find -print0 with xargs -0 or read -d ''. [[ "$s" =~ ^[0-9]+$ ]] matches a digits-only string. Capture groups: if [[ "$s" =~ ^([A-Z]+)-([0-9]+)$ ]]; then echo "${BASH_REMATCH[1]}" "${BASH_REMATCH[2]}"; fi.

Question 29 Bash Scripting For Automation

What file extension is conventionally used for Bash scripts?

Bash (Bourne Again SHell) is a Unix shell and command language written as a free software replacement for the Bourne shell (sh). It is the default login shell on most Linux distributions and macOS (until Catalina). Bash arithmetic only handles integers. Use awk or bc: awk -v a=1.5 -v b=1.4 'BEGIN{exit !(a>b)}', or (( $(echo "$a > $b" | bc -l) )). joined=$(IFS=,; echo "${arr[*]}") or in Bash 4.4+ joined="${arr[*]}" with IFS=, set locally. For arbitrary joining use printf '%s\\n' "${arr[@]}" | paste -sd ,. .sh. While not strictly required for execution, .sh signals intent and lets tooling treat the file as a shell script.

Question 30 Bash Scripting For Automation

How do you test a script safely?

[[ $EUID -eq 0 ]] (root has EUID 0). Alternatively, [[ "$(id -u)" -eq 0 ]]. Use this to gate operations needing privileges. Forces a variable to integer type: arithmetic is performed on assignment, e.g. declare -i n=1+2 gives n=3. Useful for counters, though ((n++)) is often clearer. After unquoted parameter, command, and arithmetic expansions, Bash splits the result into words on each character in IFS. Always quote expansions ("$var") to prevent it. Use bash -n for syntax, shellcheck script.sh (linter) for issues, and run with set -x on a copy or in a sandbox. For destructive commands, dry-run with echo prefixed or use rm -i during testing.

Question 31 Bash Scripting For Automation

How do you loop over array indices and values?

for i in "${!arr[@]}"; do echo "$i=${arr[i]}"; done. ${!arr[@]} expands to the indices of arr. ${var#pattern} removes shortest prefix, ${var##pattern} removes longest prefix, ${var%suffix} removes shortest suffix, ${var%%suffix} removes longest suffix. Forces a variable to integer type: arithmetic is performed on assignment, e.g. declare -i n=1+2 gives n=3. Useful for counters, though ((n++)) is often clearer. cmd >> log.txt 2>&1. Without >>, > truncates the file each run.

Question 32 Bash Scripting For Automation

How do you break out of a loop early or skip an iteration?

Creates a unique temporary file or directory safely: tmp=$(mktemp) or tmpd=$(mktemp -d). Avoids race conditions and predictable paths in /tmp. break exits the innermost loop; continue skips to the next iteration. Both accept an optional integer level, e.g. break 2 exits two levels. Internal Field Separator: the characters Bash uses for word splitting after unquoted expansions and for splitting records in read. Default is space, tab, newline. name() { commands; } or function name { commands; }. The body shares the caller's shell environment (no local scope by default).

Question 33 Bash Scripting For Automation

How do you append to an array?

read -rp "Prompt: " answer. -r disables backslash escaping, -p provides a prompt. Makes a pipeline's return status the value of the rightmost command to exit non-zero (or zero if all succeed), instead of the last command only. Combined with set -e it surfaces failures hidden mid-pipeline. arr+=(four five). To append a single element: arr+=("six"). Bash (Bourne Again SHell) is a Unix shell and command language written as a free software replacement for the Bourne shell (sh). It is the default login shell on most Linux distributions and macOS (until Catalina).

Question 34 Bash Scripting For Automation

How do you loop over an array safely?

arr+=(four five). To append a single element: arr+=("six"). Internal Field Separator: the characters Bash uses for word splitting after unquoted expansions and for splitting records in read. Default is space, tab, newline. source (alias: .) executes the file in the current shell, so variable assignments and cd persist. ./script.sh runs in a new subshell, so its changes do not affect the parent. for item in "${arr[@]}"; do echo "$item"; done. Quoting and [@] preserve elements that contain spaces.

Question 35 Bash Scripting For Automation

What is a variable in Bash?

printf '%s\\n' "error" >&2 or echo "error" >&2. Send errors to stderr so they can be redirected separately from normal output. A named storage location for a value, assigned with NAME=value (no spaces around =) and referenced as $NAME or ${NAME}. The prompt prefix used by set -x for trace output. Setting PS4='+ ${BASH_SOURCE}:${LINENO}: ' gives line-numbered traces that pinpoint where in a script each command runs. The special variable $? holds it. Zero means success; non-zero indicates failure.

Question 36 Bash Scripting For Automation

How do you write a multi-line string in Bash?

Internal Field Separator: the characters Bash uses for word splitting after unquoted expansions and for splitting records in read. Default is space, tab, newline. Use a here-document: cat <<EOF\nline 1\nline 2\nEOF. Quote the delimiter (<<'EOF') to disable parameter expansion inside. case "$(uname -s)" in Linux) ;; *) echo "Linux only"; exit 1 ;; esac. Or use [[ "$(uname)" == "Linux" ]] with a guard. The arguments passed to a script or function, accessed as $1, $2, ..., with $0 being the script name and $# the count of arguments.

Question 37 Bash Scripting For Automation

What is a common pitfall when reading `getopts` after the loop?

-e exists, -f is regular file, -d is directory, -L is symlink, -r readable, -w writable, -x executable, -s size > 0, ! negates. Example: [[ -f "$file" ]]. You usually want to shift processed options with shift $((OPTIND-1)) so that "$@" contains the remaining positional arguments. After unquoted parameter, command, and arithmetic expansions, Bash splits the result into words on each character in IFS. Always quote expansions ("$var") to prevent it. The special variable $? holds it. Zero means success; non-zero indicates failure.

Question 38 Bash Scripting For Automation

What does `until` do?

Word-splitting expands it to [ = "bar" ], a syntax error. Always quote: if [ "$foo" = "bar" ] or use [[ "$foo" == "bar" ]]. joined=$(IFS=,; echo "${arr[*]}") or in Bash 4.4+ joined="${arr[*]}" with IFS=, set locally. For arbitrary joining use printf '%s\\n' "${arr[@]}" | paste -sd ,. Loops as long as its condition is false: until ping -c1 host; do sleep 5; done. Bash (Bourne Again SHell) is a Unix shell and command language written as a free software replacement for the Bourne shell (sh). It is the default login shell on most Linux distributions and macOS (until Catalina).

Question 39 Bash Scripting For Automation

What is the C-style `for` loop syntax?

for (( i=0; i<10; i++ )); do echo "$i"; done. Uses arithmetic expression syntax with comma-separated C-style clauses. ${var/old/new} replaces first match, ${var//old/new} replaces all, ${var/#old/new} replaces at the start, ${var/%old/new} at the end. Each positional parameter as a separate, individually quoted word: "$1" "$2" "$3" .... This is the correct way to forward arguments. A no-op that ignores its arguments and returns 0. Used as a placeholder in empty functions or branches, and as the left side of parameter expansion tricks, e.g. : "${VAR:=default}".

Question 40 Bash Scripting For Automation

How do you join an array into a single string with a delimiter?

exec > >(while IFS= read -r line; do printf '%s %s\\n' "$(date '+%F %T')" "$line"; done) 2>&1. Or pipe everything through ts '[%F %T]' from moreutils. joined=$(IFS=,; echo "${arr[*]}") or in Bash 4.4+ joined="${arr[*]}" with IFS=, set locally. For arbitrary joining use printf '%s\\n' "${arr[@]}" | paste -sd ,. .sh. While not strictly required for execution, .sh signals intent and lets tooling treat the file as a shell script. Positional parameters $1, $2, etc. inside the function body. Quoted arrays can be passed by reference via declare -n refname=array (Bash 4.3+).

Question 41 Bash Scripting For Automation

What are the basic conditional expressions in an `if` statement?

if, elif, else, fi. The condition is a list of commands; its exit status (0 = true) decides the branch. for item in "${arr[@]}"; do echo "$item"; done. Quoting and [@] preserve elements that contain spaces. basename /a/b/c.sh prints c.sh, dirname /a/b/c.sh prints /a/b. Useful for working with file paths inside scripts. name() { commands; } or function name { commands; }. The body shares the caller's shell environment (no local scope by default).

Question 42 Bash Scripting For Automation

How do you read a configuration file as Bash variables?

${var#pattern} removes shortest prefix, ${var##pattern} removes longest prefix, ${var%suffix} removes shortest suffix, ${var%%suffix} removes longest suffix. source config.sh or . config.sh. Files meant for sourcing should not run side effects and should be careful with quoting. Using the output of a command as a value: $(command) (preferred) or the legacy backticks `command`. $(...) nests cleanly and is recommended. $0 is the name used to invoke the script (may be bash, ./script.sh, or just script.sh). ${BASH_SOURCE[0]} is always the path of the script file currently executing, useful for resolving the script's own directory: dir=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd).

Question 43 Bash Scripting For Automation

How do you check whether you are running as root?

Start with #!/usr/bin/env bash and check at runtime: [[ -n "${BASH_VERSION:-}" ]] || { echo "Need bash"; exit 1; }. [[ $EUID -eq 0 ]] (root has EUID 0). Alternatively, [[ "$(id -u)" -eq 0 ]]. Use this to gate operations needing privileges. Append &: long_job &. The shell prints the job ID and PID. Use wait %1 or wait $pid to synchronize. arr+=(four five). To append a single element: arr+=("six").

Question 44 Bash Scripting For Automation

How do you run a command in the background?

Append &: long_job &. The shell prints the job ID and PID. Use wait %1 or wait $pid to synchronize. Tells the shell to exit immediately if any command, pipeline, or simple command exits with a non-zero status (unless explicitly tested in if/while/||/&& contexts). source config.sh or . config.sh. Files meant for sourcing should not run side effects and should be careful with quoting. arr+=(four five). To append a single element: arr+=("six").

Question 45 Bash Scripting For Automation

How do you schedule a Bash script to run periodically?

Use cron (crontab -e, e.g. 0 3 * * * /opt/scripts/backup.sh >> /var/log/backup.log 2>&1) or systemd timers. Cron jobs have a minimal PATH; set required variables explicitly. case "$(uname -s)" in Linux) ;; *) echo "Linux only"; exit 1 ;; esac. Or use [[ "$(uname)" == "Linux" ]] with a guard. [[ -z "$s" ]] true if empty, -n true if non-empty, == or = equal (with glob on right), != not equal, < and > lexicographic compare. A child shell spawned with ( commands ) or by backgrounding. Changes to variables, cd, and shell options do not affect the parent. Useful for isolating cd and temporary variable changes.

Question 46 Bash Scripting For Automation

How do you convert a string to an integer safely?

cmd >> log.txt 2>&1. Without >>, > truncates the file each run. Bash arithmetic only handles integers. Use awk or bc: awk -v a=1.5 -v b=1.4 'BEGIN{exit !(a>b)}', or (( $(echo "$a > $b" | bc -l) )). Bash (Bourne Again SHell) is a Unix shell and command language written as a free software replacement for the Bourne shell (sh). It is the default login shell on most Linux distributions and macOS (until Catalina). Use arithmetic: n=$((10#$s)) forces base-10 interpretation and errors on non-digits. declare -i n=$s also forces integer semantics but silently ignores bad suffixes.

Question 47 Bash Scripting For Automation

What is the difference between `sh script.sh` and `./script.sh`?

sh explicitly invokes the sh interpreter and ignores the shebang, so Bash-specific features may break. ./script.sh honors the shebang and uses the listed interpreter. A static analysis tool (shellcheck.net) that flags common bugs, quoting issues, and style problems in shell scripts. It is highly recommended; install via package manager and run in CI. Treats unset variables as an error when expanded, causing immediate exit. Helps catch typos and missing configuration. Positional parameters $1, $2, etc. inside the function body. Quoted arrays can be passed by reference via declare -n refname=array (Bash 4.3+).

Question 48 Bash Scripting For Automation

What must the first line of an executable Bash script be?

Use trap 'cleanup' EXIT. The EXIT pseudo-signal fires on any termination: normal exit, error, or signal (except SIGKILL). Enables alias expansion in non-interactive scripts. By default aliases are not expanded in scripts, so the line alias ll='ls -la' defined in ~/.bashrc won't apply. Each positional parameter as a separate, individually quoted word: "$1" "$2" "$3" .... This is the correct way to forward arguments. A shebang line, e.g. #!/usr/bin/env bash or #!/bin/bash, which tells the kernel which interpreter to invoke.

Question 49 Bash Scripting For Automation

What is the `(( ))` command?

An arithmetic-evaluation builtin returning 0 if the expression result is non-zero. Use it for integer tests: if (( a < b )); then ...; fi. [[ -z "$s" ]] true if empty, -n true if non-empty, == or = equal (with glob on right), != not equal, < and > lexicographic compare. name() { commands; } or function name { commands; }. The body shares the caller's shell environment (no local scope by default). Sets a handler to run when the shell receives a signal or exits. Example: trap 'rm -f "$tmpfile"' EXIT ensures cleanup on any exit, including signals.

Question 50 Bash Scripting For Automation

What is the syntax for a `while` loop?

Bash (Bourne Again SHell) is a Unix shell and command language written as a free software replacement for the Bourne shell (sh). It is the default login shell on most Linux distributions and macOS (until Catalina). while condition; do body; done. The condition is re-evaluated after each iteration; the loop ends when the command returns non-zero. [[ "$s" =~ ^[0-9]+$ ]] matches a digits-only string. Capture groups: if [[ "$s" =~ ^([A-Z]+)-([0-9]+)$ ]]; then echo "${BASH_REMATCH[1]}" "${BASH_REMATCH[2]}"; fi. There is no direct include: use source (executes) or parse functions with eval "$(declare -f funcname)". Modules are usually sourced.