Programmer’s Reference
Navigate, inspect files, write scripts, and diagnose a Linux host.
Bash on Linux, with GNU tools. macOS and other Unix systems may use different flags. Replace demo paths before use.
Select a term for its definition, syntax, example, and common mistakes.
Branch on command exit status: zero means success.
#!/usr/bin/env bash
count=3
if (( count > 5 )); then
printf '%s\n' Many
elif (( count > 0 )); then
printf '%s\n' Some
else
printf '%s\n' None
fiPractical useUse this construct in your own practice project.
Common mistakesBash tests exit status, not printed text; close the construct with fi.
Source: www.gnu.org documentation
Run the next command only if the previous command fails.
#!/usr/bin/env bash
test -d ./demo-project || printf '%s\n' 'Directory missing'Practical useUse this construct in your own practice project.
Common mistakesFailure means a nonzero exit status; || does not inspect output.
Source: www.gnu.org documentation
Run the next command only if the previous command succeeds.
#!/usr/bin/env bash
test -d ./demo-project && printf '%s\n' 'Directory exists'Practical useUse this construct in your own practice project.
Common mistakesChains are not always equivalent to an if/else block.
Source: www.gnu.org documentation
Loop through a list of words.
#!/usr/bin/env bash
for name in Demo Example; do
printf '%s\n' "$name"
donePractical useUse this construct in your own practice project.
Common mistakesQuote variable expansions to prevent splitting and globbing.
Source: www.gnu.org documentation
Repeat while the condition command succeeds.
#!/usr/bin/env bash
count=0
while (( count < 3 )); do
printf '%s\n' "$count"
count=$((count + 1))
donePractical useUse this construct in your own practice project.
Common mistakesMake the condition eventually fail.
Source: www.gnu.org documentation
Choose a branch by matching a shell pattern.
#!/usr/bin/env bash
state=ready
case "$state" in
ready) printf '%s\n' Ready ;;
*) printf '%s\n' Unknown ;;
esacPractical useUse this construct in your own practice project.
Common mistakesPatterns are shell patterns, not regular expressions; close cases with ;;.
Source: www.gnu.org documentation
Define a reusable shell function and return an exit status.
#!/usr/bin/env bash
describe() {
printf '%s\n' "$1"
return 0
}
describe DemoPractical useUse this construct in your own practice project.
Common mistakesreturn sets status, not an arbitrary string result; use stdout for text.
Source: www.gnu.org documentation
Leave a loop or skip to its next iteration.
#!/usr/bin/env bash
for n in 1 2 3 4; do
if (( n == 2 )); then continue; fi
if (( n == 4 )); then break; fi
printf '%s\n' "$n"
donePractical useUse this construct in your own practice project.
Common mistakesThese builtins need an enclosing loop.
Source: www.gnu.org documentation
Bash has no goto builtin or label-based jump syntax; use loops and functions.
#!/usr/bin/env bash
describe() {
if [[ $1 == ready ]]; then
printf '%s\n' Ready
return 0
fi
printf '%s\n' Waiting
}
describe readyPractical useUse this construct in your own practice project.
Common mistakesA word named goto is treated as a command name, not a language keyword.
Source: www.gnu.org documentation
Print your current directory, inspect its files, then change into a demo directory.
pwd
ls -lah
cd ~/demo-project
pwdPractical useConfirm you are inside the staging checkout before a release check.
Common mistakesCheck pwd before modifying files. Quote paths containing spaces.
Source: www.gnu.org documentation (1) · www.gnu.org documentation (2) · www.gnu.org documentation (3)
Read a command manual or Bash built-in help.
man ls
help cd
ls --helpPractical useCheck flags on the target Linux host rather than assuming macOS behavior.
Common mistakes--help is common on GNU commands but not universal. q exits most manual pagers.
Source: www.gnu.org documentation (1) · www.gnu.org documentation (2)
Inspect a file in a pager, show its last lines, and search for a fixed string.
less demo.log
tail -n 30 demo.log
grep -nF -- "ERROR" demo.logPractical useFind an error in a sanitized staging log.
Common mistakesLogs may contain sensitive data. grep treats patterns as regular expressions unless -F is used.
Source: www.gnu.org documentation (1) · www.gnu.org documentation (2)
Use double quotes to keep a variable value as one argument.
#!/usr/bin/env bash
site='Demo site'
printf '%s\n' "$site"Practical useKeep a site label containing spaces intact.
Common mistakesUnquoted expansions can split words and expand wildcards. Single quotes prevent variable expansion.
Source: www.gnu.org documentation
Send one command’s output into another and save a new report.
⚠ Caution: The > redirect replaces existing file contents.
printf "%s\n" "demo: running" | sort
printf "%s\n" "demo: running" > demo-report.txtPractical useWrite a disposable health summary in a staging directory.
Common mistakes> truncates an existing file; >> appends. A pipe normally reports the last command’s exit status.
Source: www.gnu.org documentation
Create a backup directory and copy a demo file while preserving basic metadata.
⚠ Caution: May overwrite the destination file.
mkdir -p demo-backups
cp -p -- demo.conf demo-backups/demo.confPractical useBack up a staging configuration before editing.
Common mistakescp can overwrite an existing destination. A file copy is not a consistent database backup.
Source: www.gnu.org documentation (1) · www.gnu.org documentation (2)
Inspect ownership and grant the owner read/write access to a private demo file.
⚠ Caution: Changes file access; a service using the file may lose access.
ls -l demo.env
chmod 600 demo.envPractical useKeep a local environment fixture readable only by its owner.
Common mistakesPermissions alone do not fix ownership, ACLs, or SELinux. Never use chmod 777 as a general repair.
Source: www.gnu.org documentation
Search a demo tree for configuration files without modifying them.
find ./demo-project -type f -name '*.conf' -printPractical useInventory configuration files in a staging copy.
Common mistakesQuote the wildcard so the shell does not expand it first. Confirm the root directory.
Source: www.gnu.org documentation
Request confirmation before unlinking one explicitly named file.
⚠ Caution: Deletes the named file; ensure a recoverable copy exists.
rm -i -- ./demo-output.txtPractical useClean up a disposable report after reviewing it.
Common mistakesDeleted files generally bypass a trash bin. Do not add -r or -f casually, or use an unverified variable as the path.
Source: www.gnu.org documentation
Check a Bash script’s syntax without running its commands.
bash -n deploy-demo.sh
printf "Exit status: %s\n" "$?"Practical useValidate a release helper before staging execution.
Common mistakesSyntax checks do not prove a script is safe. Read the script and test it on disposable staging data.
Source: www.gnu.org documentation