Linux Command Line & Bash Automation Flashcards

Question 1

Numeric Permission Modes (`chmod 755`)

Accepted Answer

Sets permissions using octal notation for **User**, **Group**, and **Others**:
- Read (`r`) = 4
- Write (`w`) = 2
- Execute (`x`) = 1

`755` represents `7` (4+2+1=rwx) for owner, `5` (4+0+1=r-x) for group, and `5` (4+0+1=r-x) for others.

Question 2

Octal Representation: `644`

Accepted Answer

Calculates permissions as:
- **User**: `6` (4+2+0 = `rw-`)
- **Group**: `4` (4+0+0 = `r--`)
- **Others**: `4` (4+0+0 = `r--`)

This is the standard permission for public/readable files.

Question 3

Changing Ownership (`chown`)

Accepted Answer

Changes file owner and group settings:
- `chown user:group filename` changes both the owner and group.
- `chown -R user:group directory/` recursively updates permissions for all subdirectories and files.

Question 4

Standard Redirection Operators (`>`, `>>`, `2>`)

Accepted Answer

- `command > file`: Redirects standard output (stdout), overwriting the destination file.
- `command >> file`: Appends stdout to the destination file.
- `command 2> file`: Redirects standard error (stderr) to the file.

Question 5

Combining Standard Output and Error (`&>` / `2>&1`)

Accepted Answer

Redirects both stdout and stderr to a single file:
- `command > output.log 2>&1` (POSIX compliant)
- `command &> output.log` (Bash shorthand)

Question 6

The Pipe Operator (`|`)

Accepted Answer

Passes the standard output (stdout) of the command on the left directly into the standard input (stdin) of the command on the right.

Example: `cat app.log | grep "ERROR"`

Question 7

`grep` Essential Flags

Accepted Answer

Searches plain text for regular expression patterns:
- `-i`: Case-insensitive search.
- `-v`: Invert match (returns non-matching lines).
- `-r` / `-R`: Recursive directory search.
- `-n`: Displays line numbers alongside matches.

Question 8

Stream Editor (`sed`) Text Substitution

Accepted Answer

Replaces text pattern matches line-by-line:
```bash
sed 's/old/new/g' file.txt
```
- `s`: Substitute command.
- `g`: Global flag (replaces all matches per line, not just the first).
- `-i`: Edits the file in-place instead of printing to stdout.

Question 9

`awk` Field Processing

Accepted Answer

A pattern-scanning and processing language that processes text row-by-row based on delimiters.
```bash
awk -F':' '{print $1, $3}' /etc/passwd
```
- `-F':'`: Sets field separator to `:`.
- `$1`, `$3`: Prints the 1st and 3rd field of each line.

Question 10

Process Snapshot vs. Live Monitoring (`ps` vs `top`)

Accepted Answer

- `ps aux`: Returns a static snapshot of all currently running processes.
- `top` (or `htop`): Provides an interactive, real-time updated view of system processes, CPU utilization, and memory usage.

Question 11

Process Signals (`kill -15` vs `kill -9`)

Accepted Answer

- `kill -15 ` (`SIGTERM`): Requests graceful process termination, allowing cleanup. - `kill -9 ` (`SIGKILL`): Immediately and forcefully kills the process; cannot be caught or ignored.

Question 12

Crontab Schedule Syntax

Accepted Answer

Cron schedules automated jobs using 5 time fields:
```
* * * * * command_to_execute
│ │ │ │ │
│ │ │ │ └─ Day of week (0 - 6) (0=Sunday)
│ │ │ └─── Month (1 - 12)
│ │ └───── Day of month (1 - 31)
│ └─────── Hour (0 - 23)
└───────── Minute (0 - 59)
```

Question 13

Exit Status Variable (`$?`)

Accepted Answer

Holds the return code of the most recently executed command:
- `0`: Success.
- `1-255`: Failure or error condition.

Used in scripting conditionals to check execution success.

Question 14

Positional Script Variables (`$0`, `$1`, `$#`, `$@`)

Accepted Answer

- `$0`: Script filename.
- `$1`, `$2`, ...: First and second arguments passed to the script.
- `$#`: Total number of arguments passed.
- `$@`: Expands to all positional arguments as individual words.

Question 15

Process ID Variables (`$$` and `$!`)

Accepted Answer

- `$$`: Returns the PID (Process ID) of the current shell or script.
- `$!`: Returns the PID of the most recently executed background process.

Question 16

Job Control (`&`, `Ctrl+Z`, `bg`, `fg`)

Accepted Answer

- `command &`: Runs process in the background.
- `Ctrl + Z`: Suspends current foreground process.
- `bg`: Resumes suspended job in the background.
- `fg`: Brings background job back to foreground.

Question 17

Conditional Operators (`&&` vs `||`)

Accepted Answer

- `cmd1 && cmd2`: Short-circuit AND. Runs `cmd2` **only if** `cmd1` succeeds (exit status `0`).
- `cmd1 || cmd2`: Short-circuit OR. Runs `cmd2` **only if** `cmd1` fails (exit status non-zero).

Question 18

Command Substitution (`$()`)

Accepted Answer

Executes a command and substitutes its standard output into a variable or expression.
```bash
CURRENT_DATE=$(date +%Y-%m-%d)
echo "Today is $CURRENT_DATE"
```
*Note:* Preferred over legacy backtick syntax (`` `command` ``).

Question 19

Finding Files with `find`

Accepted Answer

Searches directory trees based on attributes:
```bash
find /path -type f -name "*.log" -mtime -7
```
- `-type f`: Restricts search to regular files.
- `-name "*.log"`: Filters by file extension.
- `-mtime -7`: Matches files modified within the last 7 days.

Question 20

Soft Link vs. Hard Link

Accepted Answer

- **Soft Link** (`ln -s target link`): A symbolic shortcut pointing to the target file path. Breaks if the original file is moved or deleted.
- **Hard Link** (`ln target link`): Points directly to the file's underlying inode. Remains functional even if the original filename is removed.

Linux Command Line & Bash Automation

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