Shell Basic Commands

Quickly Write to File

1. Method 1

cat > a.txt << 'EOF'
afasdf
EOF

2. Method 2

tee a.txt <<- 'EOF'
	afsafdsf
	EOF	

Linux System Scheduled Network Time Synchronization

Reference Article

# Set scheduled task
crontab -e 
*/5 * * * * ntpdate time.nist.gov

# View scheduled task list
crontab -l

# View scheduled task log
tail -f /var/log/cron

# Several time servers
ntpdate time.nist.gov
ntpdate time.nuri.net
ntpdate 0.asia.pool.ntp.org
ntpdate 1.asia.pool.ntp.org
ntpdate 2.asia.pool.ntp.org
ntpdate 3.asia.pool.ntp.org

crontab scheduled task parameter meaning

Field	Mandatory	Allowed Values	Allowed Special Characters	Remarks
Seconds	Yes	0–59	*``,``-	Standard implementation does not support this field.
Minutes	Yes	0–59	*``,``-
Hours	Yes	0–23	*``,``-
Day of month	Yes	1–31	*``,``-``?``L``W	?``L``W only partially implemented by software
Month	Yes	1–12 or JAN–DEC	*``,``-
Day of week	Yes	0–7 or SUN–SAT	*``,``-``?``L``#	?``L``# only partially implemented by software. Linux and Spring allow 0-7, 0 and 7 are Sunday. Quartz allows 1-7, 1 is Sunday
Year	No	1970–2099	*``,``-	Standard implementation does not support this field.

vmware Network Settings

Note best to use vmware 15, vmware 16 version seems to have network problems.

Query Total Size of Current Folder

du -sh ./

Query Directory Tree of First n Levels in Current Directory Structure

Show first level directory for /d %i in (*) do @echo %i

Install lrzsz

yum install -y lrzsz

sz download rz upload

Close Java Process in Linux

kill -9 $(ps -ef | grep -w "xxx-1.0.jar" | grep -v "grep" | awk '{print $2}')

grep Command

1️⃣ Basic Command

grep [options] PATTERN FILE

• Search for lines containing PATTERN in file
• Common options:
  • -i → Ignore Case
  • -v → Invert Match (lines not containing PATTERN)
  • -n → Show Line Number
  • -r / -R → Recursive Search Directory
  • -E → Enable Extended Regular Expressions (ERE)

---

2️⃣ Basic Examples

2.1 Match Keyword

grep "error" logfile.txt

Match lines containing "error"

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2.2 Ignore Case

grep -i "error" logfile.txt

Match "error", "Error", "ERROR" etc.

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2.3 Show Line Number

grep -n "error" logfile.txt

Output Line Number + Line Content

---

2.4 Invert Match

# Output lines **not containing `error`**
grep -v "error" logfile.txt
# Records containing a but not b
awk '/a/ && !/b/' logfile.txt
grep "a" logfile.txt | grep -v "b"

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2.5 Recursive Search Directory

grep -r "TODO" ./src

Search all files in src directory and subdirectories

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3️⃣ grep -E Extended Regex

• Support more powerful regex syntax:
  • | → OR
  • + → Previous character 1 or more times
  • ? → Previous character 0 or 1 time
  • () → Grouping
• No need to use backslash escape like normal grep

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3.1 Match Multiple Keywords

grep -E "error|fail|fatal" logfile.txt

Match "error", "fail" or "fatal"

---

3.2 Match Repeated Characters

echo -e "aaa\naa\nb" | grep -E "a+"

Match 1 or more consecutive a Output:

aaa
aa

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3.3 Optional Characters

echo -e "color\ncolour" | grep -E "colou?r"

u? → u can appear 0 or 1 time Output:

color
colour

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3.4 Group Matching

echo -e "cat\nbat\nrat" | grep -E "(c|b)at"

Match "cat" or "bat" Output:

cat
bat

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3.5 Line Number + Ignore Case

grep -Eni "error|fail" logfile.txt

• -n → Line Number
• -i → Ignore Case
• -E → Extended Regex

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3.6 Complex Example

grep -E '(error|fail|fatal) [0-9]{3}' logfile.txt

Parse:

• (error|fail|fatal) → Match error, fail or fatal
• → Space
• [0-9]{3} → Match three digits
• Output entire matching line content

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4️⃣ Match pip list Example (Combined with Extended Regex)

Assuming you have Python package list:

pip list | grep -E "sentence"
# Output:
# sentence-transformers                    5.1.0

If want to match sentence-transformers, use .* wildcard:

pip list | grep -E "sentence.*transformers"

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5️⃣ Tips

1. Pattern wrapped with single quotes '...', avoid shell escape
2. Match entire word: \bpattern\b
3. Match numbers: [0-9]+
4. Match any character: .
5. Match repeated characters: + / {n,m}

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✅ Summary

• grep → Basic Search
• grep -E → Extended Regex, supports |, +, ?, ()
• Combinable options: -i, -v, -n, -r
• Common tips: Wrap with single quotes, .* wildcard, [- ] match multiple separators

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