Linux/Bash Command Reference Guide

A comprehensive reference for common Linux and Bash commands, utilities, and system information.

Table of Contents

Find

Basic Syntax

find . -type [f/d] -name "pattern"

Common Examples

Find files and directories by name:

# Find files starting with "test"
find . -type f -name "test*"

# Find directories containing "backup"
find . -type d -name "*backup*"

Replace file extensions for files starting with capital letters:

# Change .js to .jsx for files starting with capital letters
find . -type f -name "[[:upper:]]*.js" -exec sh -c 'mv "$1" "${1}x"' _ {} \;

Find and process multiple files:

# Find all Python files modified in last 7 days
find . -type f -name "*.py" -mtime -7

# Find and delete empty directories
find . -type d -empty -delete

Sed

Basic Syntax

sed -i 's/pattern/replacement/flags' file

Note: On macOS, use sed -i '' (with empty quotes)

Common Examples

Basic find and replace:

# Replace text in Ruby files (with capture group)
sed -i '' 's/\(regex\)/\1, extra string/g' app/models/*.rb

Mass find and replace across multiple files:

# Replace FactoryGirl with FactoryBot in all Ruby files
find . -type f -name "*.rb" -exec sed -i '' 's/FactoryGirl/FactoryBot/g' {} \;

Advanced sed operations:

# Delete lines containing pattern
sed -i '/pattern/d' file.txt

# Insert text at specific line number
sed -i '3i\New line text' file.txt

# Replace only first occurrence per line
sed 's/old/new/' file.txt

Bash Scripting

String Operations

Parentheses vs Braces

  • (): Commands run in a subshell (isolated environment)
  • {}: Commands grouped together in current shell

Variable Quoting

#!/bin/bash
var1="A B  C   D"
echo $var1   # Output: A B C D (word splitting occurs)
echo "$var1" # Output: A B  C   D (preserves spacing)

String Length

export stringZ="abcABC123ABCabc"
echo ${#stringZ}                 # 15
echo $(expr length $stringZ)     # 15
echo $(expr "$stringZ" : '.*')   # 15

Substrings

export stringZ="abcABC123ABCabc"
echo ${stringZ:1}          # bcABC123ABCabc (from position 1)
echo ${stringZ:7:3}        # 23A (3 characters starting at position 7)

String Replacement

Syntax: ${parameter/pattern/string}

  • Single /: Replace first match
  • Double //: Replace all matches
  • # prefix: Match at beginning
  • % prefix: Match at end
  • Empty replacement: Delete matches
export stringZ="abcABC123ABCabc"
echo ${stringZ/abc/xyz}       # xyzABC123ABCabc (first match)
echo ${stringZ//abc/xyz}      # xyzABC123ABCxyz (all matches)
echo ${stringZ/#abc/XYZ}      # XYZABC123ABCabc (beginning)
echo ${stringZ/%abc/XYZ}      # abcABC123ABCXYZ (end)

Default Parameters

# ${parameter-default}: Use default if unset
# ${parameter:-default}: Use default if unset or empty

# var is unset
echo ${var-'default'}   # default

# var is set but empty
var=""
echo ${var:-'default'}  # default

Arithmetic Expansion

i=5
echo $((i + 1))    # 6
echo $((i * 2))    # 10

Loops

Reading Files Line by Line

# Process each line of a file
while IFS= read -r line; do
    echo "Processing: $line"
    # Add your commands here
done < "filename.txt"

Looping with Find Results

# Rename files found by find
for file in $(find src -type f -name "*index.less*"); do
    mv "$file" "${file%%index.less}style.less"
done

# Process images
for f in *.png; do
    convert "$f" -resize 300x300 "${f%%.png}-300.png"
done

Processing CSV Files

i=0
for filename in ./data/*.csv; do
    i=$((i + 1))
    echo "Processing $filename... [$i]"
    # Process file here
done

Network Operations

# Test with different IP addresses
for i in {1..8}; do
    curl --header "X-Forwarded-For: 1.2.3.$i" "http://example.com"
done

Conditionals (IF-ELSE)

Basic Syntax

#!/bin/bash
if [ condition ]; then
    # commands
elif [ another_condition ]; then
    # commands
else
    # commands
fi

Test Operations

Operation Description
! EXPRESSION Expression is false
-n STRING String length > 0
-z STRING String length = 0
STRING1 = STRING2 Strings are equal
STRING1 != STRING2 Strings are not equal
int1 -eq int2 Integers are equal
int1 -gt int2 Greater than
int1 -lt int2 Less than
-d FILE File is directory
-e FILE File exists
-f FILE File exists and is regular file
-r FILE File is readable
-w FILE File is writable
-x FILE File is executable
-s FILE File exists and is not empty

Examples

#!/bin/bash
if [ -f "config.txt" ]; then
    echo "Config file exists"
fi

if [ $# -eq 0 ]; then
    echo "No arguments provided"
    exit 1
fi

# Numeric comparison (note: string vs numeric)
test "001" = "1"    # false (string comparison)
test "001" -eq 1    # true (numeric comparison)

Text Processing

Remove ^M Characters in Vim

When working with files that have Windows line endings (CRLF), you may see ^M characters in Vim.

Solution:

  1. Force DOS format reading: 
    :e ++ff=dos

    This tells Vim to read the file again with DOS format, removing CRLF endings.

  2. Set Unix format: 
    :set ff=unix
  3. Save and quit: 
    :wq

Alternative methods:

# Using dos2unix command
dos2unix filename.txt

# Using sed
sed -i 's/\r$//' filename.txt

# Using tr
tr -d '\r' < input.txt > output.txt

Convert CSV to JSON

Basic CSV to JSON Array

# Convert CSV to JSON array of objects
cat data.csv | python -c "
import csv, json, sys
print(json.dumps([dict(r) for r in csv.DictReader(sys.stdin)])
" | jq

Create Key-Value Object from CSV

# Convert CSV to single object using one column as keys
cat nace-es-dedup.csv | python -c "
import csv, json, sys
print(json.dumps([dict(r) for r in csv.DictReader(sys.stdin)]))
" | jq 'map({(.Code): .ES}) | add'

Advanced Processing Examples

# Filter and transform while converting
cat data.csv | python -c "
import csv, json, sys
data = [dict(r) for r in csv.DictReader(sys.stdin)]
filtered = [item for item in data if item.get('status') == 'active']
print(json.dumps(filtered))
" | jq '.[] | select(.price > 100)'

# Convert with custom field mapping
cat inventory.csv | python -c "
import csv, json, sys
reader = csv.DictReader(sys.stdin)
mapped = []
for row in reader:
    mapped.append({
        'id': row['item_id'],
        'name': row['item_name'],
        'cost': float(row['price'])
    })
print(json.dumps(mapped))
" | jq

System Information

Linux

Check Linux distribution and version:

# Method 1: Release information
cat /etc/*-release

# Method 2: System information
uname -a

# Method 3: Kernel version
cat /proc/version

# Method 4: OS info (newer systems)
hostnamectl

# Method 5: LSB information
lsb_release -a

Additional system info commands:

# CPU information
cat /proc/cpuinfo

# Memory information
cat /proc/meminfo
free -h

# Disk usage
df -h
lsblk

# System uptime
uptime

# Kernel modules
lsmod

Android

Check CPU Architecture (32-bit vs 64-bit)

# Primary method
cat /proc/cpuinfo

# Alternative methods
uname -m
getprop ro.product.cpu.abi

Sample output for 64-bit ARM:

Processor : AArch64 Processor rev 4 (aarch64)
processor : 0
processor : 1
processor : 2
processor : 3
Features : fp asimd evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: AArch64
CPU variant : 0x0
CPU part : 0xd03
CPU revision : 4

Hardware : Amlogic
Serial : [redacted]

Android-specific Commands

# Android version
getprop ro.build.version.release

# Device model
getprop ro.product.model

# Build information
getprop ro.build.fingerprint

# ABI information
getprop ro.product.cpu.abilist

# Check if device is rooted
su -c "echo 'Device is rooted'"

Additional Tips

Performance and Efficiency

  • Use find with -exec sparingly on large directories
  • Consider xargs for better performance with multiple files
  • Use parallel for CPU-intensive operations across multiple files
  • Quote variables to prevent word splitting: "$variable"

Safety Practices

  • Always test commands on sample data first
  • Use --dry-run options when available
  • Make backups before bulk operations
  • Use version control for important changes

Debugging

  • Add set -x to bash scripts for debugging
  • Use echo statements to verify variable values
  • Check exit codes with $?
  • Use shellcheck to validate bash scripts