Basics

echo & Navigation

echo "Hello, World!"     # print text
pwd                      # print working directory
cd /path/to/dir          # change directory
cd ..                    # go up one level
cd ~                     # go to home directory
cd -                     # go to previous directory

Listing & Creating

ls                # list files
ls -la            # long format, show hidden
ls -lh            # human-readable sizes
mkdir mydir       # create directory
mkdir -p a/b/c    # create nested directories

Copy, Move & Remove

cp file.txt copy.txt      # copy file
cp -r dir/ backup/        # copy directory recursively
mv old.txt new.txt        # rename / move
rm file.txt               # delete file
rm -r dir/                # delete directory recursively
rm -rf dir/               # force delete (no prompt)

Variables & Expansion

Variables

name="Alice"              # assign (no spaces!)
echo "$name"              # variable expansion
echo "${name}_file"       # braces for clarity
readonly PI=3.14          # constant
unset name                # delete variable

Special Variables

$0	Script name
$1 $2 ...	Positional arguments
$#	Number of arguments
$@	All arguments (separate words)
$*	All arguments (single string)
$?	Exit status of last command
$$	Current process ID
$!	PID of last background process

Command Substitution & Arithmetic

files=$(ls)               # capture output
today=$(date +%Y-%m-%d)   # command substitution
count=$((5 + 3))          # arithmetic: 8
echo $((10 / 3))          # integer division: 3
echo $((10 % 3))          # modulo: 1

String Operations

${#str}	String length
${str:0:5}	Substring (offset:length)
${str/old/new}	Replace first match
${str//old/new}	Replace all matches
${str^^}	Uppercase
${str,,}	Lowercase

Conditionals

if / elif / else

if [[ "$name" == "Alice" ]]; then
    echo "Hi Alice"
elif [[ "$name" == "Bob" ]]; then
    echo "Hi Bob"
else
    echo "Who are you?"
fi

Test Operators

-eq -ne	Integer equal / not equal
-lt -gt	Integer less / greater than
-le -ge	Integer less/greater or equal
== !=	String equal / not equal
-z "$str"	String is empty
-n "$str"	String is not empty
-f file	File exists and is regular
-d dir	Directory exists
-e path	Path exists (any type)
-r -w -x	Readable / writable / executable
&& \|\|	Logical AND / OR

Loops

for Loop

for fruit in apple banana cherry; do
    echo "$fruit"
done

for f in *.txt; do
    echo "File: $f"
done

C-style for Loop

for ((i=0; i<5; i++)); do
    echo "$i"
done

while Loop

count=0
while [[ $count -lt 5 ]]; do
    echo "$count"
    ((count++))
done

Loop Control

break	Exit the loop
continue	Skip to next iteration

Functions

Defining & Calling

greet() {
    echo "Hello, $1!"   # $1 = first arg
    return 0             # exit status
}
greet "Alice"            # Hello, Alice!

Local Variables & Return Values

add() {
    local sum=$(($1 + $2))
    echo "$sum"          # "return" via stdout
}
result=$(add 3 5)        # capture: 8

Pipes & Redirection

Pipes

ls -l | grep ".txt"      # pipe output
cat log | sort | uniq    # chain commands
cmd1 | tee out.txt       # pipe + save to file

Redirection

cmd > file	Redirect stdout (overwrite)
cmd >> file	Redirect stdout (append)
cmd < file	Redirect stdin from file
cmd 2> file	Redirect stderr
cmd 2>&1	Redirect stderr to stdout
cmd &> file	Redirect stdout + stderr
cmd << EOF	Here document (inline input)
/dev/null	Discard output: `cmd > /dev/null`

File Operations

Viewing Files

cat file.txt             # print entire file
head -n 10 file.txt      # first 10 lines
tail -n 10 file.txt      # last 10 lines
tail -f log.txt          # follow (live updates)
less file.txt            # paginated viewer

Counting & Finding

wc -l file.txt           # count lines
wc -w file.txt           # count words
wc -c file.txt           # count bytes
find . -name "*.txt"     # find by name
find . -type d           # find directories
find . -mtime -7         # modified in last 7 days

Other File Commands

touch file	Create file / update timestamp
stat file	File metadata (size, dates)
file img.png	Detect file type
diff a.txt b.txt	Compare two files
sort file.txt	Sort lines
uniq	Remove adjacent duplicates
cut -d: -f1	Extract fields by delimiter
tr 'a-z' 'A-Z'	Translate / replace characters

Text Processing

grep

grep "error" log.txt       # search for pattern
grep -i "error" log.txt    # case-insensitive
grep -r "TODO" src/        # recursive search
grep -n "func" file.go     # show line numbers
grep -c "error" log.txt    # count matches
grep -v "debug" log.txt    # invert match

sed

sed 's/old/new/' file      # replace first per line
sed 's/old/new/g' file     # replace all
sed -i 's/old/new/g' file  # edit in place
sed -n '5,10p' file        # print lines 5-10
sed '/pattern/d' file      # delete matching lines

awk

awk '{print $1}' file      # print first field
awk -F: '{print $1}' file  # custom delimiter
awk '$3 > 100' file        # filter by field value
awk '{sum+=$1} END{print sum}' file  # sum column

Permissions

chmod

chmod 755 script.sh    # rwxr-xr-x
chmod +x script.sh     # add execute
chmod -w file.txt      # remove write
chmod u+x,g-w file     # user +exec, group -write

Permission Reference

r (4)	Read
w (2)	Write
x (1)	Execute
u / g / o	User / Group / Others
755	Owner: rwx, Group/Other: r-x
644	Owner: rw-, Group/Other: r--

Ownership

chown user file.txt         # change owner
chown user:group file.txt   # change owner + group
chown -R user:group dir/    # recursive

BASH QUICK REFERENCE

BASH QUICK REFERENCE (continued)