bash shell豆知識

Last updated at 2024-06-27Posted at 2024-06-04

Replace a substring in file
(20, 'm') in file: get_session.sql to (35, 'm'). 20 can be any number.
```
sed -i "s/([0-9]\+,'m');/(35,'m');/" get_session.sql && cat get_session.sql
```

Write multipule lines to a file by command

cat <<EOL > get_session.sql
set linesize 300;
set serveroutput on;
exec session_infos(20,'m');
exit;
EOL

Positional Parameters

#: Description: Positional Parameters - print Filename and the first command-line argument
              
printf "Filename: %s, First param: %s!\n" "$0" "$1"

$ hello Tom
Filename: /home/tom/bin/hello, First param: Tom!

Printing to a Variable

printf -v filename "staff_%03d.csv" 12
echo $filename
# output: staff_012.csv It's the same as:
echo "staff_$(printf '%03d' 12)"

Exit status

The exit code is stored in the special parameter $?. If the command executed successfully (or true), the value of $? is zero.

Test

test -f /etc/abc  ## true if a regular file
[ -x "$HOME/go" ]   ## true if you can execute the file, the spaces are necessary
[ 2 -eq 1 ]  ## test whether integers equal
[ "$q" != "$b" ] ## test whether two strings equal
[ -z "" ]  ## return successfully if their arguments are empty
[ -n "" ]  ## return successfully if their arguments are nonempty
test "$str1" \< "$str2" ## < or > must be escaped to prevent them from being interpreted as redirection operators
[ -e "b.txt" -a "$a" -gt 9 ] ## Check if b.txt exists and a is greater than 9
[ -f b.txt -o -f c.txt ] ## b.txt or c.txt is a regular file
if (( total > max )); then : ...; fi ## (( … )): Evaluate an Arithmetic Expression
mkdir "$HOME/bin" && cd "$HOME/bin" || exit 1  ## create dir and cd. If failed, exit 1

case

## check if the first argument is integer. two ;; are necessary
case $1 in         
    *[!0-9]*) echo false;;   ## if it contains any char except 0-9
    *) echo true;;
esac

while loop

# print n while N <=10
n=1
while [ $n -le 10 ]
do
 echo $n
 n=$(( $n + 1 ))
done

# read from session.txt line by line, print the line if conatains "id"
while IFS= read -r line
do 
    if [[ $line == *"id"* ]]; then
        echo $line
    fi
done < session.txt

for loop

# print odd number while N <=5
for (( n=1; n<=5; ++n)); do
    if ((n%2 == 0));then
        continue
    fi
    echo $n
done

# iterate a list
for fruit in Apple Banana Orange; do
    printf "%s\n" "$fruit"
done

Brace Expansion

# generate filename p001.txt ~ p100.txt
for num in p{001..100}.txt; do 
    echo $num;
done

Arithmetic Expansion

When the shell encounters $(( expression )), it evaluates expression and places the result on the command line; expression is an arithmetic expression.
```
echo "$(( 1 + 1 ))" "$(( 2 * 3 ))" "$(( 16 / 3 ))" "$(( 16%5 ))" "$(( 6 - 9 ))" "$(( (2+3)*4 ))"
# 2 6 5 1 -3 20
```

Command Substitution

Command substitution replaces a command with its output.
New format: $( command )
old format: `command`

 # current date as YYYY-MM-DD
 cur_date=$(date +%Y-%m-%d)
 echo "$cur_date"
 # count the lines in variant txt
 txt=$(cat access.log)
 num=$(echo -n "$txt" | wc -l)  # must quote the %txt and add -n to remove newline by echo
 echo $num
 num=$(printf "%s" "$t" | wc -l)
 echo $num

Process Substitution

Process substitution creates a temporary file name for a command or list of commands. You can use it anywhere a file name is expected. The form <(command) makes the output of command available as a file name;
```
line_count=$(wc -l <(cat hw.txt) | cut -d ' ' -f 1) # first:line number, second: file path
echo $line_count
```
Variables

Variables set in a subshell are not visible to the script that called it. Subshells include command substitution, as in $(command) or `command`; all elements of a pipeline; and code enclosed in parentheses, as in ( command ).

Parameter Expansion

var=
echo "${var:-default_value}"

if a var is unset or empty then expands to default_value.

var=
echo "${var-default_value}"
unset var
echo "${var-default_value}"

if a var is unset then expands to default_value.

echo "${var:+alternate_value}" use alternate only if $var is set and is not empty
echo "${var+alternate_value}" use alternate if $var is set

${var:=default} and ${var=default}: Assign Default Values
The ${var:=default} expansion behaves in the same way as ${var:-default} except that it also assigns the default value to the variable

${var:?message} and ${var?message}: If var is empty or not set, message will be printed to the standard error, and the script will exit with a status of 1.

#!/bin/bash
: ${1?An argument is required}
: ${2?Two arguments are required}
echo "Both arguments are set: \$1 = $1, \$2 = $2"

In Bash, the : character is a built-in command that essentially does nothing and returns a status of 0 (true). It is often used as a placeholder in scripts where a command is syntactically required but no operation is needed.

String operation

# ${#var}: Length of Variable’s Contents
read -p "Please input the password(8 digit or more):" passwd
[ ${#passwd} -lt 8 ] && echo "Password is tool short" && exit 1

# ${var%PATTERN}: Remove the Shortest Match from the End
var=hello
echo "${var%l*}"  # hel
# ${var%%PATTERN}: Remove the Longest Match from the End
echo "${var%%l*}"  # he

# ${var#PATTERN}: Remove the Shortest Match from the Beginning
var=hello
echo "${var#*l}"  # lo
# ${var##PATTERN}: Remove the Longest Match from the Beginning
echo "${var##*l}"  # o

# ${var//PATTERN/STRING}: Replace All Instances of PATTERN with STRING
var=helplok
echo "${var//l?/v}"  # hevvk the question mark matches any single character
# With a single slash, only the first matching character is replaced.
echo "${var/l?/v}" # hevlok

# ${var:OFFSET:LENGTH}: Return a Substring of $var
var=hello
echo "${var:1:2}" # el
echo "${var:1}" # ello
# A negative OFFSET is counted from the end of the string. 
#  If a literal minus sign is used (as opposed to one contained in a variable),
# it must be preceded by a space to prevent it from being interpreted as a default expansion
echo "${var: -2}" # lo
echo "${var: -3: -1}" # ll , end excluded

# ${var^PATTERN}: Convert the first character to Uppercase
var=hello
echo "${var^}" #Hello
# ${var^^PATTERN}: Convert all the matched character to Uppercase
echo "${var^^}" #HELLO
echo "${var^^[a-j]}" # HEllo
# ${var,PATTERN}, ${var,,PATTERN}: Convert to Lowercase
var=HELLO
echo "${var,}" #hELLO
echo "${var,,}" #hello

Arrays

# Create a array
FRUITS=( Apple Banana Orange Grape )
# get the second element
echo "${FRUITS[1]}"
# Assign new value
FRUITS[1]=Melon
echo "${FRUITS[1]}"
# Append new values
FRUITS+=(WaterMelon Pear)
# print all
echo "${FRUITS[@]}"
# print length
echo "${#FRUITS[@]}"
# print 2~3 elements
echo "${FRUITS[@]:1:2}"

# Associative Arrays
declare -A weekMap
weekMap["m"]="Monday"
weekMap["t"]="Tuesday"
echo "${weekMap[t]}"
echo "${weekMap[@]}"

String Manipulation
```
# Concatenation
a="123"
a+="abc"
```

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