Basic Syntax

Shebang#

#!/usr/bin/env bash

Variable Assignment#

No spaces are allowed around the = sign.

name=foo

Double Quotes#

Can interpret variables.

name=foo
echo "hello, $name" # hello, foo
echo "hello, \$\$"  # hello, $$
echo "hello, \"\""  # hello, ""

Single Quotes#

Do not interpret variables, except for single quotes as special characters.

name=foo
echo 'hello, $name' # hello, $name
echo 'hello, $$'    # hello, $$
echo 'hello, ""'    # hello, ""

To output single quotes, you need to close single quotes first.

echo 'I'\''m fine'  # I'm fine

Command Substitution#

There are two ways to use backticks and $().

result=`date`

result=$(date)

It is recommended to use $(), which is more readable, with more examples as follows:

echo "dir is: $(pwd)"

count=$(ls $(pwd) | wc -l)

Here, ls did not use the -l option, but the value of count is 4, the reason is here.

Output Redirection#

Standard output redirection >, creates a new file or overwrites an existing one.

cmd > file

Append output redirection >>.

cmd >> file

Standard error redirection 2>, creates a new file or overwrites an existing one.

cmd 2> file

Standard output and error redirection to different files.

cmd > foo.log 2> bar.log

Standard output and error redirection to the same file.

ls 404 > foobar.log 2>&1

Input Redirection#

Common way, using < symbol.

echo "a b c" > foo

wc < foo

1 3 6

Inline redirection, Inline Input Redirection.

wc << FOO
apple
banana
cat
FOO

3       3      17

FOO is a custom marker, used for multi-line input.

EXPR Command#

The inhumane math operation command, spaces around the + sign cannot be less.

expr 2 + 5

The * sign is a wildcard, and it needs to be escaped.

expr 2 \* 5

Only integer division is supported, no floating-point numbers.

expr 24 / 10

Brackets#

You can use [] to perform mathematical operations.

var1=$[1+5*2]
var2=$[2*(3+2)]

BC Calculator#

A precise math operation calculator, full name Basic/Bench Calculator.

Interactive Mode#

bc

Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006, 2008, 2012-2017 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.

4*2.5
10.0
quit

The -q option does not print the above English welcome message.

bc -q

Floating Point#

You can directly use floating-point numbers for calculations.

2.5*5
12.5

Division uses the scale variable, default value is 0, indicating integer division.

scale=2
10/3

Indicates to keep 2 decimal places, scale variable only applies to division.

Pipeline Method#

foo=$(echo "scale=2; 10/3" | bc)

Inline Input Redirection#

var1=10.24

foo=$(bc << EOF
scale=2
a=2
$var1/a
EOF
)

EXIT Command#

The default exit code of the script is 0, indicating normal exit, and can be changed using exit.

exit 5

The exit code range is 0-255, mod (%256).