mirror of
https://github.com/onkelbeh/cheatsheets.git
synced 2025-06-15 14:47:53 +02:00
bash.md: update with best practices (#1897)
This commit is contained in:
tripleee
GitHub
parent
6999e947cb
commit
271e0b7c5d
197
bash.md
197
bash.md
@ -32,32 +32,39 @@ This is a quick reference to getting started with Bash scripting.
|
||||
```bash
|
||||
#!/usr/bin/env bash
|
||||
|
||||
NAME="John"
|
||||
echo "Hello $NAME!"
|
||||
name="John"
|
||||
echo "Hello $name!"
|
||||
```
|
||||
|
||||
### Variables
|
||||
|
||||
```bash
|
||||
NAME="John"
|
||||
echo $NAME
|
||||
echo "$NAME"
|
||||
echo "${NAME}!"
|
||||
name="John"
|
||||
echo $name # see below
|
||||
echo "$name"
|
||||
echo "${name}!"
|
||||
```
|
||||
Generally quote your variables unless they contain wildcards to expand or command fragments.
|
||||
|
||||
```bash
|
||||
wildcard="*.txt"
|
||||
option="iv"
|
||||
cp -$options $wildcard /tmp
|
||||
```
|
||||
|
||||
### String quotes
|
||||
|
||||
```bash
|
||||
NAME="John"
|
||||
echo "Hi $NAME" #=> Hi John
|
||||
echo 'Hi $NAME' #=> Hi $NAME
|
||||
name="John"
|
||||
echo "Hi $name" #=> Hi John
|
||||
echo 'Hi $name' #=> Hi $name
|
||||
```
|
||||
|
||||
### Shell execution
|
||||
|
||||
```bash
|
||||
echo "I'm in $(pwd)"
|
||||
echo "I'm in `pwd`"
|
||||
echo "I'm in `pwd`" # obsolescent
|
||||
# Same
|
||||
```
|
||||
|
||||
@ -128,65 +135,65 @@ Parameter expansions
|
||||
|
||||
```bash
|
||||
name="John"
|
||||
echo ${name}
|
||||
echo ${name/J/j} #=> "john" (substitution)
|
||||
echo ${name:0:2} #=> "Jo" (slicing)
|
||||
echo ${name::2} #=> "Jo" (slicing)
|
||||
echo ${name::-1} #=> "Joh" (slicing)
|
||||
echo ${name:(-1)} #=> "n" (slicing from right)
|
||||
echo ${name:(-2):1} #=> "h" (slicing from right)
|
||||
echo ${food:-Cake} #=> $food or "Cake"
|
||||
echo "${name}"
|
||||
echo "${name/J/j}" #=> "john" (substitution)
|
||||
echo "${name:0:2}" #=> "Jo" (slicing)
|
||||
echo "${name::2}" #=> "Jo" (slicing)
|
||||
echo "${name::-1}" #=> "Joh" (slicing)
|
||||
echo "${name:(-1)}" #=> "n" (slicing from right)
|
||||
echo "${name:(-2):1}" #=> "h" (slicing from right)
|
||||
echo "${food:-Cake}" #=> $food or "Cake"
|
||||
```
|
||||
|
||||
```bash
|
||||
length=2
|
||||
echo ${name:0:length} #=> "Jo"
|
||||
echo "${name:0:length}" #=> "Jo"
|
||||
```
|
||||
|
||||
See: [Parameter expansion](http://wiki.bash-hackers.org/syntax/pe)
|
||||
|
||||
```bash
|
||||
STR="/path/to/foo.cpp"
|
||||
echo ${STR%.cpp} # /path/to/foo
|
||||
echo ${STR%.cpp}.o # /path/to/foo.o
|
||||
echo ${STR%/*} # /path/to
|
||||
str="/path/to/foo.cpp"
|
||||
echo "${str%.cpp}" # /path/to/foo
|
||||
echo "${str%.cpp}.o" # /path/to/foo.o
|
||||
echo "${str%/*}" # /path/to
|
||||
|
||||
echo ${STR##*.} # cpp (extension)
|
||||
echo ${STR##*/} # foo.cpp (basepath)
|
||||
echo "${str##*.}" # cpp (extension)
|
||||
echo "${str##*/}" # foo.cpp (basepath)
|
||||
|
||||
echo ${STR#*/} # path/to/foo.cpp
|
||||
echo ${STR##*/} # foo.cpp
|
||||
echo "${str#*/}" # path/to/foo.cpp
|
||||
echo "${str##*/}" # foo.cpp
|
||||
|
||||
echo ${STR/foo/bar} # /path/to/bar.cpp
|
||||
echo "${str/foo/bar}" # /path/to/bar.cpp
|
||||
```
|
||||
|
||||
```bash
|
||||
STR="Hello world"
|
||||
echo ${STR:6:5} # "world"
|
||||
echo ${STR: -5:5} # "world"
|
||||
str="Hello world"
|
||||
echo "${str:6:5}" # "world"
|
||||
echo "${str: -5:5}" # "world"
|
||||
```
|
||||
|
||||
```bash
|
||||
SRC="/path/to/foo.cpp"
|
||||
BASE=${SRC##*/} #=> "foo.cpp" (basepath)
|
||||
DIR=${SRC%$BASE} #=> "/path/to/" (dirpath)
|
||||
src="/path/to/foo.cpp"
|
||||
base=${src##*/} #=> "foo.cpp" (basepath)
|
||||
dir=${src%$base} #=> "/path/to/" (dirpath)
|
||||
```
|
||||
|
||||
### Substitution
|
||||
|
||||
| Code | Description |
|
||||
| ----------------- | ------------------- |
|
||||
| `${FOO%suffix}` | Remove suffix |
|
||||
| `${FOO#prefix}` | Remove prefix |
|
||||
| `${foo%suffix}` | Remove suffix |
|
||||
| `${foo#prefix}` | Remove prefix |
|
||||
| --- | --- |
|
||||
| `${FOO%%suffix}` | Remove long suffix |
|
||||
| `${FOO##prefix}` | Remove long prefix |
|
||||
| `${foo%%suffix}` | Remove long suffix |
|
||||
| `${foo##prefix}` | Remove long prefix |
|
||||
| --- | --- |
|
||||
| `${FOO/from/to}` | Replace first match |
|
||||
| `${FOO//from/to}` | Replace all |
|
||||
| `${foo/from/to}` | Replace first match |
|
||||
| `${foo//from/to}` | Replace all |
|
||||
| --- | --- |
|
||||
| `${FOO/%from/to}` | Replace suffix |
|
||||
| `${FOO/#from/to}` | Replace prefix |
|
||||
| `${foo/%from/to}` | Replace suffix |
|
||||
| `${foo/#from/to}` | Replace prefix |
|
||||
|
||||
### Comments
|
||||
|
||||
@ -206,37 +213,37 @@ comment
|
||||
|
||||
| Expression | Description |
|
||||
| --------------- | ------------------------------ |
|
||||
| `${FOO:0:3}` | Substring _(position, length)_ |
|
||||
| `${FOO:(-3):3}` | Substring from the right |
|
||||
| `${foo:0:3}` | Substring _(position, length)_ |
|
||||
| `${foo:(-3):3}` | Substring from the right |
|
||||
|
||||
### Length
|
||||
|
||||
| Expression | Description |
|
||||
| ---------- | ---------------- |
|
||||
| `${#FOO}` | Length of `$FOO` |
|
||||
| `${#foo}` | Length of `$foo` |
|
||||
|
||||
### Manipulation
|
||||
|
||||
```bash
|
||||
STR="HELLO WORLD!"
|
||||
echo ${STR,} #=> "hELLO WORLD!" (lowercase 1st letter)
|
||||
echo ${STR,,} #=> "hello world!" (all lowercase)
|
||||
str="HELLO WORLD!"
|
||||
echo "${str,}" #=> "hELLO WORLD!" (lowercase 1st letter)
|
||||
echo "${str,,}" #=> "hello world!" (all lowercase)
|
||||
|
||||
STR="hello world!"
|
||||
echo ${STR^} #=> "Hello world!" (uppercase 1st letter)
|
||||
echo ${STR^^} #=> "HELLO WORLD!" (all uppercase)
|
||||
str="hello world!"
|
||||
echo "${str^}" #=> "Hello world!" (uppercase 1st letter)
|
||||
echo "${str^^}" #=> "HELLO WORLD!" (all uppercase)
|
||||
```
|
||||
|
||||
### Default values
|
||||
|
||||
| Expression | Description |
|
||||
| ----------------- | -------------------------------------------------------- |
|
||||
| `${FOO:-val}` | `$FOO`, or `val` if unset (or null) |
|
||||
| `${FOO:=val}` | Set `$FOO` to `val` if unset (or null) |
|
||||
| `${FOO:+val}` | `val` if `$FOO` is set (and not null) |
|
||||
| `${FOO:?message}` | Show error message and exit if `$FOO` is unset (or null) |
|
||||
| `${foo:-val}` | `$foo`, or `val` if unset (or null) |
|
||||
| `${foo:=val}` | Set `$foo` to `val` if unset (or null) |
|
||||
| `${foo:+val}` | `val` if `$foo` is set (and not null) |
|
||||
| `${foo:?message}` | Show error message and exit if `$foo` is unset (or null) |
|
||||
|
||||
Omitting the `:` removes the (non)nullity checks, e.g. `${FOO-val}` expands to `val` if unset otherwise `$FOO`.
|
||||
Omitting the `:` removes the (non)nullity checks, e.g. `${foo-val}` expands to `val` if unset otherwise `$foo`.
|
||||
|
||||
Loops
|
||||
-----
|
||||
@ -246,7 +253,7 @@ Loops
|
||||
|
||||
```bash
|
||||
for i in /etc/rc.*; do
|
||||
echo $i
|
||||
echo "$i"
|
||||
done
|
||||
```
|
||||
|
||||
@ -254,7 +261,7 @@ done
|
||||
|
||||
```bash
|
||||
for ((i = 0 ; i < 100 ; i++)); do
|
||||
echo $i
|
||||
echo "$i"
|
||||
done
|
||||
```
|
||||
|
||||
@ -277,9 +284,9 @@ done
|
||||
### Reading lines
|
||||
|
||||
```bash
|
||||
cat file.txt | while read line; do
|
||||
echo $line
|
||||
done
|
||||
while read -r line; do
|
||||
echo "$line"
|
||||
done <file.txt
|
||||
```
|
||||
|
||||
### Forever
|
||||
@ -318,12 +325,12 @@ myfunc "John"
|
||||
```bash
|
||||
myfunc() {
|
||||
local myresult='some value'
|
||||
echo $myresult
|
||||
echo "$myresult"
|
||||
}
|
||||
```
|
||||
|
||||
```bash
|
||||
result="$(myfunc)"
|
||||
result=$(myfunc)
|
||||
```
|
||||
|
||||
### Raising errors
|
||||
@ -470,14 +477,14 @@ Fruits[2]="Orange"
|
||||
### Working with arrays
|
||||
|
||||
```bash
|
||||
echo ${Fruits[0]} # Element #0
|
||||
echo ${Fruits[-1]} # Last element
|
||||
echo ${Fruits[@]} # All elements, space-separated
|
||||
echo ${#Fruits[@]} # Number of elements
|
||||
echo ${#Fruits} # String length of the 1st element
|
||||
echo ${#Fruits[3]} # String length of the Nth element
|
||||
echo ${Fruits[@]:3:2} # Range (from position 3, length 2)
|
||||
echo ${!Fruits[@]} # Keys of all elements, space-separated
|
||||
echo "${Fruits[0]}" # Element #0
|
||||
echo "${Fruits[-1]}" # Last element
|
||||
echo "${Fruits[@]}" # All elements, space-separated
|
||||
echo "${#Fruits[@]}" # Number of elements
|
||||
echo "${#Fruits}" # String length of the 1st element
|
||||
echo "${#Fruits[3]}" # String length of the Nth element
|
||||
echo "${Fruits[@]:3:2}" # Range (from position 3, length 2)
|
||||
echo "${!Fruits[@]}" # Keys of all elements, space-separated
|
||||
```
|
||||
|
||||
### Operations
|
||||
@ -485,7 +492,7 @@ echo ${!Fruits[@]} # Keys of all elements, space-separated
|
||||
```bash
|
||||
Fruits=("${Fruits[@]}" "Watermelon") # Push
|
||||
Fruits+=('Watermelon') # Also Push
|
||||
Fruits=( ${Fruits[@]/Ap*/} ) # Remove by regex match
|
||||
Fruits=( "${Fruits[@]/Ap*/}" ) # Remove by regex match
|
||||
unset Fruits[2] # Remove one item
|
||||
Fruits=("${Fruits[@]}") # Duplicate
|
||||
Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate
|
||||
@ -496,7 +503,7 @@ lines=(`cat "logfile"`) # Read from file
|
||||
|
||||
```bash
|
||||
for i in "${arrayName[@]}"; do
|
||||
echo $i
|
||||
echo "$i"
|
||||
done
|
||||
```
|
||||
|
||||
@ -522,11 +529,11 @@ Declares `sound` as a Dictionary object (aka associative array).
|
||||
### Working with dictionaries
|
||||
|
||||
```bash
|
||||
echo ${sounds[dog]} # Dog's sound
|
||||
echo ${sounds[@]} # All values
|
||||
echo ${!sounds[@]} # All keys
|
||||
echo ${#sounds[@]} # Number of elements
|
||||
unset sounds[dog] # Delete dog
|
||||
echo "${sounds[dog]}" # Dog's sound
|
||||
echo "${sounds[@]}" # All values
|
||||
echo "${!sounds[@]}" # All keys
|
||||
echo "${#sounds[@]}" # Number of elements
|
||||
unset sounds[dog] # Delete dog
|
||||
```
|
||||
|
||||
### Iteration
|
||||
@ -535,7 +542,7 @@ unset sounds[dog] # Delete dog
|
||||
|
||||
```bash
|
||||
for val in "${sounds[@]}"; do
|
||||
echo $val
|
||||
echo "$val"
|
||||
done
|
||||
```
|
||||
|
||||
@ -543,7 +550,7 @@ done
|
||||
|
||||
```bash
|
||||
for key in "${!sounds[@]}"; do
|
||||
echo $key
|
||||
echo "$key"
|
||||
done
|
||||
```
|
||||
|
||||
@ -640,12 +647,14 @@ pwd # still in first directory
|
||||
### Redirection
|
||||
|
||||
```bash
|
||||
python hello.py > output.txt # stdout to (file)
|
||||
python hello.py >> output.txt # stdout to (file), append
|
||||
python hello.py 2> error.log # stderr to (file)
|
||||
python hello.py 2>&1 # stderr to stdout
|
||||
python hello.py 2>/dev/null # stderr to (null)
|
||||
python hello.py &>/dev/null # stdout and stderr to (null)
|
||||
python hello.py > output.txt # stdout to (file)
|
||||
python hello.py >> output.txt # stdout to (file), append
|
||||
python hello.py 2> error.log # stderr to (file)
|
||||
python hello.py 2>&1 # stderr to stdout
|
||||
python hello.py 2>/dev/null # stderr to (null)
|
||||
python hello.py >output.txt 2>&1 # stdout and stderr to (file), equivalent to &>
|
||||
python hello.py &>/dev/null # stdout and stderr to (null)
|
||||
echo "$0: warning: too many users" >&2 # print diagnostic message to stderr
|
||||
```
|
||||
|
||||
```bash
|
||||
@ -708,6 +717,10 @@ printf "1 + 1 = %d" 2
|
||||
|
||||
printf "This is how you print a float: %f" 2
|
||||
#=> "This is how you print a float: 2.000000"
|
||||
|
||||
printf '%s\n' '#!/bin/bash' 'echo hello' >file
|
||||
# format string is applied to each group of arguments
|
||||
printf '%i+%i=%i\n' 1 2 3 4 5 9
|
||||
```
|
||||
|
||||
### Transform strings
|
||||
@ -728,14 +741,14 @@ printf "This is how you print a float: %f" 2
|
||||
#### Example
|
||||
|
||||
```bash
|
||||
echo "Welcome To Devhints" | tr [:lower:] [:upper:]
|
||||
echo "Welcome To Devhints" | tr '[:lower:]' '[:upper:]'
|
||||
WELCOME TO DEVHINTS
|
||||
```
|
||||
|
||||
### Directory of script
|
||||
|
||||
```bash
|
||||
DIR="${0%/*}"
|
||||
dir=${0%/*}
|
||||
```
|
||||
|
||||
### Getting options
|
||||
@ -743,7 +756,7 @@ DIR="${0%/*}"
|
||||
```bash
|
||||
while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in
|
||||
-V | --version )
|
||||
echo $version
|
||||
echo "$version"
|
||||
exit
|
||||
;;
|
||||
-s | --string )
|
||||
@ -768,10 +781,12 @@ END
|
||||
|
||||
```bash
|
||||
echo -n "Proceed? [y/n]: "
|
||||
read ans
|
||||
echo $ans
|
||||
read -r ans
|
||||
echo "$ans"
|
||||
```
|
||||
|
||||
The `-r` option disables a peculiar legacy behavior with backslashes.
|
||||
|
||||
```bash
|
||||
read -n 1 ans # Just one character
|
||||
```
|
||||
|
||||
Loading…
x
Reference in New Issue
Block a user