category: CLI
layout: 2017/sheet
tags: [Featured]
-updated: 2019-10-02
+updated: 2020-07-05
keywords:
- Variables
- Functions
---------------
{: .-three-column}
+### Introduction
+{: .-intro}
+
+This is a quick reference to getting started with Bash scripting.
+
+- [Learn bash in y minutes](https://learnxinyminutes.com/docs/bash/) _(learnxinyminutes.com)_
+- [Bash Guide](http://mywiki.wooledge.org/BashGuide) _(mywiki.wooledge.org)_
+
### Example
```bash
echo {A,B}.js
```
-| `{A,B}` | Same as `A B` |
+| Expression | Description |
+| ---------- | ------------------- |
+| `{A,B}` | Same as `A B` |
| `{A,B}.js` | Same as `A.js B.js` |
-| `{1..5}` | Same as `1 2 3 4 5` |
+| `{1..5}` | Same as `1 2 3 4 5` |
See: [Brace expansion](http://wiki.bash-hackers.org/syntax/expansion/brace)
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)
```bash
STR="Hello world"
echo ${STR:6:5} # "world"
-echo ${STR:-5:5} # "world"
+echo ${STR: -5:5} # "world"
```
```bash
### Substitution
-| Code | Description |
-| --- | --- |
-| `${FOO%suffix}` | Remove suffix |
-| `${FOO#prefix}` | Remove prefix |
-| --- | --- |
-| `${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 suffix |
-| `${FOO/#from/to}` | Replace prefix |
+| Code | Description |
+| ----------------- | ------------------- |
+| `${FOO%suffix}` | Remove suffix |
+| `${FOO#prefix}` | Remove prefix |
+| --- | --- |
+| `${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 suffix |
+| `${FOO/#from/to}` | Replace prefix |
### Comments
### Substrings
-| `${FOO:0:3}` | Substring _(position, length)_ |
-| `${FOO:-3:3}` | Substring from the right |
+| Expression | Description |
+| --------------- | ------------------------------ |
+| `${FOO:0:3}` | Substring _(position, length)_ |
+| `${FOO:(-3):3}` | Substring from the right |
### Length
-| `${#FOO}` | Length of `$FOO` |
+| Expression | Description |
+| ---------- | ---------------- |
+| `${#FOO}` | Length of `$FOO` |
### Manipulation
echo ${STR^^} #=> "HELLO WORLD!" (all uppercase)
```
-
### Default values
-| `${FOO:-val}` | `$FOO`, or `val` if not set |
-| `${FOO:=val}` | Set `$FOO` to `val` if not set |
-| `${FOO:+val}` | `val` if `$FOO` is set |
-| `${FOO:?message}` | Show error message and exit if `$FOO` is not set |
+| Expression | Description |
+| ----------------- | -------------------------------------------------------- |
+| `${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 `$FOO` is unset (or null) |
-The `:` is optional (eg, `${FOO=word}` works)
+Omitting the `:` removes the (non)nullity checks, e.g. `${FOO-val}` expands to `val` if unset otherwise `$FOO`.
Loops
-----
### Reading lines
```bash
-< file.txt | while read line; do
+cat file.txt | while read line; do
echo $line
done
```
### Arguments
-| Expression | Description |
-| --- | --- |
-| `$#` | Number of arguments |
-| `$*` | All arguments |
-| `$@` | All arguments, starting from first |
-| `$1` | First argument |
+| Expression | Description |
+| --- | --- |
+| `$#` | Number of arguments |
+| `$*` | All positional arguments (as a single word) |
+| `$@` | All positional arguments (as separate strings) |
+| `$1` | First argument |
+| `$_` | Last argument of the previous command |
+
+**Note**: `$@` and `$*` must be quoted in order to perform as described.
+Otherwise, they do exactly the same thing (arguments as separate strings).
See [Special parameters](http://wiki.bash-hackers.org/syntax/shellvars#special_parameters_and_shell_variables).
| --- | --- |
| `(( NUM < NUM ))` | Numeric conditions |
-| Condition | Description |
-| --- | --- |
-| `[[ -o noclobber ]]` | If OPTIONNAME is enabled |
-| --- | --- |
-| `[[ ! EXPR ]]` | Not |
-| `[[ X ]] && [[ Y ]]` | And |
-| `[[ X ]] || [[ Y ]]` | Or |
+#### More conditions
+
+| Condition | Description |
+| -------------------- | ------------------------ |
+| `[[ -o noclobber ]]` | If OPTIONNAME is enabled |
+| --- | --- |
+| `[[ ! EXPR ]]` | Not |
+| `[[ X && Y ]]` | And |
+| `[[ X || Y ]]` | Or |
### File conditions
echo "String is empty"
elif [[ -n "$string" ]]; then
echo "String is not empty"
+else
+ echo "This never happens"
fi
```
```bash
# Combinations
-if [[ X ]] && [[ Y ]]; then
+if [[ X && Y ]]; then
...
fi
```
```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 ${!Fruits[@]} # Keys of all elements, space-separated
```
### Operations
### Commands
-| `history` | Show history |
+| Command | Description |
+| --------------------- | ----------------------------------------- |
+| `history` | Show history |
| `shopt -s histverify` | Don't execute expanded result immediately |
### Expansions
-| `!$` | Expand last parameter of most recent command |
-| `!*` | Expand all parameters of most recent command |
-| `!-n` | Expand `n`th most recent command |
-| `!n` | Expand `n`th command in history |
+| Expression | Description |
+| ------------ | ---------------------------------------------------- |
+| `!$` | Expand last parameter of most recent command |
+| `!*` | Expand all parameters of most recent command |
+| `!-n` | Expand `n`th most recent command |
+| `!n` | Expand `n`th command in history |
| `!<command>` | Expand most recent invocation of command `<command>` |
### Operations
-| `!!` | Execute last command again |
-| `!!:s/<FROM>/<TO>/` | Replace first occurrence of `<FROM>` to `<TO>` in most recent command |
-| `!!:gs/<FROM>/<TO>/` | Replace all occurrences of `<FROM>` to `<TO>` in most recent command |
-| `!$:t` | Expand only basename from last parameter of most recent command |
-| `!$:h` | Expand only directory from last parameter of most recent command |
+| Code | Description |
+| -------------------- | --------------------------------------------------------------------- |
+| `!!` | Execute last command again |
+| `!!:s/<FROM>/<TO>/` | Replace first occurrence of `<FROM>` to `<TO>` in most recent command |
+| `!!:gs/<FROM>/<TO>/` | Replace all occurrences of `<FROM>` to `<TO>` in most recent command |
+| `!$:t` | Expand only basename from last parameter of most recent command |
+| `!$:h` | Expand only directory from last parameter of most recent command |
`!!` and `!$` can be replaced with any valid expansion.
### Slices
-| `!!:n` | Expand only `n`th token from most recent command (command is `0`; first argument is `1`) |
-| `!^` | Expand first argument from most recent command |
-| `!$` | Expand last token from most recent command |
-| `!!:n-m` | Expand range of tokens from most recent command |
-| `!!:n-$` | Expand `n`th token to last from most recent command |
+| Code | Description |
+| -------- | ---------------------------------------------------------------------------------------- |
+| `!!:n` | Expand only `n`th token from most recent command (command is `0`; first argument is `1`) |
+| `!^` | Expand first argument from most recent command |
+| `!$` | Expand last token from most recent command |
+| `!!:n-m` | Expand range of tokens from most recent command |
+| `!!:n-$` | Expand `n`th token to last from most recent command |
`!!` can be replaced with any valid expansion i.e. `!cat`, `!-2`, `!42`, etc.
```
```bash
-$((RANDOM%=200)) # Random number 0..200
+$(($RANDOM%200)) # Random number 0..199
```
### Subshells
```bash
python hello.py < foo.txt # feed foo.txt to stdin for python
+diff <(ls -r) <(ls) # Compare two stdout without files
```
### Inspecting commands
#=> "This is how you print a float: 2.000000"
```
+### Transform strings
+
+| Command option | Description |
+| ------------------ | --------------------------------------------------- |
+| `-c` | Operations apply to characters not in the given set |
+| `-d` | Delete characters |
+| `-s` | Replaces repeated characters with single occurrence |
+| `-t` | Truncates |
+| `[:upper:]` | All upper case letters |
+| `[:lower:]` | All lower case letters |
+| `[:digit:]` | All digits |
+| `[:space:]` | All whitespace |
+| `[:alpha:]` | All letters |
+| `[:alnum:]` | All letters and digits |
+
+#### Example
+
+```bash
+echo "Welcome To Devhints" | tr [:lower:] [:upper:]
+WELCOME TO DEVHINTS
+```
+
### Directory of script
```bash
### Special variables
-| `$?` | Exit status of last task |
-| `$!` | PID of last background task |
-| `$$` | PID of shell |
-| `$0` | Filename of the shell script |
+| Expression | Description |
+| ---------- | -------------------------------------- |
+| `$?` | Exit status of last task |
+| `$!` | PID of last background task |
+| `$$` | PID of shell |
+| `$0` | Filename of the shell script |
+| `$_` | Last argument of the previous command |
See [Special parameters](http://wiki.bash-hackers.org/syntax/shellvars#special_parameters_and_shell_variables).
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