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chapter19.txt
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chapter19.txt
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# OTHER
## Use `read` as an alternative to the `sleep` command
Surprisingly, `sleep` is an external command and not a `bash` built-in.
**CAVEAT:** Requires `bash` 4+
**Example Function:**
```sh
read_sleep() {
# Usage: read_sleep 1
# read_sleep 0.2
read -rt "$1" <> <(:) || :
}
```
**Example Usage:**
```shell
read_sleep 1
read_sleep 0.1
read_sleep 30
```
For performance-critical situations, where it is not economic to open and close an excessive number of file descriptors, the allocation of a file descriptor may be done only once for all invocations of `read`:
(See the generic original implementation at https://blog.dhampir.no/content/sleeping-without-a-subprocess-in-bash-and-how-to-sleep-forever)
```shell
exec {sleep_fd}<> <(:)
while some_quick_test; do
# equivalent of sleep 0.001
read -t 0.001 -u $sleep_fd
done
```
## Check if a program is in the user's PATH
```shell
# There are 3 ways to do this and either one can be used.
type -p executable_name &>/dev/null
hash executable_name &>/dev/null
command -v executable_name &>/dev/null
# As a test.
if type -p executable_name &>/dev/null; then
# Program is in PATH.
fi
# Inverse.
if ! type -p executable_name &>/dev/null; then
# Program is not in PATH.
fi
# Example (Exit early if program is not installed).
if ! type -p convert &>/dev/null; then
printf '%s\n' "error: convert is not installed, exiting..."
exit 1
fi
```
## Get the current date using `strftime`
Bash鈥檚 `printf` has a built-in method of getting the date which can be used in place of the `date` command.
**CAVEAT:** Requires `bash` 4+
**Example Function:**
```sh
date() {
# Usage: date "format"
# See: 'man strftime' for format.
printf "%($1)T\\n" "-1"
}
```
**Example Usage:**
```shell
# Using above function.
$ date "%a %d %b - %l:%M %p"
Fri 15 Jun - 10:00 AM
# Using printf directly.
$ printf '%(%a %d %b - %l:%M %p)T\n' "-1"
Fri 15 Jun - 10:00 AM
# Assigning a variable using printf.
$ printf -v date '%(%a %d %b - %l:%M %p)T\n' '-1'
$ printf '%s\n' "$date"
Fri 15 Jun - 10:00 AM
```
## Get the username of the current user
**CAVEAT:** Requires `bash` 4.4+
```shell
$ : \\u
# Expand the parameter as if it were a prompt string.
$ printf '%s\n' "${_@P}"
black
```
## Generate a UUID V4
**CAVEAT**: The generated value is not cryptographically secure.
**Example Function:**
```sh
uuid() {
# Usage: uuid
C="89ab"
for ((N=0;N<16;++N)); do
B="$((RANDOM%256))"
case "$N" in
6) printf '4%x' "$((B%16))" ;;
8) printf '%c%x' "${C:$RANDOM%${#C}:1}" "$((B%16))" ;;
3|5|7|9)
printf '%02x-' "$B"
;;
*)
printf '%02x' "$B"
;;
esac
done
printf '\n'
}
```
**Example Usage:**
```shell
$ uuid
d5b6c731-1310-4c24-9fe3-55d556d44374
```
## Progress bars
This is a simple way of drawing progress bars without needing a for loop
in the function itself.
**Example Function:**
```sh
bar() {
# Usage: bar 1 10
# ^----- Elapsed Percentage (0-100).
# ^-- Total length in chars.
((elapsed=$1*$2/100))
# Create the bar with spaces.
printf -v prog "%${elapsed}s"
printf -v total "%$(($2-elapsed))s"
printf '%s\r' "[${prog// /-}${total}]"
}
```
**Example Usage:**
```shell
for ((i=0;i<=100;i++)); do
# Pure bash micro sleeps (for the example).
(:;:) && (:;:) && (:;:) && (:;:) && (:;:)
# Print the bar.
bar "$i" "10"
done
printf '\n'
```
## Get the list of functions in a script
```sh
get_functions() {
# Usage: get_functions
IFS=$'\n' read -d "" -ra functions < <(declare -F)
printf '%s\n' "${functions[@]//declare -f }"
}
```
## Bypass shell aliases
```shell
# alias
ls
# command
# shellcheck disable=SC1001
\ls
```
## Bypass shell functions
```shell
# function
ls
# command
command ls
```
## Run a command in the background
This will run the given command and keep it running, even after the terminal or SSH connection is terminated. All output is ignored.
```sh
bkr() {
(nohup "$@" &>/dev/null &)
}
bkr ./some_script.sh # some_script.sh is now running in the background
```
<!-- CHAPTER END -->