bash manual

Albeit some commands will work on the command line, they do not work in a script
Example:

sed --quiet '/searchword/!p' /home/user/somefile.txt > /home/user/outfile.txt

You can check your code with shellcheck

Generate a beep: printf '\a'

The value of $- after

1. a command is exexuted by the user in a terminal on the command line is himBHs
2. a script is exexuted by the user in a terminal on the command line is hB
3. a command is exexuted by in a user owned cron job is hB
4. a script is exexuted by in a user owned cron job is hB

These settings are set via the bash set command or by bash itself

Variables
You can use variable names consisting of variables by using the eval statement or an array
See How can I use variable variables (indirect variables, pointers, references) or associative arrays?

RSYNC=/usr/bin/rsync
${RSYNC} -a source destination
 
NAME=somename
echo $(echo $NAME         | wc -c)
9
echo $(echo $(echo $NAME) | wc -c)
9
echo $(echo ${NAME}       | wc -c)
9

Some solutions

somevariable=hello; echo "${#somevariable}" (5)
somevariable=hello123456; numberofcharacters=$(echo -n $somevariable | wc -c); echo $numberofcharacters (11)
somevariable=hello123456; interimvariable="${somevariable//[^[:alpha:]]/}"; echo "${#interimvariable}" (5)
somevariable=hello123456; awk -F '[0-9]' '{print NF-1}' <<< "$somevariable" (6)
somevariable=hello123456; interimvariable=$(echo "$somevariable" | sed  's/[^0-9]//g'); echo ${#interimvariable} (6)
somevariable=hello123456; echo "$somevariable" | grep -o "[0-9]" | grep -c "" (6)

When

variablename=$(some piped command)

(command substitution) does not work because of the quotes needed in the pipe command using process subsitution can be a solution

read somevariable < <(some piped command)

Long lines can be broken up by adding a “\” on the place where you want the line to braek and hit ENTER. Mpre than one Space after the “\” is not allowed

A new line (skips one line)

echo -e ""

Two new lines (skips two lines)

echo -e "\n"

${1,,}

is a parameter expansion that converts the value of the variable $1 to lowercase

Read data from a file containing variable=value or variable=“value” lines and put them in variables with the same name

source /etc/os-release && DEBIAN="Debian version: $VERSION_ID $VERSION_CODENAME"

$ STORAGE=/dev/sda1
$ DISK="${STORAGE::-2}"
$ echo $DISK
$ /dev/sd

Get the linenumber a string is on

awk '/somestring/{ print NR; exit }' somefile.txt

Script

#! /bin/bash
if [[ $(date +%_H) -eq 0 ]]; then echo "_H -eq 0"; fi
if [[ $(date +%_H) == 0 ]]; then echo "_H == 0"; fi
if [[ $(date +%H) -eq 0 ]]; then echo "H -eq 0"; fi
if [[ $(date +%H) -eq 00 ]]; then echo "H -eq 00"; fi
if [[ $(date +%H) == 0 ]]; then echo "H == 0"; fi
if [[ $(date +%H) == 00 ]]; then echo "H == 00"; fi
 
if [[ $(date +%_H) -eq 9 ]]; then echo "_H -eq 9"; fi
if [[ $(date +%_H) == 9 ]]; then echo "_H == 9"; fi
if [[ $(date +%H) -eq 9 ]]; then echo "H -eq 9"; fi
if [[ $(date +%H) -eq 09 ]]; then echo "H -eq 09"; fi # Error 09 is not allowed it should be 9 (09 is a string)
if [[ $(date +%H) == 9 ]]; then echo "H == 9"; fi
if [[ $(date +%H) == 09 ]]; then echo "H == 09"; fi

Cronjob

41 0 27,28,29 6 *  /home/user/scripts/tests/stringornumber.sh

Result received in e-mail

_H eq
H eq 0
H eq 00
H == 00

Conclusion

0 is always a number not a string
00 can be a number or a string
09 is always a string

This will round to three digits. The comma might be a dot according to your LC_NUMERIC locale setting

printf %g 453,1427934923

453,143

This will also print the thousand separators

printf "%'d" 4531427934923

4.531.427.934.923

if

Test a function in a (library) file

. $HOME/lib/libthing.sh; FUNCTIONNAME

Try to comment out a line with a #. If the line does not change in a comment line the problem is upstream in the code. This will help finding missing quotes
Turn on debugging mode with

set -x

SC1090
Code:

. /home/user/externalfile.sh
^-- SC1090: Can't follow non-constant source. Use a directive to specify location.

Solutions:

• Add “# shellcheck source=/home/user/externalfile.sh” before the line with the code (did not work for us)
• “Use the -x option to check the external file as well (did not work for us)
• Leave the message be

SC1091
Code:

. /home/user/externalfile.sh
^-- SC1091: Not following: /home/user/externalfile.sh was not specified as input (see shellcheck -x)."

This is strange. We expected SC1090. SC1091 is for an other issue. SC1090 occurred on the line after the line which caused SC1091
Solutions:

• Use the -x option to check the external file as well (did not work for us)
• Leave the message be

Code:

echo "Some text.\\nSome more text"
                 ^-- SC2028: echo won't expand escape sequences. Consider printf.

Solution: The -e option has to be added to the echo command

echo -e "Some text.\\nSome more text"

Anacron
Array
Braces
Calculations
case
cron
Check if X windows is running
Command exit codes
Color
Command line arguments
Cursor movement
Editing text
for
Functions
if
Notifier
Shell Function Definitions - see man bash (search for it)
search end replace
reading a line
true and false
Use of spaces
while
zenity

TEST=Hello; echo “${#TEST}“
Output: The number of characters held in TEST. In this case: 5

• Make a script /home/user/load.sh
  
• Make it executable for the user with chmod 740
• Put

  #! /bin/bash
  uptime | cut -d "a" -f 4 | tr , " " | tr -s " " | cut -d " " -f 2-4 | tr " " "\n"

  in it

• Make a new Generic Monitor in the panel
• Put /home/user/load.sh in the Command field
• Adjust the font size to you needs
• Set the Period to 2 seconds

php -r 'echo "Hello World\n";'

(wd: /somepath)
(wd now: ~)

wd stands for working directory

If you get one of

test.sh: 3: [: ==: unexpected operator
test.sh: 3: test.sh: [[: not found

then

#! /bin/bash

is probably missing in the first line of test.sh, your script

Bash Guide
Bash Hackers
Bash Pitfalls
The Open Group Base Specifications, IEEE, Shell & Utilities
The Open Group Base Specifications, IEEE, Shell Command Language

The difference between [ and [[
Use expansions in a shell script

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