Bash scripting

Keyboard shortcuts

Command Line Editing
Commands For Moving
Mastering The Linux Shell – Bash Shortcuts Explained (Now With Cheat Sheets)
Bash Keyboard Shortcuts

[CTRL + P]      Print previous command (Up arrow)
[CTRL + N]      Print next command (Down arrow)
[CTRL + R]      History search
[ALT  + R]      Cancel changes, put back like in the history
[!    + !]      Repeat last command
[!    + <cmd>]  Run last command starting with <cmd>
[!    + *]      Run all argument of previous command
[!    + $]      Run last argument of previous command
[ALT  + .]      Print last argument of previous command
[^<p1> + ^<p2>] Run last command, replace <p1> with <p2>
[CTRL + F]      Move forward one character (Right arrow)
[CTRL + B]      Move backward one character (Left arrow)

[ALT  + F]      Move forward one word
[ALT  + B]      Move backward one word

[CTRL + A]      Move to the start of the line
[CTRL + E]      Move to the end of the line

[CTRL + X + X]  Toggle between start of word and cursor
[CTRL + H]      Delete previous character from the cursor
                (= Rubout, like Backspace)
[CTRL + D]      Delete current character from the cursor (Delete)
[CTRL + K]      Cut from the cursor to the end of the line
[CTRL + U]      Cut from the cursor to the start of the line
                (like [CTRL + X + BACKSPACE]

[ALT  + D]      Cut from the cursor to the end of the current word
[CTRL + W]      Cut from the cursor to the previous whitespace

[CTRL + Y]      Paste
[ALT  + T]      Swap current word with previous (or [ESC + T])
[CTRL + T]      Swap the last two characters before the cursor (typo)

[ALT  + U]      Upper-case word and move to the end of the word
[ALT  + L]      Lower-case word and move to the end of the word
[ALT  + C]      Capitalize word and move to the end of the word

Job ID / Percent sign (%)

Percent sign (%) in front of shell command

$ tail -f /var/log/messages
[CTRL + Z]
[1]+  Stopped                 tail -f /var/log/messages

$ tail -f /var/log/maillog
[CTRL + Z]
[2]+  Stopped                 tail -f /var/log/maillog

$ jobs
[1]-  Stopped                 tail -f /var/log/messages
[2]+  Stopped                 tail -f /var/log/maillog

$ fg %2
tail -f /var/log/maillog
[CTRL + Z]
[2]+  Stopped                 tail -f /var/log/maillog

$ fg %1
tail -f /var/log/messages
[CTRL + Z]
[1]+  Stopped                 tail -f /var/log/messages

$ kill %1 %2
[1]-  Terminated              tail -f /var/log/messages
[2]+  Terminated              tail -f /var/log/maillog

Process List (ps)

$ ps auxf
$ ps auxd

#!/usr/bin/env

Howto Make Script More Portable With #!/usr/bin/env As a Shebang

#!/usr/bin/env bash

rather than

#!/bin/bash

Bash Conditional Expressions
The Set Builtin

Parameter

Parameter Purpose
$0, $1, $2, etc. Positional parameters, passed from command line to script, passed to a function, or set to a variable.
$# Number of command-line arguments or positional parameters
$* All of the positional parameters, seen as a single word.
$@ Same as $*, but each parameter is a quoted string, that is, the parameters are passed on intact, without interpretation or expansion. This means, among other things, that each parameter in the argument list is seen as a separate word.
Parameter Purpose
$0, $1, $2, ... The positional parameters starting from parameter 0. Parameter 0 refers to the name of the program that started bash, or the name of the shell script if the function is running within a shell script. See the bash man pages for information on other possibilities, such as when bash is started with the -c parameter. A string enclosed in single or double quotes will be passed as a single parameter, and the quotes will be stripped. In the case of double quotes, any shell variables such as $HOME will be expanded before the function is called. You will need to use single or double quotes to pass parameters that contain embedded blanks or other characters that might have special meaning to the shell.
$* The positional parameters starting from parameter 1. If the expansion is done within double quotes, then the expansion is a single word with the first character of the IFS special variable separating the parameters, or no intervening space if IFS is null. The default IFS value is a blank, tab, and newline. If IFS is unset, then the separator used is a blank, just as for the default IFS.
$@ The positional parameters starting from parameter 1. If the expansion is done within double quotes, then each parameter becomes a single word, so that “$@” is equivalent to “$1” “$2” … If your parameters are likely to contain embedded blanks, you will want to use this form.
$# The number of parameters, not including parameter 0.

Internal Variables

Advanced Bash-Scripting Guide: 9.1. Internal Variables

Variable Purpose
$BASHPID Process ID of the current instance of Bash. This is not the same as the $$ variable, but it often gives the same result.
$PS1 This is the main prompt, seen at the command-line.
$PS2 The secondary prompt, seen when additional input is expected. It displays as “>”.
$PS3 The tertiary prompt, displayed in a select loop.
$PS4 The quartenary prompt, shown at the beginning of each line of output when invoking a script with the -x [verbose trace] option.
$PWD Working directory (directory you are in at the time).
$OLDPWD Old working directory.
$HOME PHome directory of the user.
$- Flags passed to script.
$! PID (process ID) of last job run in background.
$_ Special variable set to final argument of previous command executed.
$? Exit status of a command, function, or the script itself.
$$ Process ID (PID) of the script itself. The $$ variable often finds use in scripts to construct “unique” temp file names.

get pid in shell (bash)

$ ( echo $$; echo $BASHPID )
11436
8408

Default values

Expansion Purpose
${PARAMETER:-WORD} If PARAMETER is unset or null, the shell expands WORD and substitutes the result.The value of PARAMETER is not changed.
${PARAMETER:=WORD} If PARAMETER is unset or null, the shell expands WORD and assigns the result to PARAMETER. This value is then substituted. You cannot assign values to positional parameters or special parameters this way.
${PARAMETER:?WORD} If PARAMETER is unset or null, the shell expands WORD and writes the result to standard error. If WORD is not present a message is written instead. If the shell is not interactive, it exits.
${PARAMETER:+WORD} If PARAMETER is unset or null, nothing is substituted. Otherwise the shell expands WORD and substitutes the result.
APP_NAME=${APP_NAME:-"default_name"}

$ export FOO=first
$ echo "The ${FOO:-second} choice"
The first choice

$ unset FOO
$ echo "The ${FOO:-second} choice"
The second choice

$ export FOO=
$ echo "The ${FOO:-second} choice"
The second choice

Default values
bash assign default value

Parameter Expansion

Parameter expansion
Parameter Expansion

  • Simple usage
  • Indirection
  • Case modification
  • Variable name expansion
  • Substring removal
  • Search and replace
  • String length
  • Substring expansion
  • Use a default value
  • Assign a default value
  • Use an alternate value
  • Display error if null or unset

Bash : removing part of a string (= Parameter Expansion)

Abschneiden von Mustern

Eine gewöhnungsbedürftige, aber doch sehr nette Funktion ist das Herausschneiden bestimmter Muster aus der Zeichenkette einer Variablen.

Funktion Erklärung
${variable%muster} Entfernt rechts das kleinste passende Stück.
${variable%%muster} Entfernt rechts das größte passende Stück.
${variable#muster} Entfernt links das kleinste passende Stück.
${variable##muster} Entfernt links das größte passende Stück.
From the beginning:
path1="/usr/local/bin/bash"
${PARAMETER#PATTERN}  => shortest matching: ${path1#/*}  => usr/local/bin/bash
${PARAMETER##PATTERN} => longest matching:  ${path1##*/} => bash

From the end:
path2="x/usr/local/bin/bash"
${PARAMETER%PATTERN}  => shortest matching: ${path2%/*}  => x/usr/local/bin
${PARAMETER%%PATTERN} => longest matching:  ${path2%%/*} => x

var="Memory Used: 19.54M"
var=${var#*: }            # Remove everything up to a colon and space
var=${var%M}              # Remove the M at the end

Lists of Commands, Command Sequences

Lists of Commands
Writing Better Shell Scripts – Part 2
Meaning of colon in Bash after a double pipe
Bourne Shell Builtins
Is there a difference between how two ampersands and a semi-colon operate in bash?

  • Two logical short-circuits are the double ampersand (&&) and double pipe (||) operators.
  • The && only allows the command that comes after it in the series to be executed if the previous command exited with a status of 0.
  • The || operator does the opposite by only allowing the next command to be executed if the previous one returned a non-zero exit status.
  • The ; just separates one command from another.
  • The : is a null statement, so it does nothing.
  • The . executes a script in the current shell, not starting a new shell
$ true; echo $?
0

$ false; echo $?
1


=== && ===
$ true && echo "hello"
hello

$ false && echo "hello"
<no output>


=== || ===
$ true || echo "hello"
<no output>

$ false || echo "hello"
hello


=== ; ===
$ true; echo "hallo"
hallo

$ false; echo "hallo"
hallo


=======================

[ -n STRING ] => True if the length of "STRING" is non-zero.

=== return value ===
$ [ -n "hallo" ]; echo $?
0

$ [ -n "" ]; echo $?
1


=== if then ===
$ if [ -n "hallo" ]; then echo "welt"; fi
welt

$ if [ -n "" ]; then echo "welt"; fi
<no output>


=== && ===
$ [ -n "hallo" ] && echo "welt"
welt

$ [ -n "" ] && echo "welt"
<no output>


=== || ===
$ [ -n "hallo" ] || echo "welt"
<no output>

$ [ -n "" ] || echo "welt"
welt

Bang dollar-sign

Advancing in the Bash Shell

$ ./app a b c d e f

$ !*
a b c d e f
-bash: a: command not found

$ !$
f
-bash: f: command not found

Brace Expansion

Advancing in the Bash Shell

$ cp filename filename-old
$ cp filename-old filename
$ cp filename{,-old}
$ cp filename{-old,}
$ cp filename{-v1,-v2}

Exit code

Exit and Exit Status
Exit Codes With Special Meanings
Understanding Exit Codes and how to use them in bash scripts

$ bla
-bash: bla: command not found

$ echo $?
127
Exit Code Number Meaning Example Comments
1 Catchall for general errors let “var1 = 1/0” Miscellaneous errors, such as “divide by
zero”
and other impermissible operations
2 Misuse of shell builtins (according to Bash documentation) empty_function() {} Missing keyword
or command, or permission problem (and diff return code
on a failed binary file comparison
).
126 Command invoked cannot execute /dev/null Permission problem or command is not an executable
127 “command not found” illegal_command Possible problem with $PATH or a typo
128 Invalid argument to exit exit 3.14159 exit takes only integer args in the
range 0 – 255 (see
first footnote)
128+n Fatal error signal “n” kill -9 $PPID of script $? returns
137 (128 + 9)
130 Script terminated by Control-C Ctl-C Control-C is fatal error signal
2, (130 = 128 + 2, see above)
255* Exit status out of range exit -1 exit takes only integer args in the
range 0 – 255

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