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Code/Bash/Bash-Basics.md

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+# Bash scripting basics
+
+Kommt immer zu begin eines neuen Scripts
+
+```
+#!/bin/bash
+
+```
+
+Berechtigung um ausgeführt zu werden
+
+```
+chmod +x FILENAME
+
+```
+
+## Description Numeric Comparison String Comparison
+
+```
+Shell comparison example:   [ 100 -eq 50 ]; echo $? [ "GNU" = "UNIX" ]; echo $?
+less than                         -lt                       <
+greater than                      -gt                       >
+equal                             -eq                       =
+not equal                         -ne                       !=
+less or equal                     -le                       N/A
+greater or equal                  -ge                       N/A
+boolean
+true=0
+false=1
+
+```
+
+```
+Operator        Description
+! EXPRESSION    The EXPRESSION is false.
+-n STRING       The length of STRING is greater than zero.
+-z STRING       The lengh of STRING is zero (ie it is empty).
+[ -a FILE ]     True if FILE exists.
+[ -b FILE ]     True if FILE exists and is a block-special file.
+[ -c FILE ]     True if FILE exists and is a character-special file.
+[ -d FILE ]     True if FILE exists and is a directory.
+[ -e FILE ]     True if FILE exists.
+[ -f FILE ]     True if FILE exists and is a regular file.
+[ -g FILE ]     True if FILE exists and its SGID bit is set.
+[ -h FILE ]     True if FILE exists and is a symbolic link.
+[ -k FILE ]     True if FILE exists and its sticky bit is set.
+[ -p FILE ]     True if FILE exists and is a named pipe (FIFO).
+[ -r FILE ]     True if FILE exists and is readable.
+[ -s FILE ]     True if FILE exists and has a size greater than zero.
+[ -t FD   ]     True if file descriptor FD is open and refers to a terminal.
+[ -u FILE ]     True if FILE exists and its SUID (set user ID) bit is set.
+[ -w FILE ]     True if FILE exists and is writable.
+[ -x FILE ]     True if FILE exists and is executable.
+[ -O FILE ]     True if FILE exists and is owned by the effective user ID.
+[ -G FILE ]     True if FILE exists and is owned by the effective group ID.
+[ -L FILE ]     True if FILE exists and is a symbolic link.
+[ -N FILE ]     True if FILE exists and has been modified since it was last read.
+[ -S FILE ]     True if FILE exists and is a socket.
+STRING1       =    STRING2      STRING1 is equal to STRING2
+STRING1      !=    STRING2      STRING1 is not equal to STRING2
+INTEGER1    -eq    INTEGER2     INTEGER1 is numerically equal to INTEGER2
+INTEGER1    -gt    INTEGER2     INTEGER1 is numerically greater than INTEGER2
+INTEGER1    -lt    INTEGER2     INTEGER1 is numerically less than INTEGER2
+
+```
+
+## Example if then else in bash
+
+```
+#!/bin/bash
+num_a=400
+num_b=200
+if [ $num_a -lt $num_b ]; then
+    echo "$num_a is less than $num_b!"
+else
+    echo "$num_a is greater than $num_b!"
+fi
+
+```
+
+## IF ELSE && CASE
+
+```
+if      Perform a set of commands if a test is true.
+else    If the test is not true then perform a different set of commands.
+elif    If the previous test returned false then try this one.
+&&      Perform the and operation.
+||      Perform the or operation.
+case    Choose a set of commands to execute depending on a string matching a particular pattern.
+fi      End of if
+
+```
+
+## Variabeln
+
+```
+name='Someone'       -Nur Text kann man in einem String speichern.
+username=$(whoami)   -funktionen werden mit $() angegeben
+day=$(date +%A)      -parameter können ohne Probleme in die Klammer schreiben.
+
+```
+
+## Command Exec
+
+der befehl kommt ohne syntax einfach auf eine leere zeile z.b.
+
+```
+fortune
+cowsay hallo
+
+```
+
+ich kann die beiden commands aber auch zusammensetzen
+
+```
+fortune | cowsay
+
+```
+
+## stderr stdout
+
+The difference between stdout and stderr output is an essential concept as it allows us to a threat, that is, to redirect each output separately. The > notation is used to redirect stdout to a file whereas 2> notation is used to redirect stderr and &> is used to redirect both stdout and stderr. The cat command is used to display a content of any given file