# 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