Bash programming
- Manfred Preyer
- vor 4 Tagen
- 5 Min. Lesezeit
A well-formed Bash script starts with a shebang (#!/usr/bin/bash), which tells the system to use the Bash interpreter . To run it, you need to make the script file executable using the chmod +x command .
Variables and User Input
Access value of variable with a $ sign. To make your scripts interactive, the read command is used to prompt the user for input.
Conditional Logic (if, then, else, fi)
Conditionals allow scripts to make decisions. For example, you can check if a file exists or if a user is root before executing commands . The test command (or [ ]) is used for these checks .
Loops are essential for automating repetitive tasks.
for loop: Iterates over a list of items .
while loop: Repeats a block of code while a condition is true .
until loop: Repeats a block of code until a condition becomes true .
Functions:
Functions let you group reusable pieces of code for better organization, similar to other programming languages .
Key RHEL-Specific Considerations
Bash Version and Features: RHEL uses specific versions of Bash. For instance, RHEL 6 included version 4.1, which introduced several important changes compared to earlier versions:
=~ Operator Changes: The behavior of the regular expression matching operator (=~) in [[ ]] conditional expressions changed. It's recommended to store the regex pattern in a variable to avoid issues .
set -e Behavior: The set -e option (which makes the script exit on error) is more aggressive and can cause a script to exit if any command in a pipeline fails .
Compatibility Levels: Bash 4.0 introduced compatibility options (compat31) via the shopt builtin, allowing you to revert to the behavior of older versions for specific tasks .
Practical Scripting Examples for RHEL
Here are some basic scripts illustrating common tasks.
A Simple System Info ScriptThis script, when made executable (chmod +x), will display system information .
bash
#!/usr/bin/bash
echo "Hello, this is my first RHEL script."
echo "Listing block devices:"
lsblk
echo "Filesystem free space:"
df -h
Using a for Loop to Retrieve HostnamesThis script demonstrates a for loop, automating the hostname command with different options .
bash
#!/usr/bin/bash
for OPTION in "" "-f" "-s"; do
echo "Getting hostname with option: ${OPTION}"
hostname ${OPTION}
echo "------------------------"
done
A Conditional Check for Script ArgumentsThis is a common pattern to ensure a script has received the correct input (e.g., a filename) .
bash
#!/usr/bin/bash
if [ $# -ne 1 ]; then
echo "Usage: $0 <filename>"
exit 1
fi
echo "The argument is: $1"
The official RHEL documentation and man pages (man bash, man test) are your best resources for detailed information on built-in commands and syntax specifics .
Cheatsheet for Bash
File & Directory Operations
Command | Description |
pwd | Print current working directory |
ls | List files (-l long, -a all, -lh human-readable) |
cd | Change directory (cd ~ home, cd - previous) |
mkdir | Create directory (-p create parents) |
rmdir | Remove empty directory |
rm | Remove files (-r recursive, -f force) |
cp | Copy files/dirs (-r recursive) |
mv | Move/rename files/dirs |
touch | Create empty file or update timestamp |
ln -s | Create symbolic link |
File Content Viewing
Command | Description |
cat | Display entire file |
less | View file page by page (/ search, q quit) |
head | Show first 10 lines (-n 20 for 20 lines) |
tail | Show last 10 lines (-f follow live updates) |
grep | Search inside files (-i case-insensitive, -r recursive) |
Permissions & Ownership
Command | Description |
chmod | Change file permissions (chmod 755 script.sh) |
chown | Change file owner (chown user:group file) |
umask | Set default permissions for new files |
Permission numbers: 4=read, 2=write, 1=executeExample: 755 = owner:rwx, group:r-x, others:r-x
Process Management
Command | Description |
ps | Show processes (ps aux for all) |
top / htop | Interactive process viewer |
kill | Terminate process by PID (-9 force kill) |
killall | Kill all processes by name |
jobs | List background jobs |
fg | Bring job to foreground |
bg | Resume job in background |
& | Run command in background (command &) |
nohup | Run command immune to hangups |
Package Management (RHEL)
Command | Description |
dnf install <pkg> | Install package |
dnf remove <pkg> | Remove package |
dnf update | Update all packages |
dnf search <term> | Search for package |
dnf info <pkg> | Show package info |
rpm -ivh <file.rpm> | Install local RPM |
rpm -qa | List all installed RPMs |
Network Commands
Command | Description |
ping | Test network connectivity |
curl | Transfer data from/to server |
wget | Download files |
ss | Socket statistics (replaces netstat) |
ip a | Show IP addresses (replaces ifconfig) |
hostname | Show/change hostname (-I for IP) |
dig / nslookup | DNS lookup |
User Management
Command | Description |
whoami | Show current user |
id | Show user/group IDs |
sudo | Run command as root |
su - | Switch to root user |
useradd | Create new user |
passwd | Change password |
who | Show logged-in users |
Bash Built-in Syntax
Variables
bash
VAR="value" # Set variable
echo $VAR # Access variable
echo ${VAR} # Safer access
read VAR # Read user input into variable
Conditionals (if)
bash
if [ condition ]; then
# code
elif [ condition ]; then
# code
else
# code
fi
Common test conditions:
Condition | Meaning |
[ -f file ] | File exists |
[ -d dir ] | Directory exists |
[ -z string ] | String is empty |
[ -n string ] | String is not empty |
[ "$a" = "$b" ] | Strings equal |
[ "$a" != "$b" ] | Strings not equal |
[ $a -eq $b ] | Integers equal |
[ $a -ne $b ] | Integers not equal |
[ $a -lt $b ] | Less than |
[ $a -gt $b ] | Greater than |
[ -e file ] | File exists (any type) |
[ -x file ] | File is executable |
Loops
bash
# For loop
for item in list; do
echo $item
done
# For loop with range
for i in {1..10}; do
echo $i
done
# While loop
while [ condition ]; do
# code
done
# Until loop
until [ condition ]; do
# code
done
Case Statement
bash
case $VAR in
pattern1)
# code
;;
pattern2)
# code
;;
*)
# default code
;;
esac
Functions
bash
function_name() {
local var="local scope" # Local variable
echo "Hello $1" # $1 = first argument
return 0 # Return exit code
}
function_name "John" # Call function
Special Variables
Variable | Description |
$0 | Script name |
$1-$9 | Positional arguments |
$# | Number of arguments |
$@ | All arguments as separate words |
$* | All arguments as one string |
$? | Exit code of last command |
$$ | PID of current shell |
$! | PID of last background command |
Arithmetic Operators
$((5 + 3)) # Addition
$((10 - 4)) # Subtraction
$((6 * 7)) # Multiplication
$((20 / 5)) # Division
$((10 % 3)) # Modulo
Logical (within [[ ]])
Operator | Meaning |
&& | AND |
|| | OR |
! | NOT |
== | Pattern match |
=~ | Regex match |
Keyboard Shortcuts
Shortcut | Action |
Ctrl+C | Interrupt/kill current command |
Ctrl+Z | Suspend current command |
Ctrl+D | Exit shell/EOF |
Ctrl+L | Clear screen |
Ctrl+A | Move to beginning of line |
Ctrl+E | Move to end of line |
Ctrl+U | Delete from cursor to beginning |
Ctrl+K | Delete from cursor to end |
Ctrl+R | Reverse search history |
!! | Repeat last command |
!$ | Last argument of previous command |
!n | Run command number n from history |
Text Processing (One-liners)
Command | Description |
echo "text" | wc -l | Count lines |
echo "text" | wc -w | Count words |
echo "text" | sed 's/old/new/g' | Replace text |
echo "text" | awk '{print $1}' | Print first column |
sort file | Sort lines |
uniq | Remove duplicates (needs sort) |
cut -d: -f1 /etc/passwd | Extract first field (delimiter :) |
Common RHEL Script Patterns
Check if running as root:
bash
if [ $EUID -ne 0 ]; then
echo "This script must be run as root"
exit 1
fi
Check if a file exists:
bash
if [ -f "/etc/myconfig.conf" ]; then
echo "Config exists"
else
echo "Config missing"
fi
Loop through all .conf files:
bash
for file in /etc/*.conf; do
echo "Processing $file"
done
Get script directory (reliable method):
bash
SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
Quick Reference: test vs [[ ]]
Syntax | When to use |
[ ] | POSIX-compatible, simple tests |
[[ ]] | Bash-specific, more features (regex, pattern matching) |
(( )) | Arithmetic evaluation |
Prefer [[ ]] in Bash scripts for better safety and features.