Bash

git clone git@github.com:eric2003/ModernBash.git

window git bash

cd d drive

cd /d
cd /d/work/bash_work/ModernBash/codes/simple/01/

grep

$ ls -l /usr/bin | grep bash
-rwxr-xr-x 1 root   root     1396520  1月  7  2022 bash
-rwxr-xr-x 1 root   root        6818  1月  7  2022 bashbug
-rwxr-xr-x 1 root   root        4414 11月 11  2021 dh_bash-completion
lrwxrwxrwx 1 root   root           4  8月 30  2022 rbash -> bash

echo example 1

$ echo Hello World! > hello.txt
$ cat hello.txt
Hello World!

echo example 2

$ echo Hello World! > hello.txt
$ echo Hello World! > hello.txt
$ echo Hello World 1! >> hello.txt
$ echo Hello World 2! >> hello.txt
$ cat hello.txt
Hello World!
Hello World 1!
Hello World 2!
$ wc -w hello.txt
8 hello.txt

list

$ MY_LIST=(one two three four five)
$ echo $MY_LIST
$ echo ${MY_LIST}
$ echo ${MY_LIST[@]}
$ echo ${MY_LIST[0]}
$ echo ${MY_LIST[1]}
one
one
one two three four five
one
two

cat /etc/shells

$ cat /etc/shells
# /etc/shells: valid login shells
/bin/sh
/bin/bash
/usr/bin/bash
/bin/rbash
/usr/bin/rbash
/usr/bin/sh
/bin/dash
/usr/bin/dash

Set Command

set Command Options

The -e Option

When a query returns a non-zero status, the -e flag stops the script. It also detects errors in the currently executing script.

#!/bin/bash
set -e
mkdir newfolder
cat filenotindirectory
echo 'The file is not in our directory!'
---------------------
$ bash testprj.sh
cat: filenotindirectory: No such file or directory

The -C Option

Using the -C flag ensures that we cannot overwrite an existing file with the same name:

#!/bin/bash
touch myfile
echo 'An existing file' > myfile
set -C
echo 'Editing an existing file' > myfile
typical results:
$ bash testprj.sh
testprj.sh: line 5: myfile: cannot overwrite existing file

The -f Option

As we know, we can easily search for files using wildcard characters such as ?, *, or []. This method is similar to regex, where we try to find similar texts using patterns. The Bash shell uses the wildcards we specify to generate patterns and match them with filenames. This feature is called globbing. Let’s try using globbing by searching for files with the .txt extension:

#!/bin/bash
touch files.txt
ls *.txt
set -f
ls *.txt
typical results:
$ bash testprj.sh
files.txt  README.txt
ls: cannot access '*.txt': No such file or directory

The -x Option

We use the -x parameter when debugging our scripts to determine the output of individual commands.

To illustrate, let’s create a Bash script that displays a countdown from 3 to 0:

#!/bin/bash
set -x
n=3
while [ $n -gt 0 ]; do
    n=$[ $n-1 ]
    echo $n
    sleep 1
done
typical results:
$ bash testprj.sh
+ n=3
+ '[' 3 -gt 0 ']'
+ n=2
+ echo 2
2
+ sleep 1
+ '[' 2 -gt 0 ']'
+ n=1
+ echo 1
1
+ sleep 1
+ '[' 1 -gt 0 ']'
+ n=0
+ echo 0
0
+ sleep 1
+ '[' 0 -gt 0 ']'

The -a Option

We can export variables or functions with this flag, making them reusable in subshells or scripts. First, let’s define variables in our terminal:

1.
$ set -a
$ name='May'
$ age=22

2.
#!/bin/bash
echo $name $age

3.
$ bash testprj.sh
may 22

The -u Option

We use this flag to ensure that Bash does not overlook the non-existent variables in our script. We can see that in normal circumstances, Bash ignores the unassigned variables and runs our script without any errors:

1.
#!/bin/bash
set -u
x='Bash scripting is fun'
echo $x $y

2.
$ bash testprj.sh
testprj.sh: line 4: y: unbound variable

The +[argument] Option

Running the set command with the +[argument] option unsets the option’s functionality. In essence, it nullifies the effect of the -[argument] option.

We will see some examples of shell scripts run with the set +[argument] commands and their outputs.

Firstly, we consider set +e:

1.
#!/bin/bash
set +e
mkdir newfolder
cat filenotindirectory
echo 'The file is not in our directory!'

2.
$ bash testprj.sh
cat: filenotindirectory: No such file or directory
The file is not in our directory!

Other Special Variables

$0 - The name of the Bash script.
$1 - $9 - The first 9 arguments to the Bash script. (As mentioned above.)
$# - How many arguments were passed to the Bash script.
$@ - All the arguments supplied to the Bash script.
$? - The exit status of the most recently run process.
$$ - The process ID of the current script.
$USER - The username of the user running the script.
$HOSTNAME - The hostname of the machine the script is running on.
$SECONDS - The number of seconds since the script was started.
$RANDOM - Returns a different random number each time is it referred to.
$LINENO - Returns the current line number in the Bash script.

tr command

cat command

How to Use the Linux cat Command With Examples

Using the cat Command to Create a File

Using the cat command you can quickly create a file and put text into it. To do that, use the > redirect operator to redirect the text in the file.

cat > filename.txt

The file is created, and you can begin populating it with text. To add multiple lines of text just press Enter at the end of each line. Once you’re done, hit CTRL+D to exit the file.