The use of -e, &, and && in bash scripts
1.&和&&
There are several different uses for the ampersand symbol in Linux bash scripts. Here are some common ones:
Background run command:
When you add an & to the end of a command, it puts the command in the background for execution. This means that you can continue to execute other commands on the same terminal while the command is running. For example:
python script.py & The command above starts the script.py script in the background.
Logical operator:
&& can be used as a logical operator, but it is more common to use &&. && 'is used to join two commands so that the second command is executed only after the first command is successfully executed (i.e., the exit status is 0).
command1 &&command2 is not commonly used as a logical operator because it does not check the exit status of the previous command.
Bit operation:
In bash scripts, & can also be used as the "by bit and" operator in bit operations. For example:
((result=var1 & var2)) The above command computes the bit-sum results of var1 and var2 and assigns them to result.
Job Control:
In bash, & is also related to job control. You can use the jobs command to see the list of jobs running in the background, use the fg command to bring background jobs to the foreground, or use the bg command to continue running suspended jobs in the background.
jobs # lists job fg %1 # brings job number 1 to the foreground bg %1 # Continues job number 1 in the background
File descriptor:
In advanced scripting, & can also be used to redirect file descriptors. For example, 2>&1 redirects standard error (file descriptor 2) to standard output (file descriptor 1).
command > Output.txt 2>&1 The above command redirects both the standard output and standard error of the command to the 'output.txt' file.
The meaning of an & can vary greatly depending on the context. When writing the script, make sure you clearly know what each & does to avoid confusion and potential errors.
2.-e
In Linux bash scripts, -e is often used with the echo command or the test/[command, which have different meanings and purposes.
The -e option in the echo command:
When the -e option is used in the echo command, it enables echo to interpret characters escaped by backslashes \. For example:
echo -e "Line 1\nLine 2" The command above outputs two lines of text, with "\n" interpreted as a newline.
test/[-e option in the command:
In the test or its alias [command, -e is used to check whether a file or directory exists. If the specified file or directory exists, the expression returns true (exit status is 0). For example:
if [ -e /path/to/file ]; then echo "File exists." else echo "File does not exist." fi "Then echo" file exists." Else echo "file does not exist." fi" Then echo "file exists." Else echo" file does not exist.
Be sure to pay attention to the context when using -e, as its meaning is completely different in different commands.
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