April 23

8.6 Pipelines and job control

1、cat 1.txt |wc -l ; cat 1.txt |grep 'aaa'

2. ctrl+z pauses a task ( ctrl +c terminates)

3. jobs view background tasks

4. bg[id] transfers the task to the background ( short for background) to make it run in the background

5. fg[id] transfers the task to the foreground ( short for foreground) command to resume

6. Add & after the command and throw it directly to the background to continue running

pipe character

1.1 The pipe character "|" is to pass the result of the previous command to the following command

job control

1.2 How to pause the operation and exit when we are editing a file, we can enter ctrl + z to pause the task

1.3 jobs View tasks that are suspended or running in the background , the first column has an ID number

1.4 bg[id] transfers the task to the background to run;

1.5 fg[id] transfers the task to the foreground to run

1.6 Add & after the command and throw it directly to the background, sleep 100 refers to suspending for 100 seconds, and the background continues to run

8.7-8.8 Shell variables

The environment variable PATH is a variable preset by the shell , and the preset variables are usually capitalized. A variable is to use a simpler string to replace certain settings and data with special meaning. System default environment variables: PATH , HOME , LOGNAME

1.1 PATH , HOME , PWD , LOGNAME // System variables (can be viewed with echo , such as echo $PATH )

1.2 env command // View system environment variable information

1.3 The set command has many more variables, and includes user -defined variables //shell scripts

1.4 Custom variables

The variable name rules can be: alphanumeric underscore , the first position cannot be a number

Variable values that have special symbols need to be enclosed in single quotes

1.4.1 Custom variable a=1 , example

1.4.2 Variable name rules: letters and numbers are underlined, and the first position cannot be a number . Example

1.4.3 When the variable value has special symbols, it needs to be enclosed in single quotation marks , for example:

1.5 Accumulation of variables, example:

If the variable content itself has single quotes or other symbols such as $ , this requires double quotes

1.6 Global variable export ( valid for the next sub- bash , but not for the upper level)

When we customize a variable on a terminal, it is only valid for the shell under the current terminal. If a subshell is opened under this terminal , the variable will not be synchronized in the past. This is called a non-global variable . If you want all subshells to synchronize this custom variable, use the global variable command : export (only valid from top to bottom, not vice versa )