Skip to content

Latest commit

 

History

History
98 lines (64 loc) · 2.41 KB

README.md

File metadata and controls

98 lines (64 loc) · 2.41 KB
Raw

Shells

  • Currently running shell stores important config info and environment variables.

  • printenv - show environment variables for current shell.

  • PATH - list of directories searched in order to find commands to be executed.

  • There are 2 types of environment variables - global and local - global can be accessed by anything executing in shell, but local can be accessed only by the script in which it was defined (shell variables).

    # in shell
COUNT_LOCAL=24
# creates local or shell variable

echo $COUNT_LOCAL
# 24

bash
# create sub-shell

echo $COUNT_LOCAL
# no value

exit

export COUNT_GLOBAL=24
# create global or environment variable

echo $COUNT_GLOBAL
# 24

bash

echo $COUNT_GLOBAL
# 24

exit

unset COUNT_GLOBAL
# unset variable

  • .bashrc file acts as a startup file for the Bash shell.

    vim ~/.bashrc
# modify file if reqd
# for shell customization

source ~/.bashrc
# rerun all commands in file in current shell

  • stdin, stdout and stderr are three data streams used in Linux:

    • stdin - used to send info to a program
    • stdout - contains all normal output from a program
    • stderr - used to display errors

  • These data streams can be redirected to other files:

    ls /etc/ > ~/dir-contents.txt
# > used to redirect stdout to file

# writing to this file again can overwrite it

ls /tmp >> ~/dir-contents.txt
# >> to append instead of overwrite

head < /etc/passwd
# < used to redirect stdin to 'head' command

find / -name sample.txt 2> errors.txt
# 2> used to redirect stderr to file

find / -name sample.txt 2>/dev/null
# redirect stderr to /dev/null to ignore errors
# this does not print any errors

find / -name sample.txt &> all.txt
# redirect both stdout and stderr to same file

  • Pipes can be used to connect stdout of one command to stdin of another:

    ls -la /etc/ | less

ls -la /etc/ | head -n 20 | tail -n 5

  • In shell, history lets us check previously executed commands; we can execute a previously executed command in the format !n, where n is the history number from history output.

  • !! and !-1 are two shorthands to execute the last executed command.

  • Command substitution can be done using backticks to execute a part of the command first in a sub-shell, then the rest of the command.