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Shell Basics

Shell Basics

Documentation

  • man - By Default the system documentation/manuals are documented in the man pages
# Usage:
man <COMMAND>
  • help - Lists all the internal commands of shell
# Usage:
help

Bash structure

  • ~/.bashrc - configure the shell, define command aliases and set command shell options
  • ~/.bash_profile - initialization commands that set environment variables, a shell’s prompt
  • ~/.bash_logout - cleanup operations and other commands that you want the shell to execute whenever a user logs out of a shell
  • ~/.bash_aliases - if storing many aliases commands

Shell Script 

set -x # Enable RAW i/o
set -v # Enable RAW i/o 
set -e # abort on error(non-zero exit code)
set -u # detect unset variables
variable=${VARIABLE_NAME:-default} # default value if variable empty
$(( (1+1)/5 )) # Arthematic operation
${1..10} # Sequence
${var%suffix} # trim a suffix named 'suffix'
${var#prefix} # trim a prefix named 'prefix'
${foo,bar} # expand into multiple values
diff /etc/hosts < (ssh somehost cat /etc/hosts) # parenthesis to execute a sub-process
{..} # Include code in between this block, as a good practice

cat <<EOF
touch somefile
echo foo > somefile
EOF


# Bash Script strict mode
#!/bin/bash
set -euo pipefile
IFS=$'\n\t'

Redirection

File Descriptors - every file has an associated number called FD. Default ones are

  • 0 - STDIN
  • 1 - STDOUT
  • 2 - STDERR

Types of redirection

  • > - STDOUT redirection to file, overwrites at destination
# Usage:
ls -lrt > test.out
  • >> - STDOUT redirection to file, appends at destination
# Usage:
ls -lrt >> test.out
  • < - STDIN redirection from a file
# Usage:
wc < input_file.out
  • File Descriptors based redirection
# Usage:
# Redirect the STDERR to STDOUT
ls -lrt 2>&1
  • | - redirects STDOUT of one command into the STDIN of a second command
# Usage:
# Redirect the STDOUT to STDIN
ls -lrt | wc

Navigation

  • Ctrl + a - Go to beginning
  • Ctrl + e - Go to end
  • Alt + b - Go back one word
  • Alt + f - Go forward one word
  • Ctrl + w - Delete a word backward
  • Alt + d - Delete next word
  • Ctrl + u - Delete to beginning of line OR delete line
  • Ctrl + k - Delete to end
  • Ctrl + r - Search backwards(recursive)
  • Ctrl + s - Search forwards
  • Ctrl + l - clears the screen
  • Alt + t - transpose the last 2 words/characters

Job Management

  • & - background process.
  • ctrl+z - pauses the process temporarily, and places it in SUSPENDED mode.
  • fg - used to bring a background / paused process back to the foreground and continue its execution.
  • bg - resume any SUSPENDED jobs and run them in background
  • jobs - check the job status of all the jobs
  • kill - used for killing a process
  • Status: SUSPENDED, CONTINUED, RUNNING, STOPPED
# Usage:
sleep 100 & # Run the command as a backgroud process
jobs # view all the jobs
jobs -l # view all the jobs along with their job ID & process PID
fg %<JOB_ID> # Resume the job in fore-ground which has the JOB_ID
bg %<JOB_ID> # Resume the job in the back-ground which has the JOB_ID
ctrl+z # Pauses executing of the current running job
ctrl+c # terminates the current executing process
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