Red Hat System Administration III - Unit 3 - Bash Scripting and Tools

Bash Overview

• Start scripts with #!/bin/bash
• Make it executable with chmod
• Put it in a directory in $PATH
  • ~/bin - for user's private programs
  • /usr/local/bin - locally developed scripts by other uses on the system
  • /usr/local/sbin - locally developed scripts used by root
• Variable syntax:
  • $my_var
  • ${my_var}
• Command substitution:
  • today=$(date +%Y-%m-%d); echo $today
• Quoting & escaping:
  • echo "Today is $X" - interpolate
  • echo 'Today is $X' - literal

• Repetition can be achieved with: for var in commands; do command; done

  for i in {1..9}; do echo "This is $i"; done

• Conditional branching:

  • utilise non-zero value returns for false
  • Syntax: test EXPRESSION or test [ EXPRESSION ]
  • Use $? to get last return code
  • Non-zero or zero-length string operators: test -{n|z} STRING
  • String comparison: ==, !=
  • Numeric comparison: -eq, -ne
  • File status operations: test -{e|f|d|r|w|x|s} FILE
    • -s - check for empty files
  • Logical operators:
    • -o - OR
    • -a - AND
    • ! - NOT
  • man test to get info on Bash operators
• Use env to get list of environment variables
• Positional params:
  • script.sh TEST DEMO - $0 $1 $2
  • $@- all positional params
  • $# - count of params
• Running commands on remote machine:
  • Save file locally: ssh user@host 'command1; command2' > log.local
  • Save file remotely: ssh user@host 'command1; command2 > log.remote'

Text Processing Tools

diff

• Used to compare two files
• Args:
  • -c - display surrounding lines for context
  • -u - unified output format
  • -r - perform a recursive comparison of files

patch

• Changes files from patches created with diff -u:
  • $ diff -u samsreport.sh samsreport-2.sh >> my_patch
• Patch file using the patch command

$ patch samsreport.sh < my_patch
patching file samsreport.sh

• Use -b option to specify a backup before patching. Will create a file with an .orig suffix:
  • > samsreport.sh.orig

grep

• Search for patterns of text
• Args:
  • -i perform case-insensitive search
  • -n precede return lines with line of numbers
  • -r perform a recursive search of lines, starting with the named directory
  • -c display a count of lines matching
  • v return lines that don't match
  • -l list names of files with one line containing the pattern

cut

• Get first column delimitered by ::
  • > cat /etc/passwd | cut -d : -f 1

wc

• -l displays number of lines
• -w displays number of words
• -c displays number of bytes
• -m displays nubmer of characters

sort

• -n - sort numerically instead of char
• -k - set the sort field
• -t - specify a field separator

tr

Translate command

> echo "HELLO" | tr 'A-Z' 'a-z'
hello

sed

• Edits a stream of textual data
  • Use g to make it global, use d to delete lines

> echo "dogs and cats" | sed '/ddogs/d'
dogs and cats
> echo "dogs and cats" | sed '/dogs/d'

Regular Expression

• ^ - line begins
• $ - line ends
• . - any single character
• [z] - a single character that is x, y or z
• [^xyz] - opposite

Password Aging

Use change to change password information for a user

Defaults stored in /etc/login.defs:

• -m - min days
• -M - max days
• -W - warn days

Defaults stored in /etc/default/useradd:

• -I - inactive days
• -E - expiry