UNIX / Linux Shell

File Manipulation:
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> file create (voerwrite) file
>> file append to file
>file 2>&1 both output and errors to file
< file read from file
a | b pipe output from 'a'as input to 'b'
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Common Constructs:
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while read f
do
echo "Line is $f"
done
Comments: Read text file line by line
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$ grep foo myfile
afoo
foo
foobar

Comments: find matching line
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$ cut -d: f5/etc/passwd
Dilbert

Comments: get filed with delimiter
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foo= 'ls'

Comments: get output of command
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case $foo in
a)
echo "foo is A"
;;
b)
echo "foo is B"
;;
*)
echo "foo is not A or B"
;;
esac

Comments: case is a good way to avoid iterating through many if/elif/elif constructs.
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doubleit() {
expr $1\*2
}
doubleit 3 # returns 6-

Case: function declaration and calling syntax
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for i in *
do
echo "File is $i"
done

Case: A for loop iterates through its input (which is subject to globbing)
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Useful Variables
$IFS Internal File Separator
$? return code from last program
$SHELL what shell is running this script?
LANG Language; C is US English
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Test Operators

if [ "$x -lt "$y" ]; then
# do something
fi
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Numeric Tests
lt less than
gt greater than
eq equal to
ne not equal
ge greater or equal
le less or equal
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File Tests
nt newer than
d is a directory
f is a file
r readable
w writeable
x executable
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String Tests
= equal to
z zero length
n not zero length
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Logical Tests
&& logical AND
|| logical OR
! logical NOT
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Argument Variables
$0 program name
$1 1st argument
$2 2nd argument
$3 3rd argument
$4 4th argument
$5 5th argument
$6 6th argument
$7 7th argument
$8 8th argument
$9 9th argument
$* all arguments
$# No.of arguments
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Files: Contents / Attributes
find.-size 10k -print files over 10kb
find.-name "*.txt" -print find text files
find/foo -type d -ls is all directories under /foo
three= 'expr 1+2' simple maths
echo "scale = 5; 5121/1024" | bc better maths
egrep "(foo|bar)" file find "foo" or "bar" in file
awk '{print $5 }'file find "foo" or "bar" in file
sed s/foo/bar/g file replace "foo" in file with "bar"
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Variable Substitution
${v:-def} $v, or "def" if unset
${v:=def} $v (set to "def" if unset)
${v:?err} $v, or "err" if unset
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Conditional Execution
c1||c2 run c1; if it fails, run c2
c1 && c2 run c1; if it works, run c2
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Common utilities and switches
ls -lSr list files, biggest last
ls -ltr list files, newest last
ls -lh human-readable filesizes
du -sk* directory sizes (slow)
sort -n sort numerically (not alpha)
ps -ef list my commands
wget URL download URL
time cmd stopwatch on 'cmd'
touch file create file
read x read "x" from keyboard
cmd |\tee file.txt cmd output to stdout and also to file.txt
nice cmd run cmd with low priority
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Networking
ifconfig -a list all network interfaces
netstat -r show routers
ssh u@host log in to host as user "u"
scp file.txt\u@host copy file.txt to host as user "u"
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General Admin
less file display file, page by page
alias l='ls -l' create "l" as alsias for "ls -l"
tar cf t.tar\list_of_files create a tar archive t.tar from the listed dirs/files
cal 3 1973 display a calendar ( Mar 73)
df -h show disk mounts
truss -p PID show syscalls of PID
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