bash script cheat sheet

bash script cheat sheet

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#!/bin/bash
set -o errexit
set -o pipefail
set -o nounset
set -o xtrace
# set -eox pipefail #safety for script
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shell script
$0 represent the shell script file name itself
$1 Starting with $1, are actual command line arguments sent to the shell script.
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 clear your bash history
>~/.bash_history
Another option is link ~/.bash_history to /dev/null
ln -sf /dev/null ~/.bash_history

set +o history #stop logging bash history
set -o history #start logging
# put a space before command

# start the command with a space
# not be recorded in history
# up/down arrow keys will not show history

set -eux #safety for script
set -o pipefail: returns error from pipe `|` if any of the commands in the pipe fail (normally just returns an error if the last fails)
set -o errexit (set -e): exit script when command fails
set -o nounset (set -u): exit script when it tries to use undeclared variables

set -u #The shell shall write a message to standard error when it tries to expand a variable that  is  not
       set and immediately exit.
set +x #Use the plus sign(+) before any of the flags to disable
set -x #enables a mode of the shell where all executed commands are printed to the terminal,used for debugging printing every command
set -e #stop a script immediately when something goes wrong.When you're debugging a script, you probably don't want a partially functional script to keep on running, causing havoc or producing incorrect results
set -f #disable automatic file name generation,Globbing can be useful in finding files
set -C #disable Bash's default behavior of overwriting files,configures Bash to not overwrite an existing file when output redirection using >, >&, and <> is redirected to a file

bash -e myScript
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# remote public github content run
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"
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#-c string If  the  -c  option  is  present, then commands are read from string.
#If there are arguments after the  string,  they  are assigned to the positional parameters, starting with $0.

/bin/sh -c 'curl -L https://istio.io/downloadIstio | sh -' #single quotes
/bin/sh -eu -xv -c 'cmd1 | cmd2' #debug.
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#!/bin/sh

# bash lvm-create-vol.sh 3 1
# pvdisplay --- Physical volume ---
# vgdisplay --- Volume group ---
# lvdisplay --- Logical volume ---
VOL_GROUP="vgvagrant"  #Defines the volume group to create volumes on
if [ $1 ];
then
    VOLUMES=$1;
else
    echo "Number of volumes ";
    exit;
fi

if [ $2 ];
then
    SIZE=$2;
else
    echo "Size of volumes in (GB)";
    exit;
fi

VOL=0
while [ $VOL -lt $VOLUMES ];
do
    #create volumes
    lvcreate -L $SIZE -n vol$VOL $VOL_GROUP;
    VOL=$(( $VOL  1 ));
done

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#Syntax for Command Substitution
#The old-style uses backticks (also called backquotes) to wrap the command being substituted
#The new style begins with a dollar sign and wraps the rest of the command in parenthesis

#using the output of one command as an argument to another command
$ today=$(date +%d-%b-%Y) && echo $today
$ echo "Today is $(date +%d-%b-%Y)"
#The inner command, rpm -qa | grep httpd, lists all the packages that have httpd in the name
#The outer command, rpm -ql, lists all the files in each package
$ rpm -ql $(rpm -qa | grep httpd)
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# not a separate external command, but rather a shell built-in
#takes a string as its argument, and evaluates it as if it s typed on a command line
$ COMMAND="ls -lrt"
vagrant@vagrant:~$ eval $COMMAND

#This command can be used in any script also where the variable name is unknown until executing the script. 
#In bash, however, this can be accomplished with variable indirection using the syntax
${!varname}

$ cat tryeval.sh
#!/bin/bash
#Initialize the variable x and y
x=5
y=15

#The first command variable is used to assign `expr` command to add the values of $x and $y
c1="`expr $x + $y`"

#The second command variable is used to assign `echo` command
c2="echo"

#`eval` will calculate and print the sum of $x and $y by executing the commands of $c1 and $c2 variables
eval $c2 $c1
$ bash tryeval.sh
20


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#from windows to linux copy problem fix
$ make
Makefile:21: *** missing separator.  Stop.
$ perl -pi -e 's/^  */\t/' Makefile
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#Makefile gnu fortran
F90 = gfortran
TARGET=ff
OBJECTS = $(TARGET).o
FCFLAGS = -Ofast -g -pg -fbounds-check

#all: clean run

#compile
$(TARGET): $(OBJECTS)
	$(F90) $(FCFLAGS) -o $(TARGET) $(OBJECTS)
%.o: %.f90
	$(F90) $(FCFLAGS) -c $^
run:
	./$(TARGET)
	gprof $(TARGET) gmon.out > ff_analysis.txt

.PHONY:clean

clean:
	rm -rf $(TARGET) $(OBJECTS) *.out *.o
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# The double-bracket syntax serves as an enhanced version of the single-bracket syntax
# the double-bracket syntax features shell globbing.if $stringvar contains the phrase “string” anywhere, the condition will return true. 
if [[ "$stringvar" == *string* ]]; then
#match both “String” and “string”, use the following syntax
#only general shell globbing is allowed. Bash-specific things like {1..4} or {foo,bar} does not work. 
if [[ "$stringvar" == *[sS]tring* ]]; then
#The second difference is that word splitting is prevented. omit placing quotes around string variables
if [[ $stringvarwithspaces != foo ]]; then

#without the double-bracket syntax
if [ -a *.sh ]; then 
#return true if there is one single file in the working directory that has a .sh extension.
return false,If there are none
hrow an error and stop executing the script,If there are several .sh files,because *.sh is expanded to the files in the working directory
#with the double-bracket syntax
if [[ -a *.sh ]]; then
#return true if there is a file in the working directory called “*.sh

#the double-bracket syntax allows regex pattern matching using the “=~” operator.
#the and operator has precedence over the or operator, meaning that “&&” or “-a” will be evaluated before “||” or “-o”.
#returns true if $num is equal to 3 and $stringvar is equal to “foo”.
if [[ $num -eq 3 && "$stringvar" == foo ]]; then

#Double-parenthesis syntax
#true if $num is less than or equal to 5.
if (( $num <= 5 )); then
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for i in {0..10}; do echo $i; done
for i in `seq 0 2 10`; do echo $i; done
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